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Improving Airblast Spray Applications

Plenty of factors can contribute to inefficiencies in pesticide spray applications. Complex interactions between equipment, environmental conditions and physical properties of the material being sprayed influence how much of the material hits the target and how much is wasted.

Spray applications in tree nut orchards for pest and disease control are typically done with airblast sprayers. Cost of the spray material and application plus the potential loss of crop yield from pests and disease make achieving high target deposition and coverage critical.

Dr. Peter Larbi, assistant UCCE specialist in the Agricultural Application Engineering program (AgAppE Lab) at the Kearney Agricultural Research and Extension Center in Parlier, spoke on the components of a good spray application at the 2020 UC Pistachio Day. Larbi is also working on an Expert System model for spray applications to help growers achieve optimum spray coverage. Larbi said that significant material loss can result from drift and ground fall out because of variability in tree canopy profile and size. Lack of calibration and maintenance of spray equipment and operator error also contribute to spray inefficiency. Material that does not contribute to pest or disease control represents a production cost without a return, he said.

 

Spray Application Basics

Airblast sprayers use high volume, high velocity air to transport spray material. They atomize the tank mix liquid and the fan air transports the droplets toward the target. Atomization produces a spray consisting of a spectrum of droplet sizes. Three things can happen to the droplets: they can drift beyond the canopy, they can hit the ground, or they can be intercepted by the tree canopy, i.e. they can hit the target. Research in citrus has shown that 18 to 26 percent of applied spray material is generally off-target. Six to 14 percent is lost to drift, 9 to 20 percent hits the ground.

Environmental factors, such as wind, air temperature and humidity can negatively affect the efficiency of the spray application. Added to those are improper operating settings of the airblast sprayer, including nozzle design, operating pressure and the physical properties of the tank solution.

Maintenance of the sprayer is critical for proper function, Larbi said. Parts, including the agitator in the tank, pressure gauges, pumps, nozzles and fan must be operating correctly.

The rate and speed of the airflow also determine how much of the spray material reaches the target. These factors must be managed with the tree canopy size and foliage density in mind.

The operator of the sprayer is one of the primary arbiters in an effective application. This person should be trained and knowledgeable about sprayer operation, be familiar with and follow best management practices, be attentive to the machinery and respond quickly when a problem arises.

 

Accurate Calibration

The airblast sprayer must be calibrated accurately to deliver the material. Calibration should be done at the beginning of the spray season and changes made when conditions, such as foliage density, warrant. Larbi noted that not all calibration steps are always necessary. Adjustments can be made to the components of the sprayer that are affected by the change in conditions.

The spray application must be timed according to weather conditions and to the target pest for best control. The spray must be directed at the target canopy and adequately penetrate the canopy for optimal coverage. If in excess, penetration can lead to spray droplets exiting the target canopy without depositing on it.

It is important, Larbi said, to take into account specific orchard conditions before starting a spray application. Adjustments in fan speed, ground speed and nozzles can achieve better efficiency. Making sure the droplets hit the target and are being retained there is important, and different situations may call for changes in the tank mix formulation, including the use of adjuvants to improve deposition. Target coverage can be assessed with the use of spray cards (see Larbi’s related article on March/April 2020 edition of Progressive Crop Consultant magazine for details.) These can show if the spray made it to the target, but do not show deposition.

 

Expert System Model

Modeling and simulation tools developed by Larbi focus on efficiency – which is the amount of spray material reaching the target versus the amount of spray delivered by the airblast sprayer. Efficiency entails spray deposition. Larbi’s spray evaluation section of the Expert System can be used to simulate several different spray application scenarios by changing values of inputs and observing spray deposition. These scenarios may represent different optional settings a grower could use in the application. By comparing the outcomes—on-target deposition—of the various settings, the grower can identify the setting that would result in optimum spray material deposition.

Computer modeling utilizes the tree characteristics, application parameters, weather and orchard conditions to develop the optimal settings. Larbi said that validating a model with data from an actual field experiment provides confidence to trust the model’s predictions and to make decisions based on the model.

Tree height, foliage density, and canopy diameter are the tree characteristics. Application parameters are the airflow rate, nozzle type and number of nozzles, operating pressure and ground speed. Temperature, relative humidity and wind are the weather parameters. Orchard conditions include number of trees per row, tree and row spacing and missing trees. Other parameters in the modeling are output per side, total volume applied, total area covered, application rate and number of trees sprayed.

Using the model, inputting the parameters of the sprayer application and orchard design, is one way of improving the efficiency of the spray operation. In contrast, Larbi said that guessing the outcome of an airblast spray application for its canopy deposition, drift and ground fall-out is almost impossible.

Modeling the simulation tools for predictions can improve decision making for better planning, Larbi said.  CitrusSprayEx or similar tools can help. For more information on the Expert System, go to ucanr.edu/sites/CSEESDeploy/CitrusSprayEx_Resources/.

Outsmarting Birds in Vineyards: Know Your Birds and Keep them Guessing

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There are a number of options for controlling—or at least limiting the damage of—birds in vineyards, but experts say the best defense is to build a strategy that includes multiple points of protection and mix it up.

The extent of bird damage within a vineyard depends on vineyard site location, varietal and other variables, as well as bird species. UCCE Viticulture Farm Advisor Glenn McGourty in Mendocino County said birds can be a particular problem on vineyards located along the edges of wildland areas. Particularly in those situations, the best protection is to create a barrier around the vines such as netting. He said woven polypropylene netting works best in his region and experience.

Birds begin to feed on grapes in vineyards as fruit turns color and starts to ripen. In McCourty’s north coast winegrape region, early ripening red varieties such as Pinot Gris, Cielogiolo and Pinot noir are among the favorites, he said. Birds feed around the clock during the day but are particularly voracious in the early morning.

 

Protecting the Crop

UC Cooperative Extension Wildlife Specialist Roger Baldwin said netting appears to be the most effective, and most expensive, option for controlling grapes in problem areas.

“Netting is used in areas where the grower expects substantial grape loss in the vineyard, and the crop is relatively high in value,” Baldwin said.

Another effective option is using birds of prey as a natural bird deterrent. Baldwin said preliminary research shows that falcons can prove to be a good deterrent and provide “fairly substantial reduction” in crop loss.

“Falconry is a pretty effective tool but it too is pretty expensive. While it’s a little less expensive than netting, it also should be used in higher value cropping areas,” Baldwin said.

McGourty agreed falcons can be effective in his high end vineyard area, particularly in larger vineyards.

“Falcons work quite well, but are expensive, starting at about $20,000, so you need about 500 acres to make it pencil out,” McGourty said. While netting and falconry are two effective options for controlling birds, they can be cost-prohibitive for smaller or lower value vineyards.

One lower-cost control is to put a human in the vineyards with a shotgun, but that comes with both regulatory and public/neighbor relations considerations.  Invasive pests, such as starlings don’t require specific depredation permits, while most native and migratory species, including house finches and robins, will require permits through state and federal wildlife agencies to remove birds through shooting or trapping.

 

Starlings, an invasive bird common in many vineyard regions can
cause significant damage to grapes (photo courtesy UC Regents.)

 

Many growers rely on frightening devices that use auditory or visual hazing, such as propane cannons or electronic sound transmitters. Visual devices such as reflective mylar streamers, scare-eye balloons and even air dancers can provide some benefit for brief periods of time. Baldwin said auditory devices must also provide variety and target specific bird populations in the vineyard to maintain their effectiveness.

 

Know Your Birds

Baldwin said an overall program should take into account what type of birds are in the vineyard and the corresponding damage, which will vary by species. Starlings can cause extensive damage, for instance, plucking off and eating the whole berry and damaging neighboring berries with their feet. House finches tend to peck at berries and tear them open, which can lead to secondary disease problems such as bunch rot from juices dripping on berries below. It is important to have someone who is experienced at identifying bird species looking at the damage and keep track of what is in the vineyard year to year, as with most pest issues, he said.

 

House finches tend to peck at berries and tear them open, which can
lead to secondary disease problems such as bunch rot from juices dripping on berries below (photo courtesy UC Regents.)

 

Generally, Baldwin noted, migratory birds are more susceptible to frightening devices because they typically only loiter in one place for a week or two. For more resident or semi-permanent populations, the UC IPM manual suggests combining visual devices such as mylar strips, with auditory devices such as propane cannons. The key to success is to mix things up so that birds don’t become habituated to the hazing device, Baldwin said.

“Loud noises like propane cannons and shell crackers can be effective at deterring birds from a given area for a short period of time,” he said. “What we generally recommend is growers would mix and match some of these tools. Maybe use propane canons for five to six days then when birds habituate, move to electronic distress calls, then incorporate visual hazing devices, and so on. You can’t just put mylar streamers out there and think you are going to solve the problem, and you can’t just put propane cannons and think that will solve the problem; you have to be smart.”

One auditory hazing device, Bird Gard, relies on what the Sisters, Ore.-based company calls “bioacoustics” to randomly produce calls of naturally occurring local dominant predator birds combined with resident pest bird distress and alarm calls. All sounds are customized to repel the specific birds within a particular vineyard. Since the species of pest birds can change during the season, each unit contains a changeable sound card that is customized to vineyard location and the type of pest bird in the vineyard.

“Bird Gard has been around for over 30 years. In our early stages of development, we learned pretty quickly that birds habituate to the same sound played over and over,” said Quay Richerson, California sales director for Bird Gard. “So, we developed a microprocessor within our circuit board that randomizes the order in which sounds playback, the frequency of the sounds, and the intermittent time-off period. The keyword there is randomized. The sounds have to keep changing all the time for it to have the effectiveness we desire.”

Quay said that to protect the crop as sugars come on, growers should ensure the Bird Bard units are operating two weeks before veraison and run through harvest.

Baldwin said growers can usually get three to four weeks of protection with a strategic program, so they should wait to put defenses out until close to when feeding begins to get control through harvest. The decision on when to start implementing bird strategies is largely site specific. Individual growers should look at the previous year’s bird damage and crop value to figure out if and when it is time to spend the money on bird control.

“It’s best to begin before birds start coming and feeding in those fields, but you don’t want to begin too soon because each control measure will only last so long,” Baldwin said.

 

New Technologies

While some bird control measures are as old as farming, newer measures are looking at integrating technologies to outsmart one of grape production’s smart pests. New research is being done on devices that create background noise that interferes with birds’ ability to communicate with each other. Feeding repellents are also in trial, though that technology is in its infancy. In addition, drone technology is also being researched.

Dr. Page Klug, Supervisory Research Wildlife Biologist with the USDA National Wildlife Research Center at the North Dakota Field Station, has conducted evaluations of unmanned aircraft systems (UAS) as a tool to protect agricultural crops from bird damage.

UAS are known to elicit behavioral and physiological responses in wildlife and have been proposed as a means to protect crops from birds. Klug evaluated behavior responses of blackbirds to fixed wing and rotary wing drones on a number of platforms and hazing methods.

The birds showed no response to the fixed wing UAS but did show a response to the rotary UAS and responses were more pronounced with lower altitude approaches.

Klug concluded that the rotary UAS has the potential to modify bird behavior in a way that may reduce crop damage, but emphasized in her research that no studies have been done to assess potential effectiveness.

Klug said that to be effective in protecting crops from blackbird depredation, modifications to the physical UAS might be needed. Modifications include the addition of an audio system to produce distress or alarm calls or firearm discharge sounds, adding lasers or lights or shapes that mimic an aerial predator.

In addition, a fully automated UAS may be a more effective strategy. This modification could potentially reduce labor, Klug wrote, The UAS could also be programmed to fly patterns which would be most likely to deter birds. Environmental conditions also come into play with UAS use as low temperatures can affect battery packs. Klug noted that their evaluations were done with specific UAS models and other types of drones and responses by birds to approaching UAS can vary based on the specific platform and are likely species and context specific.

While growers have access to a number of measures for controlling what can be one of a vineyard’s most perplexing pests, an effective program is not “set it and forget it” experts said. An effective program should be customized and managed according to each specific site, taking into consideration the bird pests present and the size and value of the vineyard and grapes.

Preparing for the Invasive Spotted Lanternfly Threat

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In 2014, Pennsylvania reported the occurrence of a large planthopper, the spotted lanternfly (SLF), in Berks County.  SLF (Lycorma delicatula) is a hemipteran insect of the family Fulgoridae and is thought to have arrived in the US on a shipment of stones in 2012. Ever since it was found in Pennsylvania, it rapidly spread to many states with infestations currently present in Delaware, Maryland, New Jersey, Pennsylvania, Virginia, and West Virginia. SLF has also been found, without established populations, in Connecticut, Massachusetts, New York, and North Carolina as of March 2020. SLF is native to China and has a wide host range including fruit trees such as apple, apricot, cherry, and peach; ornamental or woody trees such as birch, black walnut, dogwood, lilac, maple, pine, poplar, and tree of heaven; and grapes. The tree of heaven, which is also an invasive species in the United States, is a favorite host of SLF.

Spotted lanternflies were first reported in the U.S. in Pennsylvania and have since spread significantly on the East Coast. The tree of heaven, which is also an invasive species in the United States, is a favorite host of SLF.

 

Considering the history of the spread of invasive pests from other areas to California, the presence of tree of heaven throughout California, and the importance of the grape industry and others susceptible hosts to the California economy, it is important to be aware of this pest and strategies to mitigate its potential negative impact.

 

Biology

SLF adults are about 1” with tan-colored forewings that have black spots and markings and hind wings having red, black, and white coloration and black spots. The abdomen is yellow with dark bands. Females deposit eggs in batches of 30 to 50 and cover them with a yellowish-brown waxy protective deposit. Eggs are the overwintering stage of SLF. Nymphs emerge in spring and go through four instars. First to third instar nymphs are black with white markings and the fourth instar is red with black and white markings.

Female SLF deposit eggs in batches of 30 to 50 and cover them with a yellowish brown waxy protective deposit.

 

Damage

SLF can occur in large numbers and suck plant sap with their needle-like mouthparts, reduce the plant vigor, and can cause mortality in severe cases. Copious amounts of honeydew secreted by SLF promotes the development of sooty mold, which affects photosynthesis when on foliage or quality when on fruits. Since SLF infests several landscape trees and populations build in large numbers, it can also be a nuisance in urban and landscape areas.

 

Lifecycle of the spotted lanternfly.

 

Spread

SLF can fly and spread to long distances through wind currents. They typically land on large trees and then distribute to other hosts. Their ability to deposit eggs on non-living surfaces like rocks, vehicles, and packages increases the risk of their accidental spread through materials shipped or vehicles moving from the infested areas. A recent modeling study identified California as a region highly suitable for the establishment of SLF.

 

Control

Scientists on the east coast and in California are currently working on biocontrol agents such as parasitic wasps (referred to as parasitoids) that attack SLF eggs and nymphs. If these parasitoids are specific to SLF and do not pose a risk to native insect populations or beneficial insects, they can be reared and mass-released for areawide control of the pest.  There are certain chemical pesticides and biological pesticides based on entomopathogenic fungi such as Beauveria bassiana can be used to treat SLF infestations. However, the best strategy is to reduce the risk of its introduction and spread.

 

Although adults may be detected and removed, eggs deposited on packages, vehicles, and other surfaces could be dispersed to new areas and become sources of infestation.

 

As the SLF not only infests agriculturally important hosts, but also infests hosts in urban and landscape areas, both the agricultural community and the general public should be vigilant and join hands to prevent its invasion and spread in California. When there is an areawide problem, collective actions have a major impact on addressing them. A few important points to note are:

  • Be aware of the pest and able to identify the eggs, nymphs, and adults. This link to a video provides an overview of the pest, its biology, damage, and control: https://www.youtube.com/watch?v=45103-PFI4M.
  • Check vehicles and packages arriving from the infested regions in the United States for egg masses.
  • If found, immediately report to the local Agricultural Commissioner, University of California Cooperative Extension, or California Department of Food and Agriculture office.

 

Useful Resources

https://academic.oup.com/jipm/article/6/1/20/2936989

https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=15861

https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=26349

The Brown Marmorated Stink Bug Is (Still) Invading California

The brown marmorated stink bug (BMSB), Halyomorpha halys, has caused significant yield losses in fruit and nut crops around the world. Its appearance in California around fifteen years ago was no surprise after it had already invaded large portions of the country, especially the mid-Atlantic states. Here, we give background information and an overview of the brown marmorated stink bug biology, its current status within California and its potential to impact pistachio production.

Invasion History of the Brown Marmorated Stink Bug

Originally, BMSB was known only in China, Korea, and Japan. With increasing global trade and transport, it started, like many other species, spreading to new parts of the world. Outside of its native range, it was first identified from samples in Allentown, Pennsylvania, in 2001, but the earliest confirmed sighting of the invasive stink bug already occurred five years prior. That invasive species are present for years before their official recognition is common since they generally arrive in low numbers and have to build up their populations before they can cause any damage—which then generally attracts attention and alarm.

In the case of the brown marmorated stink bug, regular interceptions, for example, in the UK and New Zealand show that it probably traveled in transport crates or shipping containers to the US, and later to Europe, Canada and Chile. Shipping containers provide BMSB adults (the overwintering stage) with shelter and protection. Other favored overwintering sites are other human-made structures, including homes, garages, barns, etc. This, in combination with their likely arrival at trade hubs such as large cities, and their tendency to form overwintering aggregations that can consist of hundreds or even thousands, has led to them being classified as a ‘nuisance pest’. Indeed, unlucky homeowners have struggled with up to 25,000 BMSB hiding in their walls, attics, and other living spaces during the winter. Of concern for farmers in California is BMSB movement from urban shelters into agricultural crops.

Biology of the Brown Marmorated Stink Bug

The brown marmorated stink bug biology is similar to many of our native stink bugs and shares many traits with leaffooted bugs and smaller ‘true bugs’. They have an egg, nymph, and adult stage. Adult BMSB are about half an inch long, with a brown body and white striped antennae and legs.

An adult of the brown marmorated stink bug. (Photo courtesy of W. Wong)

In California, they can be confused with Euschistus species or the predatory Rough-shouldered stink bug Brochymena; the website www.StopBMSB.org has a helpful compendium with pictures and detailed descriptions. After mating, adult female BMSB lays up to ten egg masses, often consisting of about 28 lightly blue-green colored eggs, over the span of her life, which can last several months. The nymphs undergo five instars until they reach the adult stage. The red-brown and black first instar nymphs can be seen sitting around the egg mass after hatching, feeding on the symbiotic microorganisms that will make it possible to digest their various host plants. After that, they start wandering off in search of food. Second to fifth instar nymphs are black and white in appearance and can walk rather long distances for their small size, for example fifth instars can walk 65 ft within only four hours.

A nymph of the brown marmorated stink bug investigating a pistachio. (Photo courtesy of K. Daane)

Both the nymphs and the adults feed by inserting their needle-like mouthparts into a variety of plant tissues including stems, leaves, and especially reproductive structures, secreting digestive enzymes and sucking up the liquified plant material. The mechanical damage and specifically the chemical changes due to the excreted enzymes can lead to discoloration, deformation and the abortion of fruiting structures, all of which make the crop unmarketable. Many stink bug species are known mainly as secondary pests in various crops. Often, an individual species has different host plants to fulfill their nutrient requirements and can therefore behave as a pest in different crops or take refuge in a naturally occurring host.

The brown marmorated stink bug has more known host plants than other stink bugs; in the US alone, more than 170 plant species have been reported, many of them economically important crops. These include vegetables, leguminous crops, fruits, nuts, and ornamentals. In the first big outbreak year, 2010, damage caused by BMSB to apple production of the mid-Atlantic states led to economic losses of $37 million. Other examples, from Georgia and Russia, include the destruction of the hazelnut crop, which is highly important for these regions, to such an extent that the government paid citizens for every bucket full of stink bugs. In contrast to those stories, BMSB crop damage has been relatively quiet in California and the rest of the West Coast.

The Brown Marmorated Stink Bug in California

Not long after the brown marmorated stink bug was reported on the East Coast, in 2002, the first individual was discovered in a storage unit in California. Like most invasive species, there can be years between the first interceptions of individuals and the establishment of a reproducing population. Consequently, the first established brown marmorated stink bug populations in California were not reported until 2006 in the Los Angeles area and 2013 in Sacramento. The following year, 2014, monitoring efforts were put into place and, since then, the invasive stink bug has been shown to be established, or at least been detected, in most California counties (https://cisr.ucr.edu/invasive-species/brown-marmorated-stink-bug). However, presence and abundance are two separate matters. The warm climate allows the invasive insect to complete two generations per year and BMSB damage has been reported in several almond orchards near Modesto, Calif. The stink bugs have also been sighted in commercial peaches, but most sightings still occur in urban centers rather than agricultural areas, often associated with the common ornamental tree, the tree of heaven (which is also from Asia). Good news is that in many monitoring sites, trap catches have actually been decreasing for several years. The current situation therefore sees BMSB spreading in California, but at a low density apart from localized larger populations. The bad news is that, in addition to almonds, many California specialty crops are either known or potential hosts.

Adult Euschistus (top) and Brochymena (bottom) species, lookalikes of the brown marmorated stink bug that can be found in California pistachios. Note that Euschistus is missing the brown-white banded antennae and Brochymena has ‘rough shoulders’ as well as an uneven front. (Photos courtesy of K. Daane)
Adult Euschistus (right) and Brochymena (left) species, lookalikes of the brown marmorated stink bug that can be found in California pistachios. Note that Euschistus is missing the brown-white banded antennae and Brochymena has ‘rough shoulders’ as well as an uneven front. (Photos courtesy of K. Daane)

Damage Potential in Pistachios

To assess the threat of the brown marmorated stink bug to California’s pistachio production, we conducted trials under Central Valley conditions by caging terminal branch endings with pistachio clusters were caged, just after bud-break, and exposing the developing nuts to BMSB for a five-day feeding period. Trials were conducted throughout the season to account for hardening of the pistachio shell and changing temperatures. As a comparison, clusters were also exposed to adults of two native species that feed on pistachios, the flat green stink bug Chinavia hilaris and a leaffooted bug, Leptoglossus zonatus.

Native true bugs common in pistachios can be grouped by their size: ‘small bugs’ like mirids are more abundant early in the season and cannot pierce the pistachio shell later in the season, while ‘large bugs’ like stink bugs and leaffooted bugs continue to cause damage during mid- and late-season. The insertion of their needle-like mouthparts and the secretion of digestive enzymes can result in external damage, brown to black lesions of the outer fruit layer, the ‘epicarp lesions’ that can stain the outer shell and lower market value. Especially after mid- to late-season feeding, epicarp lesions often appear with a delay or not at all, hiding the internal damage: if the insect’s mouthparts reach the endosperm tissue, it can become necrotic or lead to aborted nuts. Along with direct damage, the feeding can lead to fungal infections, ‘stigmatomycosis’, that result in blackened, foul-smelling kernels.

We found that, one to two weeks after feeding, BMSB caused similar amounts of external nut damage (i.e., epicarp lesions) as did the native species tested. However, by following clusters development and damage throughout the season, we noticed more epicarp lesions formed later in the season in the BMSB exposed cages. Shortly before harvest in September, there were significantly more damaged nuts per cluster (based on epicarp lesions) in BMSB cages than in the green stink bug or leaffooted bug cages, independent of when the feeding occurred during the season. This indicates that adults of the brown marmorated stink bug can cause more external damage than our native large bug pests. However, the more important internal damage criteria such as the number of necrotic kernels, aborted nuts or kernels with stigmatomycosis were not different between these large bug pest species tested.

External and internal stink bug induced pistachio damage: epicarp lesions (marked with red ‘X’) right, and kernel necrosis, left. (Photos courtesy of J. Stahl and K. Daane)

The brown marmorated stink bug may generally cause more crop damage than other large bugs because of their feeding behavior or saliva composition – this is still being investigated, but the main factor that makes them such an important pest are the sheer numbers in which they occur in affected areas, such as Virginia. In California’s Central Valley, this is generally not the case, at least at this time. One potential explanation for this phenomenon are the hot and dry summers in the Central Valley, as well as the large-structured agriculture, with thousands of contiguous acres of commercial agriculture, that may make it difficult for BMSB nymphs to switch host plants to access all their required nutrients.

To point this out, in another trial we caged first instar BMSB nymphs on different California specialty crops throughout the last two seasons and showed high nymphal mortality. This could explain the low overall abundance of BMSB in the Central Valley—it’s just too hot and dry. They were generally more likely to reach the adult stage on almond than on pistachio, which could be explained by the close relation of almond to one of their favorite host plants, peach. This is also in line with the records of brown marmorated stink bugs in California almonds, but so far not in pistachio. Still, even on almond survivorship was low.

To conclude, the brown marmorated stink bug has the potential to cause at least as much damage in pistachios as our native stink bug and leaffooted bug species. This is, however, dependent on its abundance in the respective areas, which is currently low.

What You Can Do if You Suspect You Have Brown Marmorated Stink Bugs

In case BMSB becomes a bigger issue in California, there are a number of measures that can be taken. The first line of defense is monitoring. Take into account that border rows are generally more affected than the inside, especially if the orchard or field is close to woodlands or preferred host plants, like the tree of heaven. Common sampling programs include sweep and beat samples as well as visual counts, but those methods have not proven to be as reliable as pheromone traps. There are many different trap types available of which the most effective one is the black pyramid trap, and the most economic one a clear sticky trap that can be mounted on a pole. The lure most commonly used is a blend of the aggregation pheromone of BMSB and the closely related oriental stink bug (Plautia stali).

Trap counts can be used to determine insecticide applications as opposed to calendar-based applications, but the system is still being optimized because it is more difficult to relate trap catches with BMSB densities in the orchard than it is for other pests. Pheromones and insecticides can also be combined in ‘attract and kill’ methods using for example ‘bait trees’ that are equipped with pheromone lures and are sprayed in regular intervals. This can ideally reduce pest populations with only a small area affected by the insecticides, thereby protecting natural enemies, decreasing the risk of secondary pest resurgence, and reducing costs. Research efforts to make these systems commercially available are currently underway on the east coast.

When the brown marmorated stink bug first started threatening yields in the mid-Atlantic states, growers applied insecticides registered for native stink bugs such as pyrethroids and neonicotinoids, which unfortunately failed to provide complete control of this invasive species. Part of the failure was due to the combination of a very mobile insect and products with a short residual activity. It is also easier to kill the overwintering adults than the subsequent generations, so once population densities are high during the season, application efficacy is reduced.

One reason to back off of pesticide treatments for low densities of BMSB are its natural enemies. Many predators are feeding on different life stages of this bug, although the levels of control in California are still not clearly known. There are a number of native parasitic wasps that attack the egg stage of stink bugs, including this invasive stink bug. However, since the brown marmorated stink bug is a novel host for them, they have yet to adapt to it; often, their offspring are not able to develop within the eggs of the brown marmorated stink bug. Currently, BMSB control in the United States by resident natural enemies is not sufficient to reduce population sizes significantly and prevent crop losses. There is, however, a parasitic wasp that could make a difference: Trissolcus japonicus, also known as ‘the samurai wasp’.

A female of the samurai wasp Trissolcus japonicus parasitizing
eggs of the brown marmorated stink bug. (Photo courtesy of Warren H. L. Wong)

The Samurai Wasp

To the naked eye, the samurai wasp looks just like any other of our native egg parasitoids that attack stink bugs: it is smaller than a grain of sand, mostly black, and completely harmless. But unlike its relatives, it has the same area of origin as BMSB and is very well-adapted to this host. In Asia, the samurai wasp is the most important natural enemy keeping the brown marmorated stink bug in check. Like its host, it has made its way to North America and Europe. After first being discovered in the eastern part of the US in 2014, it spread, presumably with multiple new introductions from Asia, to the West, and has recently been recovered in the Los Angeles area. Across the country, release and redistribution efforts are underway and a national consortium of researchers is working on the development of an optimal strategy to use the samurai wasp to control BMSB. The samurai wasp is unlikely to be a silver bullet but will be one of many factors helping with the suppression and sustainable management of this invasive pest.

In California, most growers have so far been luckier than their colleagues on the east, and there are no indications that that will change soon; but the brown marmorated stink bug has been full of surprises and considering that it has the potential to severely impact SJV nut production, everyone should continue to be calm but vigilant.

For a list of references please email judithmstahl@berkeley.edu.

Characterization and Interaction of Fusarium Races and Rhizoctonia on Disease Development in Cotton

Objectives of Proposed Research

  1. To survey and molecularly identify Fusarium oxysporum f. sp. vasinfectum (FOV) races and other seedling and wilt pathogens in commercial and grower cotton fields in California.
  1. To further evaluate the seedling and wilt capabilities of FOV races with different inoculation methods using susceptible and resistant Pima and Upland germplasm.
  1. To further evaluate the interactions of different FOV races and Rhizoctonia solani and their impact on disease development in cotton.

Additional match funding has been approved from the California State University Agricultural Research Institute (ARI) for both years of the project thanks to support letters provided by CCGGA and Cotton Inc. With the additional funding, we were able to expand on our current proposed work and add the following objectives to the ARI proposal.

  1. To use representative identified FOV races for phenotypic evaluation of selected Upland cotton germplasm
  2. To determine the effects of pH, temperature, and moisture on disease development in cotton when inoculated with FOV4.

Objective 1: To survey and molecularly identify Fusarium oxysporum f. sp. vasinfectum (FOV) races and other seedling and wilt pathogens in commercial and grower cotton fields in California.  

Prior to this proposal, Fusarium isolates were collected in 2017 and 2018. Isolate information is provided in Table 1. All isolates were identified using two PCR assays and DNA sequencing of the translation elongation factor (EF-1α) gene. The first PCR assay produced a 208 bp amplicon unique to FOV races 3, 4, and 7, while the second multiplex PCR assay genotyped FOV isolates into two genotypes, N (396 bp), and T (583 bp). These genotypes were identified based on the absence (N type) or presence (T type) of the insertion of the transposable element Tfo1 in the phosphate permease (PHO) gene unique to FOV race 4. Although not shown these isolates have been genotyped with newly developed primers. We are repeating the genotyping currently for verification of results.

Table 1: Isolates of Fusarium collected in seven locations in the San Joaquin Valley of California in 2017 and 2018.

For the current proposed research, 11 locations across the San Joaquin Valley and six locations in the El Paso, Texas region were sampled beginning in mid-May 2019. To date, single spore isolations for 110 Fusarium isolates have been completed (Table 2). All isolates were collected from symptomatic cotton seedlings. Additionally, 18 isolates of Rhizoctonia solani were also isolated from symptomatic cotton seedlings (Table 2). Other fungal species were isolated and are currently being identified morphologically. It appears that there may be some additional Fusarium species that are not FOV. Isolates will be genotyped similar to 2017 and 2018 isolates.

Table 2: Isolates of Fusarium spp. and Rhizoctonia solani collected in 2019

A preliminary baiting method using collected soil from a cotton field in Dos Palos, Calif. was completed. This assay was modified for the isolation of Pythium spp. from field soil using soybean as bait. From this assay isolates of what appear to be FOV, Pythium, and R. solani were all baited using the susceptible Pima cultivar DP-340. Soil has been collected from a number of locations across CA and will be used in the baiting method to isolate other potential pathogens not isolated from collected plant material. Soil samples from some of the first locations where FOV race 4 was identified in CA but are no longer in production for cotton were also collected. This assay might allow us to determine if the pathogen FOV race 4 is still present in these locations, despite being out of production for at least a decade in some cases.

Two undergraduate students have been trained and have been conducting the work mentioned above under the guidance of Dr. Ellis and her previous graduate student. Another student has also started to isolate DNA from the single spore isolations for identification using new PCR primers. Additionally, DNA sequencing of isolates will be done using the translation elongation factor and internal transcribed spacer region.

Objective 2: To further evaluate the seedling and wilt capabilities of FOV races with different inoculation methods using susceptible and resistant Pima and Upland germplasm.

Three assays will be compared to further evaluate seedling and wilt capabilities of FOV races/genotypes. A rolled towel assay was developed in our lab, and will be compared to the root-dip inoculation method and an infested-oat-seed method that was modified from a protocol by Beccera et al. (2012). Protocols for these methods have all been established and tested in preliminary studies. A rolled towel assay using eight representative Fusarium isolates was completed to evaluate possible variation in aggressiveness towards cotton by different FOV4 genotypes and F. solani isolates. The assay was set up using Pima cultivar DP-340. The results from two runs of the assay are provided in Figure 1 and 2, below. There was a significant difference among isolate and experiment (P<0.0001), but there was not a significant difference for the interaction for isolate and experiment.

To calculate the disease severity index (DSI), lesion length and total plant length were measured with a ruler for each seedling and then the lesion length was divided by the total plant length and multiplied by 100. Seed that did not germinate and were colonized by FOV were given a 100% index rating (Ellis et al., 2011).
For the ordinal scale a 1-to-5 scale was used, where 5 = no germination, complete colonization of the seed; 4 = germination, complete colonization of the seed, and 75% or more of the seedling root with lesions; 3 = germination, some colonization of seed, and 20 to 74% of the root with lesions; 2 = germination, little colonization of the root, and 1 to 19% of the root with lesions; 1 = germination, healthy seedling with no visible signs of colonization.

Additionally, these same isolates or a similar set will be used in the comparison of different greenhouse assays. We plan to use varieties of both Pima and Upland cotton with varying levels of plant host resistance in the assays. Finally, we have started to screen previous isolates collected from 2017 and 2018 using the root dip inoculation method. This will also be done for a majority of isolates collected in 2019. Once pathogenicity for the majority of the isolates is tested using the root dip assay and genotyping is completed a representative set of isolates can be used in our screening efforts.

Objective 3: To further evaluate the interactions of different FOV races and Rhizoctonia solani and their impact on disease development in cotton.

The graduate student for this objective has been currently evaluating environmental parameters of our CA FOV and R. solani isolates such as pH and temperature. Infested-oat inoculum has been prepared to begin the interaction study with FOV race 4 and R. solani. Furthermore, we also plan to co-inoculate with different FOV race 4 genotypes and F. solani.

 

 

 

 

 

Controlling Herbicide-Resistant and Perennial Weeds in California Cotton

Cotton is susceptible to weed interference, especially following emergence, as many weed species can outgrow and outcompete the newly germinated seedlings. This includes a weed native to California – Palmer amaranth (Amaranthus palmeri)- whose season-long germination phenology and high rate of photosynthesis enhances its ability as a crop competitor. Palmer amaranth interference significantly affects the growth and yield of most agronomic crops, with cotton being one of the more sensitive commodities. In addition to direct impacts on yield, Palmer amaranth can also interfere with harvest efficiency. Research has suggested that mechanical harvesting of cotton with Palmer amaranth at densities greater than six plants per 30 feet of row was impractical because of the potential for damage to equipment. Additional reports noted that the frequency of work stoppages increased as Palmer amaranth densities increased because of the need to repeatedly dislodge weed stems from the harvester.

Figure 1b. Palmer amaranth infestation in an almond orchard in Merced County (2019).
Figure 1a. Glyphosate-resistant Palmer amaranth in cotton in Madera County (2019).

 

 

 

 

 

 

 

 

Currently, glyphosate is the predominant herbicide applied in California cotton for weed control. According to data derived from the California Department of Pesticide Regulation (CDPR) pesticide use reports, glyphosate was applied to 438,305 cotton acres in 2016, which is eight times more treated acreage than the next most commonly applied active ingredients (paraquat and oxyfluorfen). The use of glyphosate is not limited solely to cotton; glyphosate is an important component of weed control programs in a diverse array of crops, including almonds, alfalfa, corn, grapes, pistachios, and walnuts. The extensive use of glyphosate across commodities and over time has resulted in the selection for glyphosate-resistance in six species in California, including Palmer amaranth (Figures 1a and 1b, above).

Pesticide use reports indicate that California cotton growers do not regularly use residual herbicides on their planted acres; pendimethalin and flumioxazin were applied to less than half of California’s cotton acres in 2016, suggesting that growers are relying, heavily, on post-emergence measures (including glyphosate, hand-weeding, and cultivation) for weed control. Palmer amaranth has an exceptionally high growth rate, which allows the species to rapidly exceed height limits for chemical control. For example, glufosinate applications should be made to small (<3” in height) Palmer amaranth to prevent weed escape and regrowth.

In 2019, a trial was undertaken in Fresno, Calif., to describe the growth of Palmer amaranth in response to emergence date and to determine how quickly Palmer amaranth can overcome most herbicide label height limits. Palmer amaranth seed was collected in September of 2018 from a population growing alongside an agronomic crop field in Merced County. Seed were planted into 1.7-gallon pots containing all-purpose garden soil on April 21, April 28, May 30 and June 18, 2019. Palmer amaranth emerged on April 24th, May 2, June 2 and June 21 and were thinned to a density of one plant per pot (10 pots total per planting date). Palmer amaranth growth and development was recorded for each individual pot every second day until 20 days after emergence (DAE). Growing degree days (GDD) were calculated for each observation window using UC IPM models and Palmer growth regressed against GDD to predict critical stages (3 and 6 inches in height) for Palmer management.

All Palmer amaranth in this study reached a height of 3 inches by six to 10 DAE (Figure 2, below). Palmer emerging on April 24th and May 2nd reached a height of six inches 14 to 16 DAE, whereas Palmer amaranth emerging on June 2nd and June 24th reached a height of 6 inches 12 DAE. Plant heights at 20 DAE were 11.5, 8.5, 20.0 and 21.3 inches for the April 24th, May 2nd, June 2nd and June 21st emergence dates, respectively.

Figure 2. Palmer amaranth height (inches) two to 20 DAE as affected by emergence date.

To standardize Palmer growth across all observation periods, plant heights were regressed against accumulated GDD using a second-order polynomial model; a threshold base temperature of 50 degrees F was used in the computation (Figure 3, below). Results indicated that the observed SJV Palmer amaranth population requires 175 to 180 GDD to achieve a height of 3 inches and 270 to 275 GDD to reach a height of 6 inches. This model can serve as a basis for predicting Palmer amaranth development in the future. Understanding the relationship between the accumulation heat units and plant growth makes it possible to predict when Palmer could become too large for control during a growing season regardless of yearly variation in temperature.

Figure 3. Palmer amaranth height (inches) regressed against GDD. Y = 5E-05×2 + 0.0084x – 0.0247 where Y = inches and x = GDD.

If Palmer amaranth escapes herbicide (or cultivation) treatments, hand-weeding may be needed to prevent Palmer amaranth from producing seed that can be returned to the soil seedbank. Remember: female Palmer amaranth can produce up to a million seed per plant, which can support an infestation for many years to come. When hand-weeding, plants should, ideally, be removed entirely from the field to prevent them from becoming re-established. Even plants that are cut off at or near the base of the stem can re-sprout and achieve reproductive maturity.

Escapes are not uncommon as Palmer amaranth can grow rapidly and outpace many control efforts. If plants become established in the field and hand-weeding is necessary, be sure to remove as much of the weed biomass as possible to prevent plants from growing and achieving reproductive maturity.

Field Bindweed Perennialization

Field bindweed (Convolvulus arvensis) is another species that has become problematic in California cotton, particularly in crop rotation systems that are characterized by drip irrigation and reduced tillage. In addition to negatively impacting cotton yield, bindweed can serve as an alternate host for the silverleaf whitefly, the honeydew from which is a primary source of sugars that can result in sticky cotton lint.

Field bindweed is a deep-rooted (up to 20 feet) and spreading perennial vine, Management guidelines often suggest that field bindweed is susceptible to control at the seedling stage, although there is limited information to suggest when newly emerged field bindweed vines assume the characteristics of perennial plants. Personal communications between weed scientists have indicated that field bindweed seedlings could survive defoliation attempts as soon as 3 WAE.

Field bindweed in cotton in Merced County (2019).

In 2019, a trial was undertaken in Fresno, Calif., to describe the growth of seedling field bindweed and to determine when the vines take on the characteristics of perennial plants; specifically, the study was designed to evaluate at what stage field bindweed can regrow from root buds following above-ground biomass removal. Field bindweed seed collected in Merced County in 2018 was scarified using boiling water to induce germination. Seed were planted into 1.7-gallon pots containing all-purpose garden soil on April 17 and June t, 2019, representing two runs of the trial. Bindweed emerged on April 20 and June 4, respectively. Four replicate bindweed seedlings were physically defoliated (by removing all aboveground biomass at the soil line) at either 2, 4, 6, or 8 WAE and their compensatory growth measured two weeks after the cutting treatment (WAT). A second set of seedlings were destructively harvested at 2, 4, 6, and 8 WAE to describe biomass accumulation at the time of cutting.

Results indicate that the ability of field bindweed to regrow following defoliation increased with plant age (Table 1, see below). Field bindweed seedlings defoliated at 2 WAE did not re-sprout by two weeks following cutting; no viable above- or below-ground tissue was observed and recorded. Thirty-eight percent of field bindweed seedlings defoliated at 4 and 6 WAE survived the cutting treatment and re-sprouted. One average, 0.5 to 3.0 grams of stem/leave and root tissue were recovered at 2 WAT. One hundred percent of the field bindweed defoliated at 8 WAE survived the cutting treatment and produced 13.1 and 35.9 grams of above- and below-ground tissue, respectively.

While most management practices are focused on controlling rhizomatous vines, the seed of field bindweed should not be ignored. Bindweed seed can remain viable in the soil for decades (Weaver and Riley 1982) suggesting that infestations can re-occur even if rhizomes are successfully eradicated from a site. Anecdotal evidence indicated that newly emerged seedlings could take on the characteristics of perennial vines, rapidly, following germination. Results from this study suggest that field bindweed seedlings may not remain sensitive to certain control measures for more than 4 weeks after emergence. Studies to examine seedling development and responses to contact and systemic herbicides will be conducted during the fall of 2019/winter of 2020.

Field Bindweed Response to Trifluralin and Pendimethalin

Results from previous studies in processing tomatoes have shown that trifluralin pre-plant incorporated (PPI) can suppress perennial field bindweed vines (Sosnoskie and Hanson 2015). However, most cotton growers do not regularly apply this active ingredient in their systems; with respect to pre-emergence herbicides, pendimethalin (which is in the same chemical family as trifluralin) is more commonly used.

Studies were initiated at the UC Westside Research and Extension Center in Five Points California in May 2019 to describe the response of field bindweed to trifluralin and pendimethalin relative to an untreated check. Trifluralin (24 oz/A Treflan) and pendimethalin (24 oz/A Prowl H2O) were applied on May 24 and physically incorporated to a depth of three inches. Individual plots were 13.5 feet in width and 50 feet in length. An untreated check (UTC) was also included. Bindweed pressure in the trial was considered to be significant; approximately half of the study site was covered in vines two weeks before the initiation of the trial. To ensure sufficient contact between the herbicide and the soil surface, the trial location was repeatedly disked to remove standing vegetation. Bindweed cover and flowering was assessed weekly from June 6 until July 16.

Figure 4. Bindweed cover (% of area occupied by vines) in response to trifluralin and pendimethalin.

Few pre-emergence or pre-plant incorporated herbicides are registered for the suppression of perennial field bindweed vines. Trifluralin, a dinitroaniline microtuble inhibitor, has been shown to inhibit vine emergence while pendimethalin has not. Results from the 2019 trial demonstrated that vine cover in the trifluralin treatments was reduced by 50 percent or more relative to the untreated check and pendimethalin treatments (Figure 4, above). There were no differences between pendimethalin and the UTC. By July 16, mean bindweed cover in the trifluralin plots was 45 percent, whereas cover in the pendimethalin and UTC plots were 88 percent and 93 percent respectively. Flowering didn’t commence until June 27 in all treatments (<1% – trifluralin, 11% – pendimethalin, 27% – UTC) (Figure 5, below). Trifluralin also reduced flowering potential on July 8; however, by July 16, 90 percent of emerged vines were flowering in all treatments. Pendimethalin and trifluralin control a similar spectrum of weeds; if field bindweed is a concern in a field, growers may want to consider the use of trifluralin for vine suppression.

Figure 5. Bindweed flowering (% vines flowering) in response to trifluralin and pendimethalin.

Continuing Research

A field trial to evaluate the combined effects of residual and postemergence herbicides and cultivation on vine control and cotton growth is ongoing and will be reported on at a later date. Results describing bindweed control in response to fall applied herbicides will also be presented later.

Economic Trends in Almond Production

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The University of California Agricultural Issues Center (UC AIC) and the Department of Agricultural and Resource Economics at UC Davis work with UC Cooperative Extension Farm Advisors and Specialists to compile cost studies for crops and livestock produced in California. These costs and return studies are used by growers, bankers, crop consultants and many others to aid in a range of farm decisions from what to plant to production specifics. Often policy makers and researchers use these cost studies as well.  The current and archived cost studies can be found at: https://coststudies.ucdavis.edu/

UC AIC recently released new cost and return studies for almond production in California. These 2019 regional cost and return studies for almonds are available for the Sacramento Valley, and the Northern and Southern San Joaquin Valley. This recent update of almond studies presents an opportune time to explore trends in almond cost and returns for the most recent two decades.

Before digging into graphs and figures, it’s important to discuss the elements of the cost study. The cost and return studies are meant to be used as a guide for growers, and actual costs and returns will vary depending on the specifics of the operation, growing conditions, and orchard characteristics. Therefore, it is necessary to specify underlying assumptions for orchards represented.  It is not feasible to represent the infinite number of almond production scenarios out there. The following are some of the basic assumptions of 2019 cost study for the Northern San Joaquin Valley. For the full list of assumptions for each study listed in the charts, see the cost and return studies themselves.

  • The orchard consists of 100 acres of almonds with a density of 130 trees per acre.
  • No specific variety is listed.
  • The useful life of the orchard is expected to be 25 years.
  • A new micro-sprinkler irrigation system is installed during orchard establishment.
  • The expected yield at maturity is 2200 lbs per acre at an expected price of $2.50/lb.
  • Interest rates are 5.25% for operating loans and 6% for long-term investments.
  • Land value is $25,200 per producing acre.
  • Cost of pumping irrigation water from an established well is $100 per acre-foot.
  • Cost of pollination is 2 hives per acre at $200 per hive.

The cost studies go into detail about the following cost categories, and provide a look at costs and returns at various yield and price combinations.

Operating costs: Any costs associated with almond production practices in a given year, including pesticide and fertilizer applications, irrigation water, labor, harvesting, interest on operating loan.

Cash overhead costs: Expenses paid that are not for a particular enterprise and should be assigned to the whole farm operation, such as office and accounting expenses, assessments, field sanitation, or equipment repairs.

Non-cash overhead costs: Annual depreciation and interest cost for farm investments. Examples include depreciation on farm machinery, well/irrigation systems, annual establishment costs, etc.

Establishment costs: Total pre-plant, planting and accumulated costs for non-bearing years. Establishment costs are amortized (spread out) over the useful life of the orchard.

Total costs: Sum of operating, cash overhead, interest and non-cash overhead costs.

Looking over these cost studies can help growers and crop advisors make sure they are incorporating all costs when making crop production decisions.

Trends in Almond Costs

To outline the trends in almond production costs, I use the 1998, 2002, 2006, 2011, 2016 and 2019 UC AIC cost and return studies for establishing and producing an almond orchard in the Northern San Joaquin Valley using micro-sprinkler irrigation. This provides an approximate idea of how costs have developed over time. The trends in most cost categories should be similar across the state, however there may be noticeable differences in certain aspects across regions, ex: land values, water costs, etc.

Figure 1 displays per-acre costs of almond production over time. All costs are adjusted to 2019 dollars to account for inflation. It is clear from the figure that from 1998 to 2016, inflation-adjusted total costs of almond production remained similar at around $4,500 per acre.

Figure 1: Sample Per-Acre Costs of Establishing and Producing Almonds in the Northern San Joaquin Valley Using Micro-Sprinkler Irrigation, 1998, 2002, 2006, 2011, 2016, and 2019 (in 2019 dollars)
Sources: University of California Agricultural Issues Center Sample Cost and Returns Studies: https://coststudies.ucdavis.edu/.  US Bureau of Economic Analysis, GDP Price Deflator.

Between 2016 and 2019, total costs of almond production per-acre increased substantially (See Figure 1). The driver of this is a large increase in non-cash overhead costs. The primary increases in this cost category between 2016 and 2019 are increases in establishment costs and land values. Interest rates in 2016 were 3.25 percent compared with 6 percent in 2019, increasing establishment costs substantially. According to USDA National Agricultural Statistics Service, average irrigated land values in California increased by 8 percent on average from 2016 to 2019. Factoring land values into the cost of production allows growers to consider the opportunity cost of their investment in the almond orchard. Even if a grower owns the land he or she plans to establish an orchard on, he or she might be better off renting out the orchard and investing those rental revenues elsewhere.

Figure 2: Cost Categories as a Percentage of Total Operating Costs for Almond Production in the Northern San Joaquin Valley Using Micro-Sprinkler Irrigation, 2002, 2011 and 2019
Source: University of California Agricultural Issues Center Sample Cost and Returns Studies: https://coststudies.ucdavis.edu/

Operating costs per acre also increased between 2016 to 2019. Much of the increase in operating costs was due to increasing labor and pesticide costs, as well as increases in the operating loan interest rates. Figure 2 shows various cost categories as a percentage of total operating costs for 2002, 2011 and 2019 almond production. In 2002, pesticides, labor and harvest comprised more than 60 percent of total operating costs. While that number dropped to roughly 43 percent in 2019, over time, pollination, irrigation and fertilizer costs have increased to make up a much larger portion of total operating costs for almond growers. Irrigation costs may continue increasing as a percentage of total operating costs given implementation of the Sustainable Groundwater Management Act (SGMA), however it is unclear what the effects of this regulation will be (for SGMA resources see http://groundwater.ucdavis.edu/SGMA/). Pollination fees will likely continue their trend upward as well, though growers may be able to reduce pollination costs through decreasing the number of colonies per acre, planting self-fertile varieties or making mutually beneficial contractual arrangements with the beekeeper (Goodrich, 2019; Champetier, Lee, and Sumner, 2019).

Trends in Almond Returns

Figure 3: Planted Almond Acreage by Region and Nonpareil Average Base Rate ($/lb in 2019 dollars), 2004-2018
Sources: 2018 Almond Acreage Report, USDA NASS, CDFA. Blue Diamond Payment History 2004-2018. US Bureau of Economic Analysis, GDP Price Deflator.

Figure 3 shows the Blue Diamond average base rate for nonpareil meats from 2004 to 2018 (in 2019 dollars to adjust for inflation). Since 2016, prices have been lower than the 2004-2018 average of $3 per pound. Uncertainty in trade issues have resulted in decreased demand for almonds in a number of countries (Sumner, Hanon and Matthews, 2019). For example, almond exports to China were down 24 percent between 2018 and 2019 (Almond Board of California, 2019). This decreased demand has led to lower almond prices, and with future trade agreements still uncertain, it is unclear how prices will move going forward.

Figure 4: Variety Price as a Percentage of Nonpareil Price and Nonpareil Total Yield as Percentage of Butte/Padre, Butte, Monterey, Carmel, and Fritz Yield
Sources: Blue Diamond Payment History 2013-2018. Almond Board of California Almond Almanac 2013-2019.

The prices a grower receives will vary by quality, size and variety. Figure 4 shows average variety prices as a percentage of nonpareil. In 2016 and 2017, other varieties were discounted fairly heavily in comparison to nonpareil, while in other years discounts were not quite as large. What impacts the size of these discounts? The relative supply and demand of nonpareil compared with other varieties. Figure 4 also displays nonpareil production as a percentage of total production from Butte/Padre, Butte, Monterey, Carmel, and Fritz. In 2017 and 2018, nonpareil production was relatively high compared to these other varieties. The large supply of nonpareil almonds drives down the price relative to other varieties, shrinking the associated premium.

Trends in Planted Acreage

Figure 3 also shows planted acreage from 2004 to 2018 by region along with the average price of nonpareil. The planted acreage trends by region look relatively similar. Over the last five years, the largest almond producing region (Southern San Joaquin Valley) has seen planted acreage drop off significantly. Water availability concerns as well as relatively low prices are likely the driving issues here. The Northern San Joaquin Valley has also seen acreage drop off, but not as substantially as its southern counterpart. Planting in the Sacramento Valley has stayed relatively consistent over the last decade or so.

Figure 5: Planted Almond Acreage by Variety, 1998-2018
Source: 2018 Almond Acreage Report, USDA NASS, CDFA.

Figure 5 shows planted acreage for some of the main almond varieties. Toward the middle of the series, one sees the large planted acreage for most varieties due to relatively high prices in 2004-05. Over time, acreage plantings have stabilized at lower levels. The increase in self-fertile almond acreage is noticeable in the mid 2010s. Operating cost savings from pollination and fewer equipment passes through the orchard were likely driving this trend (Champetier, Lee and Sumner, 2019). Price discounts for the Independence variety in comparison to nonpareil have stabilized, from as low as 2 to 4 percent discount in 2013-14, to on average of 11 percent over the last four years for Independence compared to nonpareil.

Closing Remarks

Overall, net returns from almond production have likely narrowed over the last decade due to increasing costs of production. Land values and interest rates have increased, increasing the costs of establishing an almond orchard. Pollination, irrigation and fertilizer costs have increased as a percentage of total operating costs, while almond prices have remained at relatively low levels over the last few years.  The fact that acreage is still being planted suggests that the potential net returns remain relatively strong compared with other crops in California.

 References:

Almond Board of California. 2019. “Almond Almanac 2019”

Champetier, A., H. Lee, and D.A. Sumner. 2019. “Are the Almond and Beekeeping Industries Gaining Independence?” Choices. Quarter 4.

Goodrich, B.K. 2019. “Contracting for Pollination Services: Overview and Emerging Issues.” Choices. Quarter 4.

Sumner, D.A., T. Hanon and W.A. Matthews. 2019. “Implication of Trade Policy Turmoil for Perennial Crops” Choices. Quarter 4. Available online:

University of California Agricultural Issues Center Sample Cost and Returns Studies. Available online: https://coststudies.ucdavis.edu/

The Start of Irrigation in Almonds: Early Season Irrigation Management Impact Tree Health All Season

One of the motivations for making good water management decisions early in the growing season is to reduce the risk of root and crown diseases that can eventually kill almond and other tree species. These diseases need three elements to infect and damage a tree: a susceptible host plant, a pathogen, and favorable environmental conditions.

A second motivation for diligent early season water management is that even in the absence of a pathogen like Phytophthora, root death due to waterlogging alone can damage or kill trees. Prolonged wet early season conditions have been linked to the “yellowing Krymsk” and “yellowing Rootpac-R” problem in young orchards. This problem which is most often associated with the ‘Monterey’ variety, often resolves itself with careful soil moisture monitoring. Similarly, in older orchards, over-watered conditions in March through May have been linked to the lower limb dieback (LLD) problem. Both conditions show that early season water management can greatly impact tree health much later in the season.

Possible lower limb dieback symptoms in July of 2019 in a mature orchard in Durham, CA. In older orchards, over-watered conditions in March through May have been linked to the lower limb dieback (LLD) problem.

You can read more about “yellowing Krymsk” at: sacvalleyorchards.com/almonds/foliar-diseases/yellowing-krymsk/ and about LLD at: thealmonddoctor.com/2014/05/16/lower-limb-dieback-almond/

Early season water management influences the environment where roots grow by affecting soil temperature and aeration and can be pivotal in how much tree decline occurs. Trees are expensive. The money and effort spent to establish them is lost, more costs lie ahead to replace them, and production is lost.

Choosing the Best Time to Start Irrigating

Each season you need to decide when to start irrigating. It can be difficult to choose the best time to start irrigation. There are a lot of different information sources you can use to make this decision. You can copy practices that you observe around you, evaluate soil moisture, consider the weather and evapotranspiration loss of the crop (ETc), or take a plant-based approach. Utilizing multiple information sources is highly recommended. However, utilizing the plant-based monitoring approach of stem water potential readings with a pressure chamber (or “pressure bomb”) has a distinct advantage over the others.

Schematic showing how water potential is measured in a severed leaf and stem (petiole) using a hand-held pump-up pressure chamber. Source: Adapted from Plant Moisture Stress (PMS) Instrument Company.

The pressure chamber directly determines the water status experienced by the trees, while the other sources, such as ET or soil moisture, although helpful, are indirect. The pressure chamber gauges the amount of positive gas pressure (in pressure units, e.g. bars) required to balance the level of water tension in a plant sample (e.g. leaf; see related graphic). The level of water tension in a leaf expresses the degree of effort utilized to pull water all the way through the tree from the soil.

Relying on an indirect information source, particularly an approach like beginning irrigation when your neighbor does, when the surface soil has dried out, or irrigating on the first hot day, could result in irrigating too soon.

Using Information to Delay the First Irrigation

Research in walnut orchards in California’s Tehama and Stanislaus counties has found that the start of irrigation can be delayed by waiting for mild to moderate water status when measured with the pressure chamber. Some observed benefits have been a minimum 10-percent reduction in energy for pumping, less tree stress during harvest season, and no impact on edible kernel yield. A managed (informed) delay in the start of irrigation may allow for deeper root activity late in the season. It’s possible that a strategy that starts the irrigation season too early promotes a shallow root system at the expense of deeper root development. This is completely contradictory to the conventional wisdom in walnut and almond production that early season irrigation allows for “banking of water” to help avoid high water stress at harvest. UC researchers plan to investigate this managed irrigation delay in almonds in the northern Sacramento Valley.

Before UC researchers begin to see results from this work in almonds, it is best to be cautious in choosing a level of stem water potential with the pressure chamber to trigger the first irrigation of the season. From everything we currently understand, waiting for a tree water status of -2 bars below the fully watered baseline before applying the first irrigation represents a low risk irrigation decision that could benefit long term tree and root health. To learn more about the pressure chamber, stem water potential, the fully watered baseline, and how to go about getting equipment and taking measurements, check out our series at: sacvalleyorchards.com/manuals/stem-water-potential/

Monitoring Weather—Crop Evapotranspiration (ETc)

If using the pressure chamber isn’t appealing, or a second source of information is desired, monitoring the weather and evapotranspiration crop losses is an option. This method is sometimes called a “water budget,” because it is analogous to budgeting money. Soil water storage in the crop root zone equates to a balance in a checking or savings account. ET equates to a debit from the account and significant rainfall or irrigation equates to a deposit or credit into the account. Water budgeting approximates the soil moisture level in the root zone rather than measuring it with soil moisture sensors.

Weekly ET reports are available during the irrigation season online or can be delivered weekly by email. ET is estimated based upon real-time weather measurements at regional CIMIS weather stations. Estimates are for trees with at least 50 percent canopy cover and need to be adjusted downward for smaller trees. Each report provides a real-time estimate of ET in inches for the past seven days, an estimate for the next seven days, and keeps a running total for the season. Accumulations begin at leaf-out for each crop which enables their use to help decide when to begin the irrigation season. It is important to know the hourly water application rate (inches/hour) of your irrigation system.

Using an example from the 2018 season, if we followed each weekly report from February 16 to May 3 for the Gerber South CIMIS weather station, it showed that cumulative ET for almonds was 7.51 inches while cumulative rainfall for the same period was 5.26 inches and resulted in a 2.25-inch soil moisture deficit. This assumed that all of the rainfall was effectively used in the orchard which is a site-specific consideration that needs to be adjusted accordingly in the water budget. Dividing this 2.25-inch soil moisture deficit by a water application rate of 0.07 inch per hour (i.e. an almond orchard with 124 trees per acre with one 16 gph microsprinkler per tree) equates to 32 hours of irrigation or the equivalent of two 16-hour irrigation sets that suit PG&E off-peak rates. Choice of set length is site-specific depending on irrigation system and soil type; however, it is best to minimize ponding conditions that can starve roots of oxygen and provide favorable disease conditions.

For help with your own ET calculations see:  sacvalleyorchards.com/et-reports/

The previous example provides context on how this deficit relates to the irrigation system capacity. It is left to the irrigation manager’s judgement to continue to delay the beginning of irrigation to protect tree and root health, begin irrigation by partially refilling the soil moisture deficit (i.e. one 16-hour irrigation set), or begin irrigation and fully replace the soil moisture deficit. If this information were paired with the pressure chamber measurements and the stem water potential measurements were still within -2 bars of the fully irrigated baseline, the manager may have more peace of mind about continuing to delay the first irrigation.

Monitoring Soil Moisture Depletion

If neither the pressure chamber nor water budgeting appeal to you or you are looking for a supplement to one or both methods, directly monitoring soil moisture is an option. Checking soil moisture by hand is a very basic method to evaluate soil moisture conditions. There are many online stores where soil augers can be purchased (examples include: JMC Backsaver, AMS samplers, Forestry Suppliers, and Ben Meadows). For interpretation of soil moisture in collected samples, the USDA-National Resource Conservation Service also offers a nicely prepared publication with color pictures titled Estimate Soil Moisture by Feel and Appearance.

There are also a wide variety of soil moisture sensors that can also be used. Refer to the UC ANR article Soil Moisture Sensor Selection is Confusing for more insight: sacvalleyorchards.com/blog/soil-moisture-sensor-selection-is-confusing/

The Judgment of the Irrigation Manager

Stem water potential readings with a pressure chamber, evapotranspiration-based water budgets and soil moisture monitoring all bring different and valuable information to the decision of when to first irrigate almonds. Stem water potential readings with the pressure chamber offer the most direct measure of tree irrigation need. In addition, adopting this practice may both save water and encourage valuable deep root development. No matter if you choose to adopt one, or all three of these monitoring approaches, the irrigation-manager’s careful judgement is most important.

Spray Calibration and Coverage: The Basics of Spray Application

Air-assisted sprayers discharge tank mix as tiny spray droplets into an airstream that transports the droplets to the target tree or vine canopy. The mixture of air and spray droplets, known as the spray cloud, expands in both the vertical and horizontal dimensions as it moves away from the sprayer’s outlet. The speed of the air reduces drastically after exiting the sprayer and then continues to reduce gradually with distance away from the sprayer. Because the sprayer is moving, however, the spray cloud appears as having been bent backwards.

If we consider the continuous forward movement of the sprayer to be in short steps of equal lengths, we can determine the time the sprayer spends in each step by dividing the step length by the travel speed. We can then multiply the time by the flow rates of the air and spray liquid to know the volumes of air and spray discharged in each step movement. The volume of spray is what is applied in each forward step and the volume of air is how much is available to carry the applied spray. The slower the travel speed, the higher the volume discharged; the faster the travel speed, the lower the volume discharged.

During the spray application, immediate tree or vine canopies adjacent to the sprayer are the target canopies (Figure 1, below). Each tree or vine is bound by a ground area equal to row spacing times tree spacing. It is sufficient and most efficient for the air to carry spray droplets to the target, not beyond. This is because spray droplets carried beyond the target tend to either fall to the ground or potentially drift away by the wind when they miss canopies in the subsequent rows. However, the air speed and volume should be enough to cause the spray to penetrate the target canopy. This means that travel speed and air volume of the sprayer should be matched to the canopy size and density. Also, the number of nozzles used should not result in too much spray applied over and/or under the canopy. Effective spraying will result if the spray is strategically directed toward the target.

Figure 1. Basics of spray application

 

Sprayer Calibration: What, Why, When and How

What is it? Sprayer calibration is the adjustments made to a sprayer based on measurements taken to ensure that the correct amount of material (spray mix or active ingredient) is applied.

Why is it important? Calibration is best practice in pesticide spray application. When done correctly, it is a sure way to know how much material you would actually be applying to your crop. Incorrect calibration or not calibrating at all can result in inaccurate application rate, ineffective application, and illegal residues on the treated crop. Accurate calibration will lead to effective pest control while minimizing waste and negative environmental impact. Above all, accurate sprayer calibration will ensure compliance with the law as represented by the pesticide label.

 When should it be done? Ideally, sprayer calibration should be done at the beginning of the growing season and whenever there is a significant change in conditions. Examples of changes in condition that may require calibration include: Change in ground condition (e.g. soil type, soil wetness, ground cover); change in target condition (e.g. crop type, canopy size, canopy density); change in spray material condition (e.g. density). Although not all the calibration steps may be necessary in response to a change, adjustments should be made to the components that are directly affected by the change. For instance, if only ground condition has changed, then only travel speed would have to be determined again and appropriate adjustments made. However, if a global positioning system (GPS) based speedometer device or mobile app is in use, then it may not be necessary to check speed again. This is because readings of GPS-based speedometers are not affected by changes in tire traction due to ground cover.

How should it be done? A major objective of sprayer calibration is the idea of optimizing the application. There are different ways to do it, but with the same or similar outcome. The steps could be as follow: 1) determine travel speed; 2) assess air profile to determine number of nozzles; 3) select nozzles; 4) measure sprayer output; 5) adjust sprayer output; and 6) assess spray coverage.

STEP 1: Determine Travel Speed
This should be done with the sprayer tank about half-full and the fan running.

Materials needed: Measuring tape, flagging flags, stopwatch, calculator, clipboard, GPS device (e.g. smartphone).

Method 1 – Manual known distance method: Measure a known distance, D, (typically 100 or 200 feet) with a measuring tape in the orchard or vineyard where you would be spraying. Use marking flags to clearly indicate the distance. Using a stopwatch, measure the time, T, it takes for the sprayer to travel the marked distance at a preselected gear setting. Repeat this for at least three times in total and determine the average time. Calculate the speed, S, as:

When two people are available, in addition to the operator, person A should stand adjacent to the starting flag with one hand up (Figure 2a, below). Provide enough distance for the sprayer to attain the speed before reaching the starting flag. Once a predetermined feature on the tractor/sprayer (e.g. front of tractor/sprayer, center of front wheel, etc.) reaches the starting point, person A should lower the raised hand to indicate to person B to start measuring the time with a stopwatch. Once the predetermined feature on the tractor/sprayer reaches the ending flag, person B should stop measuring the time and then record the elapsed time on a clipboard.

Figure 2. Various ways to determine travel speed.

When only one person is available, in addition to the operator, fix the marking flags in the sprayer’s path (Figure 2b, above). Provide enough distance for the sprayer to attain the speed before reaching the starting flag. Once the front of the tractor touches the starting flag, start measuring the time with a stopwatch. Again, once the front of the tractor touches the ending flag, stop measuring the time and then record the elapsed time on a clipboard.

When only the operator is available, fix the marking flags in the sprayer’s path. Provide enough distance for the sprayer to attain the speed before reaching the starting flag. Maintaining sitting posture, start measuring the time with a stopwatch the moment the starting flag disappears and stop measuring just when the ending flag disappears. Record the elapsed time on a clipboard.

Method 2 – Manual tree passed method: Count about 10 or more trees or vines and fix two marking flags in an adjacent mid-row in the path of the sprayer (Figure 2c, above). Additionally, you can tie pieces of flagging tape on the vine adjacent to the marking flag to aid the operator’s visibility. Note the tree spacing, TS. Providing enough distance for the sprayer to attain speed before reaching the starting flag, measure the time it takes for the sprayer to travel the marked distance at a preselected gear setting. The number of trees passed by the sprayer, NT, is the count excluding the starting tree but including the ending tree. Repeat time measurement for a total of at least three times and determine the average. Calculate the speed as:

 

Method 3 – Automatic tracking method: Drive the sprayer for about 100 ft or more while observing the speed reading on a GPS device (a smartphone with a GPS speedometer app can be used for this). Repeat the observation for at least three times in total and determine the average. If the tractor pulling the sprayer is equipped with a GPS monitor, then speed measurement may not be necessary.

STEP 2: Assess Air Profile to Determine Number of Nozzles

Materials needed: Flagging tape, digital camera (e.g. smartphone).

Method:
Attach about 4 feet of flagging tape to each nozzle on the sprayer manifolds. Start the fan and observe the aloft flagging tapes from behind the sprayer, see Figure 3, below. Take a photo of the scene with a camera for reviewing. From the photo, determine the number of nozzles and their position that are well directed on the target canopy. Turn off nozzles that miss the target canopy. To better understand why this is necessary, see Figure 4, below.

Figure 3. To assess the air profile, attach about 4 feet of flagging tape to each nozzle on the sprayer manifolds. Start the fan and observe the aloft flagging tapes from behind the sprayer. (All photos courtesy of P. Larbi.)

 

Figure 4. Various nozzle configurations with different degrees of missing target canopy.

Also, attach a piece of flagging tape to the target tree or vine canopy and the next adjacent canopy in the path of the air. Drive the sprayer across the taped locations at the determined travel speed with the air running and observe the tapes on the canopies. If the tape on both canopies are sufficiently aloft, the air might be too much (Figure 5a, below). Adjust the air if the sprayer is equipped for that, considering the target canopy size and density. Otherwise, increase the travel speed to adjust the air (Figure 5b, below). Another option is to partially cover the fan inlet using a so called ‘Cornell doughnut’ (Figure 5c and 5d, below). Ideally, an automated means of adjusting the fan intake would be best.

 

Figure 5. A look at various ways of adjusting air
volume.

STEP 3: Select Nozzles

Materials needed: Calculator, clipboard, nozzle catalog, nozzle selection mobile app

Method:
Knowing the desired application rate, AR, and row spacing, RS, determine the total sprayer output per side, SO, from all open nozzles as:

If the spray volume is intended to be uniform on each side, then divide SO by the determined number of nozzles to get the desired nozzle flow rate. Use this number to select the nozzle from a nozzle catalog.

However, SO can be split into different fractions for upper nozzles and lower nozzles. A common configuration for trees is 2/3 for upper nozzles and 1/3 for lower nozzles. Whatever the split ratio, the total should amount to the calculated SO. Various available software applications and mobile apps can be used to aid this determination.

STEP 4: Measure Sprayer Output
This should be done with the sprayer stationary. Alternate methods to that presented here exist.

Materials needed: Measuring pitcher, stopwatch, calculator, clipboard, automatic nozzle calibrator, flow meter, pressure tester.

Method:
With the sprayer running in a fixed position, confirm that the pressure gauge reading is accurate using a tool. Start spraying and collect spray water from each nozzle for 1 min using a measuring pitcher (see figure 6) and record the values in fluid ounces (oz). These should be the nozzles that will be used for the spray application. Calculate the flow rate of each nozzle as:

Repeat the measurement and determine the average.

Calculate the sprayer output by multiplying the average FR by N, for uniform application. For non-uniform application, do this separately for the upper and lower sections and sum them up.

Alternatively, you can automatically measure the flow rate by using a flow meter (e.g. SpotOn™ calibrator). Also, you can use a manifold patternator to observe uniformity of nozzle flow across all nozzles.

STEP 5: Adjust Sprayer Output
There are two adjustments that can be applied to the sprayer output in order to achieve the desired application rate, if the measure sprayer output is off. The first is adjusting the travel speed, while the second is adjusting the pressure.

Adjusting travel speed:
The adjusted travel speed, S2 can be obtained as:

where,
AR1 = application rate obtained with the actual measured sprayer output in Step 4
AR2 = desired application rate in Step 3
S1 = travel speed measured in Step 1.

Adjusting operating pressure:
The adjusted operating, P2, can be obtained as:

where,
SO1 = measured sprayer output in Step 4
SO2 = desired sprayer output in Step 3
P1 = initial operating pressure

STEP 6: Assess Spray Coverage
Attach water sensitive cards (yellow cards that turn blue when moist) to different locations
in the target canopy. Run the sprayer and apply water similar to the intended application. Evaluate the spray coverage to ensure that it is suitable. Make adjustments as necessary to obtain a suitable coverage.

Water sensitive card for spray coverage assessment

Once the proper settings have been obtained, maintain these settings in the actual application, making sure to factor in weather conditions. It is also important to clean the sprayer and maintain it in good working condition to ensure good performance.

Selecting the Right Rootstock in California Prune Production

Roots are the unsung heroes of orchard plantings. They operate out of sight and are relatively difficult to examine and characterize. The roots of course anchor the trees to the soil and take up water and essential mineral elements. They also store carbohydrates and synthesize materials. Because roots play these key roles, rootstocks can influence scion vigor, growth and performance. Rootstocks vary in their tolerance to different soil types and conditions and their resistance to soil borne diseases and nematodes

As soil treatment options become increasingly limited, more restrictive and less effective, the priority to identify a genetic solution to solve or reduce the replant issue is of increasing interest. One genetic solution is to find or develop rootstocks to help manage soil related problems, such as soil borne fungi/bacteria, nematodes and soil acidity and excess salts. Of additional interest are root and tree characteristics imparting canopy size control, good anchorage and little or no root suckering.

The California prune industry has historically utilized just five rootstocks, Myrobalan 29C, Myrobalan seedling, Marianna 2624, Lovell peach and Marianna 40. The Prune Production Manual (UC ANR # 3507) has a very good chapter describing the traditional rootstock choices. You can find the production manual for sale at anrcatalog.ucanr.edu and as an e-version through Google Books. Recognizing the need for identifying additional rootstocks for California prune production, University of California farm advisors and campus-based faculty with funding from the California Prune Board designed and planted two replicated rootstock experiments in 2011 to evaluate 15 rootstocks for prune production. These rootstocks have diverse genetic backgrounds within the Prunus family (plum, peach, almond, etc.).

Replicated rootstock trials in growers’ orchards in Butte County and Yuba Counties allow UC researchers to evaluate a total of 15 rootstocks (Table 1, below) under very different soil, irrigation, and yield potential. The Butte plot is planted on Farwell clay adobe and the lighter textured Nord Loam soil types; this ground was previously planted to almonds on Lovell (peach) rootstock. In contrast, the Yuba site is planted on more typical prune ground (Kilga clay loam) and is prune following prune. The Butte site has tighter spacing at 12.5 feet in-row and 17 feet between rows (205 trees per acre), compared to 16 feet in-row and 18 feet between rows (151 trees per acre) at Yuba. The Butte plot is drip irrigated, while the Yuba plot has micro sprinklers. The differences in soil, crop history, tree density, irrigation, and resulting vigor at the two replicated trial sites allows for a rigorous evaluation of these rootstocks.

 

Tree Survival

Roger Duncan a pomology farm advisor based in Stanislaus County, who has done extensive trialing of rootstocks for almond production, has noted that rootstock choice is like an insurance policy. Although there is no perfect rootstock, careful rootstock selection can help guard against disaster. These two trial sites have illustrated an incredible range in rootstock survival, which helps illustrate a varying ability of these rootstocks to guard against disaster at two very different sites. Following a wet winter which delayed soil preparation in 2011, both the Butte and Yuba sites experienced extensive mortality and were significantly replanted in 2012. However, even after replanting in 2012, the rootstocks have experienced very different rates of survival.

Percent tree survival since the 2012 replanting was assessed at both sites in 2019 (Table 2, below). Survival ranged from 10 percent (Empyrean 2) to 97 percent (Atlas) at the Butte site, and 37 percent (HBOK 50) to 100 percent (Viking and Lovell) at the Yuba site. The two rootstocks that are only planted at a single site each have had dramatically different results, with the disastrous 10 percent survival of Empyrean 2 at Butte, compared to the 93 percent survival of Rootpac-R at Yuba.

Table 2: Percent tree survival at the Butte (September) and Yuba (June) sites in 2019. Values followed by the same letters are not significantly different at 95 percent using Tukey’s HSD. The numerically highest and lowest values are highlighted in each column.

There are other notable differences and similarities in survival between the two sites. Myrobalan 29C, Myrobalan seedling, and HBOK 50 have all had higher survival rates at the Butte site, potentially due to bacterial canker susceptibility at the Yuba location. At both sites, Atlas and Viking, which were planted in 2012 (not available in 2011), have had excellent survival (97 to 100 percent).  Marianna 40 and Marianna 2624 have also had decent survival (80 to 87 percent). Finally, Marianna 30 has had very low survival at both sites (43 and 37 percent at Butte and Yuba, respectively).

Bacterial canker susceptibility has greatly colored the results at the Yuba location. Land with a history of the disease likely has ring nematode, and may also have sandy or low pH soils, low tree nitrogen status, or a clay/shallow hardpan. For land with a history of bacterial canker, having a prune orchard with a high rate of survival at maturity is no small feat.

Satellite image of the UCCE prune rootstock plot in Yuba County. Although tree loss was likely from multiple causes, bacterial canker was a significant player. Note gaps of six trees (number of trees in a replicate), despite rootstock treatments with large, healthy canopies surrounding these gaps (Google©, Imagery Maxar Technologies ©2019, and U.S. Geological Survey map data ©2019).

The satellite image of this plot clearly shows several gaps of missing trees that are six trees in length (i.e. the number of trees in a rootstock treatment replicate, see related photo). Although there are several causes of tree loss in the plot, certain rootstocks have had very low survival in the same areas of the orchard where other rootstocks have 100-percent survival and vigorous growth. For example, Myroblan 29C an industry standard, had a mere 63-percent survival as of 2019, while Marianna 30 and HBOK 50 fared even worse, each at 37-percent survival. In contrast, Viking, Atlas, Krymsk 86, Lovell and Rootpac-R all had between 93- and 100-percent survival.

Vigor and Yield

The first mechanical harvest in the two trials was in 2017 and yield data continues to be taken. In addition to the Butte site having tighter spacing (205 trees/acre, compared to 151 at Yuba), the tree trunks have generally been larger, and yields per tree higher than Yuba. The 2017 yield results at Butte offer a valuable glimpse into the vigor of these rootstocks, since it was a very high yielding and unthinned crop, which helps illustrate the yield potential of these rootstocks. These high, unthinned yields were unsustainable and in 2018 there was poor return bloom at the site and low yields.

The 2017 harvest at the Butte site shows that a larger trunk, measured as trunk cross sectional area (cm2), generally higher dry yield (pounds per tree), and smaller fruit size (table 3). Among the smaller trunk size and lower yielding trees were Krymsk 1, HBOK 50, Marianna 58, Empyrean 2 and Citation. Among the largest and highest yielding were Myrobalan 29C, Atlas, Viking, Marianna 30 and Lovell. Similar yield and trunk size differences between rootstocks have been found subsequently at both sites. Of course, all rootstock trials that impose the same spacing across the plot disadvantage lower vigor rootstocks that could have been placed at a higher density.

Although there are similarities in yield performance of the rootstocks at both sites, there are some notable differences between sites. Rootpac-R, which isn’t planted at the Butte site, has had middle of the pack trunk size and yield at Yuba. Another notable difference is that Krymsk 86 is a fairly average vigor and yield rootstock at Butte but is among the largest and highest yielding at Yuba.

Table 3: 2016 trunk size (trunk cross sectional area in cm2) and ‘Improved French’ prune yield characteristics for the Butte County rootstock experiment harvested 8/29/17. Values are treatment means for the five replicates. Values followed by different letters are significantly different.

Anchorage & Suckering

In addition to survival and vigor (and resulting yield potential), there are other attributes that are important to growers. Anchorage is key since growers need trees to stand up on their own and not be blown over, as well as provide a straight up-and-down trunk for shaker harvesting. Degree of lean was measured with the level feature in the iPhone at both rootstock plots with one person pushing against the tree and another person measuring the deflection. Although rootstocks at the Yuba site had greater lean (there were notable wet soil conditions at the time of measurement), the rootstocks had similar relative lean to one-another at the two sites. Marianna 58 and Krymsk 1 had among the greatest angles of deflection, while Viking and especially Krymsk 86 showed little deflection at either site.

Removing suckers is a costly and cumbersome activity. In addition, rootstock suckers may offer a route for systemic herbicides to be taken up and damage the tree. Rootstocks suckers were rated on a one to five scale, with one being the fewest suckers and five being extensive and large suckers. Again, the two sites offered many similarities in rootstock performance. Myroblan seedling had the most rootstock suckers at both sites, while many rootstocks including Atlas, HBOK 50, Viking, Citation, Marianna 58, Lovell and Marianna 40 had few, if any suckers.

Rootstocks with Potential Problems

Many of the rootstocks haven’t performed well when survival, lean, and suckering were evaluated. Many of those same underperforming rootstocks also tend to be on the lower vigor and lower yielding end of the spectrum. Krymsk 1 and Marianna 58 had among the highest lean, and in-addition Krymsk 1 had low survival at Butte and was among the lowest yielding rootstocks. HBOK 50 also had high lean, poor survival at Yuba, and poor yield at Butte. Empyrean 2 which was only at Butte, had a mere 10-percent survival by 2019. Myroblan seedling had the worst suckering and had high lean at Yuba. The industry standard Myroblan 29C has performed well at Butte but has had low survival under the bacterial canker conditions in Yuba. Marianna 30, despite been among the highest yielding rootstocks per tree at Butte has had very low survival at both sites.

Rootstocks with Evident Strengths

Krymsk 86, Viking, and Atlas have all had very high survival at both rootstock trial sites. In addition, Krymsk 86 has been among the highest yielding at Yuba, and Atlas and Viking have been high yielding at both sites. Krymsk 86 has maintained its reputation for excellent anchorage, while Viking also showed little lean at both sites. Finally, both Viking and Atlas had a sucker rating of zero at both rootstock plots.

Where is Rootstock Selection in the Industry Headed?

Atlas, Viking, and Krymsk 86 have all shined at these two rootstock plots. Krymsk 86 is the only one of these that is so far seeing significant adoption in the prune industry. Following widespread adoption of Krymsk 86 in the Sacramento Valley for almond production, Krymsk 86, known for its superior anchorage and tolerance to wet feet, has been planted now in many new prune orchards. Like the higher vigor at the Yuba site and in contrast to the middling vigor at Butte, we have heard from growers that Krymsk 86 has been a high vigor rootstock in their plantings. Just as with growing almonds on Krymsk 86, growers should test for and be wary of planting Krymsk 86 where nematodes, particularly root-knot, are present.

As described in an article in the September/October issue of Progressive Crop Consultant (progressivecrop.com/2019/10/californias-prune-orchard-of-the-future) choosing a higher vigor rootstock and/or planting at a tighter spacing leads to capturing more sunlight and having a higher yield potential. However, when hand pruning large prune trees can cost $1,000 per acre, many growers are adopting mechanical hedging, and some progressive growers have had an increased interest in low vigor rootstocks. A couple of these growers have begun trialing low vigor inducing rootstocks in high density plantings, in hopes that low vigor rootstocks will reduce pruning expenses. Low vigor, high density and potentially trellised plantings, stand in stark contrast to the vigorous rootstocks that have stood out in these two trials. The choice between high and low vigor rootstock may prove to be one of the key defining choices where the California prune industry heads in the coming years.

We want to sincerely thank an amazing team of UC researchers, as well as support from dryer managers at Sunsweet Growers Inc. This work is made possible by the generous funding support of the California Prune Board.

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