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A Review of Pythium Diseases in Row Crops

By
Steven Koike
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Large, established cauliflower plants can still become infected with Pythium (left), resulting in loss of roots, severe stunting and yield loss (all photos courtesy S. Koike.)

It is highly likely that growers, PCAs and other field professionals are familiar with the word “Pythium”. Pythium is the name of a soilborne, fungus-like organism that is notorious for primarily causing seedling diseases. Pythium is notable because many row crops are susceptible to it, the pathogen is very widely distributed and occurs in most cropped ground, and despite the use of IPM tools and strategies, Pythium problems can still show up in row crop production systems.

 

What is Pythium?

Pythium is a fungus-like organism. Previously considered to be a true fungus, molecular studies in recent years indicate that Pythium—as well as closely related organisms like Phytophthora and downy mildew—is more closely related to brown algae and diatoms. Formally, therefore, Pythium species are no longer part of the fungal taxonomic group but are classified in the kingdom Chromista, or Stramenopila. The Pythium genus contains over 200 species, most of which are not plant pathogens. There are Pythium species that are pathogens of animals (some of which can infect humans), and many species are saprophytes and only grow on dead and decaying organic material. Pythium species are mostly found in soil environments but are also present in aquatic habitats.

Plant pathogenic Pythium species are well equipped to cause problems on row crops. Most of these species form resilient, thick-walled sexual spores (oospores) that can withstand periods of unfavorable dry and warm conditions. These structures enable Pythium to persist in the soil for a long time. When favorable soil conditions are present, mostly in the form of abundant soil water, these Pythium organisms either produce hyphae that grow toward the roots or swimming spores (zoospores) that move through the soil water in search of susceptible plant tissues. Another feature that makes Pythium problematic for growers is the extremely fast growth rate of these organisms. Given suitable soil conditions, Pythium pathogens can rapidly grow from seed-to-seed, seedling-to-seedling and root-to-root.

The primary symptom of Pythium diseases is the dark discoloration and decay of roots, pictured here on lettuce.

 

 

Diverse Pythium Diseases

In contrast to many plant pathogens, Pythium causes several different types of problems on crops (Table 1). First, Pythium is a seed pathogen. Once placed in the ground, seed can be exposed to Pythium that is residing in the soil. If conditions are favorable for the pathogen, Pythium can invade and colonize the seed, causing it to rot before it can germinate. If the seed germinates, Pythium can cause a decay of the roots and shoots that just grew out of the seed. This early disease stage is often called damping-off. Damping-off is further divided into two phases. If the newly germinated seedling is infected so early and so severely that it dies before being able to break through the soil surface, this situation is called pre-emergent damping-off. However, post-emergent damping-off occurs if the diseased seedling is strong enough to emerge above the soil surface, only to succumb and collapse shortly afterwards. Collectively, seed decay, pre-emergent damping-off, and post-emergent damping-off can result in loss of plants very early in the production cycle, causing stand loss in the field.

Table 1. Categories of Pythium diseases of row crops

Healthy seedlings that escape death at the seed and newly germinated stages remain vulnerable to this pathogen; established seedlings can still be infected and become stunted and die due to diseased roots and crowns. Older, established plants have escaped the damping-off phase that kills seedlings but can be subject to infections that prune back the roots, leading to reduced plant vigor and yield. For example, Pythium can cause late infections in cauliflower and result in weakened roots and poorly yielding plants. This soilborne pathogen can even cause a foliar blight of leaves and shoots, though this type of disease is not very common. Bits of soil carrying Pythium can be splashed or moved up onto foliage and cause blights on crops such as spinach and bean. Finally, the fleshy parts of some vegetable crops are subject to Pythium pathogens. If in contact with infested soil, cucurbit fruits, sweet potato storage roots and potato tubers can develop a soft, watery rot that will result in a non-marketable commodity.

Of the hundreds of Pythium species worldwide, relatively few species infect row crops. These plant pathogens can be conveniently placed into two categories. One group consists of Pythium species that have a relatively narrow host range and infect only a few crops, with those few crops tending to mostly be within a particular plant family. Examples are Pythium mastophorum, which primarily infects celery and parsley (Apiaceae family), and Pythium uncinulatum, which reportedly only causes significant disease on lettuce (Table 2). The second group contains Pythium organisms that have very large host ranges. The two main species, P. aphanidermatum and P. ultimum, both infect scores of plants, including dozens of vegetable and row crops.

Table 2. Examples of Pythium pathogens with broad vs. narrow host ranges

Disease Development Development of Pythium diseases is straightforward. Initial inoculum is almost always linked with infested field soils and associated soil water. Pythium is a soilborne pathogen that resides in the soil primarily as dormant resting structures. Pythium inoculum is not seedborne or airborne. For Pythium to become active, grow, and produce those swimming zoospores, the soil must be wet for prolonged periods. Once susceptible seed, seedlings, and other plant parts are in close contact with Pythium inoculum, infection can take place and disease will be initiated. If wet soil conditions persist and temperatures are optimum for the pathogen, disease losses can be significant.

Pythium pathogens form thick-walled oospores that enable the pathogen to survive in soil for prolonged periods.

 

Diagnostic Considerations

Pythium is not the only soilborne pathogen that causes seedling damping-off and root rots of row crops. On spinach, damping-off and root rot can be caused by both Pythium and Fusarium; visually, one cannot distinguish between the symptoms caused by these two pathogens. Pythium and Phytophthora pathogens both cause dark, discolored roots of lettuce and cannot be differentiated in the field. Cauliflower transplants are susceptible to both Pythium and Rhizoctonia pathogens, both of which caused the roots to become discolored. Precise and accurate diagnosis of Pythium diseases will therefore require lab-based tests and assays.

When sufficient soil water is present, Pythium forms swimming spores that are released and search for host roots. Pictured here is a cluster of zoospores just prior to release.

 

Managing Pythium

Controlling diseases caused by Pythium requires the implementation of IPM practices.

Site selection: Choose to plant in fields that do not have a history of Pythium problems and have well-draining soils.

Crop rotation: If Pythium is an issue, avoid planting the same susceptible crop in the infested field. Rotate to crops that are not known to be susceptible to the Pythium species present at that location. However, remember that some Pythium species have very broad host ranges (Table 2).

Irrigation management: Because the Pythium pathogen is so strongly dependent on wet soil conditions, carefully schedule and limit irrigations to prevent overwatered, saturated soils.

Time of planting: In some cases, moving the planting date to a different time of year may help reduce losses to Pythium. For example, depending on the Pythium species of concern, planting the crop in the warmer, drier summer may be preferred to seeding the crop in the cooler, wetter spring.

Fungicides: Plant seed treated with a fungicide that is active against Pythium. Note that the fungicides used to control Rhizoctonia or Fusarium have no effect on Pythium. For some crops, applying fungicides to the emergent crop may provide additional protection. The repeated use of products having the same mode of action can result in Pythium isolates that are insensitive (=resistant) to those products; therefore, IPM strategies will require that thought be given to deploying different fungicides.

Resistant or tolerant cultivars: Unfortunately, there do not appear to be any row crop cultivars that have genetic resistance to Pythium.

Pythium plant pathogens can grow very rapidly. Pictured here are three-day-old cultures of Pythium, Phytophthora, Fusarium and Verticillium. The diameter of the petri dish is 85 mm.

Using N-Rich Reference Zones to Inform In-Season Nitrogen Fertilization Practices in California Small Grains

By
Michelle Leinfelder-Miles, Nick Clark, Taylor Nelsen, Tom Getts, Konrad Mathesius, Sarah Light, Giuliano Galdi and Mark Lundy
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Figure 1. The GreenSeeker held above a recently headed small grain and displaying the NDVI value. Values range from 0 to 1 (i.e. less-green to very green plants).

Over the last year, a team from UCCE has been working with California small grains growers on practices that can improve nitrogen (N) use efficiency. At demonstration sites, we have implemented practices that UC Grain Cropping Systems Specialist Mark Lundy has been investigating for several years, namely N-rich reference zones, a soil nitrate quick test, handheld reflectance devices and aerial imagery. We demonstrate how to use these tools to manage N fertilizers in small grain crops across variable soil and climatic conditions in the Sacramento Valley, Delta, San Joaquin Valley and Intermountain Region.

The demonstrations are funded by the CDFA Fertilizer Research and Education Program and a USDA-NRCS California Conservation Innovation Grant. Our goal is to help growers and consultants learn and implement these practices to guide N fertilization in small grains, thereby increasing crop productivity and N use efficiency while reducing potential for N loss to the environment.

 

What are “N-Rich Reference Zones”?

Reference zones are most useful to growers who can apply the majority of their seasonal N budget during or after the tillering stage of growth. Previous work has shown that N fertilizer applied during the season−between the tillering and heading stages of small grain development−results in higher yields, higher protein and increased fertilizer use efficiency compared to pre-plant applications. The reference zone is a relatively small area within the field where extra N fertilizer is added at the beginning of the season. This extra fertilizer ensures that the reference zone will not be N-limited from planting until an in-season fertilizer decision is made. When a grower is determining whether and how much N fertilizer to add in-season, measurements from both the reference zone and the broader field are compared to understand whether the broader field is sufficient in plant-available N.

 

Fertilizer N Rate and Field Variability

Fertilizer N rate and field variability are two important considerations when creating N-rich reference zones. The amount of N to apply in the N-rich zone will depend on several factors such as yield goal, protein goal and when the expected in-season fertilizer application will take place. There should be sufficient N applied to the reference zone at planting to ensure that the plants in the zone are not limited by N at the stages of growth when the in-season fertilizer is applied. Table 1 gives some examples of how much N fertilizer to apply to the N-rich zone for a range of potential yields.

Table 1. Approximate N fertilizer application rates suggested for use in N-rich reference zones based on a range of average yields and two stages of crop growth. Suggested rates ensure that crops within the reference zone are not N-limited when an in-season fertilizer application decision is being made at the crop stage indicated.

It is important to establish the N-rich zones in representative parts of the field. Areas of the field that are unique (i.e. low areas, high areas, gravel strips, etc.) should be avoided. It is also important that the zones capture field variability. If certain areas have distinct soil types or known patterns of yield or management differences, a grower should establish multiple zones to account for these sources of spatial variability if they represent large areas in the field. Soil maps (available from casoilresource.lawr.ucdavis.edu/soilweb-apps/) and historical aerial imagery can often help in identifying field patterns and good location(s) for reference zones.

 

How and When to Apply the N-Rich Zone Fertilizer

A grower can establish N-rich zones during the pre-plant fertilizer application. For example, a grower may apply 50 pounds N per acre across the field and then make another pass or two in the zone to apply an additional 50 to 100 pounds N per acre (depending on what the grower calculates is necessary, as described above.) This method might be most easily adopted by growers. We have observed, however, that if the fertilizer is placed too deep in the soil profile, the N may not be readily available to the seedling crop early in the season because it is below the root zone. Therefore, N-rich zones established by this method may not provide a reliable early-season point of comparison. Instead, we have found that broadcasting urea is the most effective way to establish N-rich zones. At our demonstration sites, we broadcasted urea after tillage or shortly after planting, but always ahead of a storm or irrigation event that could incorporate the fertilizer. Orienting the zones perpendicular to the rows or tractor passes also helps to capture field variability. When the zones are too narrow and run in the same direction as the field work, it can be hard to differentiate between a field pattern associated with equipment passes and a N effect, particularly early in the season.

 

Monitoring the Field

Once the crop begins to grow, the field should be monitored periodically to assess whether the crop is likely to respond to a N fertilizer application. A combination of the soil nitrate quick test (SNQT) and plant reflectance measurements taken from both the N-rich zones and the broader field can indicate when a top-dress fertilizer application may be beneficial. The soil nitrate quick test and plant reflectance measurements complement other important information like current crop growth stage, crop yield and protein goals, and local weather records to inform a site-specific N fertilizer recommendation.

The SNQT is a simple and low-cost test that provides a ballpark estimate of the soil nitrate-N concentration in the root zone. Nitrate is a highly plant-available form of N. Using the SNQT when N fertilizer decisions are being made will help to narrow a range of fertilizer rates appropriate for that field. More information on using the SNQT in small grains, including a sample protocol and demonstration video, is available at smallgrains.ucanr.edu/Nutrient_Management/snqt/. Over the past several years, UCCE agronomists have developed a strong relationship between the value measured using the SNQT and an estimate of fertilizer N equivalence.

Crop reflectance can be measured using a number of tools, including handheld devices, drones and satellite imagery. Common indices that result from measurements of canopy reflectance are normalized difference vegetation index (NDVI) and normalized difference red edge index (NDRE). These indices represent measurements of light reflected from the crop canopy at key wavelengths indicative of plant vigor. Relative differences in vigor among plants in the same field can be captured by comparing canopy reflectance measurements like NDVI and NDRE. We have been using handheld devices, drones and satellite imagery at our demonstration sites to compare crop reflectance values in the N-rich zones and the broader field.

One of the tools we are using is the GreenSeeker by Trimble Agriculture. This is a hand-held NDVI meter (See Figure 1) that emits light and detects how much is reflected from the crop canopy in the red and infrared wavelengths. The GreenSeeker’s canopy measurement indicates how well the plants are growing and covering the soil with greenness. This information about vigor is important early in the crop’s growth because it indicates the ability of plants to support grain production and yield potential.

We are obtaining similar information as from the GreenSeeker by measuring NDRE with a five-band multispectral camera (MicaSense RedEdge-MX) mounted on a drone (DJI Matrice M200 V2). NDRE is similar to NDVI but replaces the reflectance from the red wavelength with reflectance from the red edge wavelength. Because the drone is able to capture data from hundreds of feet above the ground, it allows us to measure a large area quickly and under conditions when entering the field is not possible. Figure 2 depicts side-by-side images from a field in Solano County where N-rich reference zones were implemented during the 2019-20 season.

Figure 2. A field in Solano County where three N-rich reference zones are visible at tillering using NDRE captured via drone (left), but not visible to the naked eye (right) (all photos courtesy M. Leinfelder-Miles.)

Another device we are using to monitor plant N is the atLEAF CHL by FT Green LLC, which is a chlorophyll meter that measures light absorbed by a single leaf (Figure 3). Like the GreenSeeker, it also emits and detects light. The atLEAF CHL, however, measures how much light passes through a single leaf instead of measuring reflected light. This information becomes increasingly valuable as an indicator of whether or not the crop has sufficient N as it begins heading out and filling grain.

Step-by-step instructions for using both the GreenSeeker and atLEAF CHL in small grains are available at ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=42903.

Since plant N is strongly related to plant greenness and chlorophyll content, measurements of NDVI, NDRE and leaf chlorophyll can serve as proxies for relative plant N status within a field. Many factors can affect absolute greenness or chlorophyll values, including variety, crop injury and environmental factors. Because of this, it is important to remember that the absolute values given by these devices are only meaningful when compared to a reference zone like the N-rich zone.

Figure 3. Small grain leaf inserted in the sampling area of the at LEAF showing a chlorophyll reading in the lower right corner of the display while the user’s back shades the device.

 

What do the Readings Mean?

Plant reflectance and transmittance measurements are best interpreted by expressing values measured in the broader field relative to the N-rich reference zones, according to the following equation:

Relative value= (Production area value)/(N-rich zone value)

The relative value is sometimes referred to as a Sufficiency Index (SI) and will usually result in a decimal value between 0 and 1. When the SI is below a certain threshold, it indicates that the production area is experiencing detectable N deficiency relative to the N-rich zone. Table 2 shows SI ranges for proximal and remotely-sensed data and the associated plant N status.

Table 2. Sufficiency Index (SI) values and associated plant N sufficiency status, calculated as the production area value divided by the N-rich zone value.

When it comes to deciding on N fertilization in California small grains, a N fertilizer response is almost certain when plant N status is “Highly Deficient”, very likely when the status is “Deficient” and uncertain when the status is “Sufficient”. The SNQT supplements the plant measurements with information about the current nitrate concentration in the root zone.

If a grower decides that a N fertilizer application is warranted based on the combination of plant and soil measurements, the next step is to figure how much N is necessary. This can be determined using a crop growth and N uptake model in conjunction with yield and protein goals. As part of our larger demonstration project, we will be releasing an online decision support tool in 2021 that integrates these components and provides customized predictions of crop response to in-season N fertilizer.

 

Summary

California farmers are under pressure to increase N use efficiency and reduce the potential for N loss to the environment. N-rich reference zones are a tool that can assist in these goals while considering and managing the risk of reduced yields. By implementing N-rich reference zones, using a suite of tools to monitor them during the season and comparing results to the broader field, a grower gets real-time knowledge to inform N fertilizer management in small grains. The information gained from implementing N-rich reference zones can help growers make fertilizer applications when increased yield and/or protein benefits are likely and avoid them when they are not. These improvements in N fertilizer decision-making can yield better economic and environmental outcomes in California small grain systems.

BMSB Targets Peach Crops

By
Cecilia Parsons
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Monitoring for BMSB in an orchard can be done with sticky panel traps with BMSB lures (photo by J. Rijal.)

Brown marmorated stink bug continues to spread in agricultural crops in California.

This large, invasive insect pest can cause significant damage to fruit crops, and peach orchards are one of this pest’s preferred feeding sites. First detected in the U.S. in the late 1990s, it has been causing crop losses in East Coast fruit orchards since 2010.

In his presentation for California Cling Peach Day, UCCE IPM advisor Jhalendra Rijal said BMSB is spreading slowly into the crop production areas of the northern San Joaquin Valley. As BMSB feed on 170 crop and non-crop host plants, the reproducing populations of BMSB have been established in residential areas of more than 16 California counties, with a majority of them being in the Central Valley. He emphasized the importance of identification of stink bug species when crop damage occurs.

Adult BMSB are about 0.75 inch in length, and larger than Consperse stink bug and red shouldered stink bug. BMSB can also be distinguished from other stink species by the two white bands on their antennae and legs.

In California, BMSB can have two generations per year. Adults will begin to emerge from overwintering sites in mid-March and may continue through May. Because of that, Rijal said the reproductive stages are staggered and both adults and nymphs can be found simultaneously in orchards. He also warned that populations can build quickly early in the season, especially when overwintering sites or non-crop hosts such as trees of heaven are present nearby.

BMSB feed on most of the plants out there with fruiting structures, Rijal said, but peach is one of the preferred hosts. BMSB can feed on all stages of the peach fruit development from early stage through ripening. Feeding causes both external and internal damage. Unlike almonds, early feeding does not cause fruit drop. Surface depression, gumming, necrotic lesions, cork-like lesions and whitish tissue beneath the surface are all signs of BMSB damage in peaches.

Damage is generally confined to the orchard edges. In a survey from 2017 to 2020, Rijal said that BMSB activity was cyclical. The severity of the damage to crops in 2020 was lower than previous years, but BMSB were found in more orchards

Monitoring for BMSB in an orchard can be done with sticky panel traps with BMSB lures. These panels are placed on a pole at the four-foot level from the ground in orchard edges. They attract both adults and nymphs. Beat trays and visual scouting can also confirm presence of this pest.

There are insecticides to reduce the BMSB population, mostly from the broad spectrum pyrethroid groups. These insecticides are often detrimental to natural enemies of other pests such as mites. Also, one spray is very likely not going to be effective if the population is present in the orchard throughout the season, similar to what has been seen on the east coast. The parasitic Samurai wasp is a specific natural enemy of BMSB and has been detected in many states including Washington, Oregon and in the Los Angeles area of California.

BMSB feeding on peach causes both external and internal damage (photo by J. Rijal.)

Biocontrol May be an Option for Grapevine Trunk Disease

By
Cecilia Parsons
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Fungal pathogens cause grapevine trunk diseases which can affect productivity and shorten the lifespan of a vineyard (photo by José R. Úrbez-Torres, Agriculture and Agri-Food Canada.)

Grape vine trunk diseases affect productivity and shorten life of a vineyard.

Akif Eskalen, UCCE Specialist and plant pathologist in the Department of Plant Pathology at UC Davis, in a presentation for Sustainable Winegrowing, explained his research into naturally occurring microorganisms for use as biocontrol against fungal pathogens that cause grape vine trunk disease.

Grapevine trunk diseases are prevalent in mature vineyards. The disease complex includes Eutypa, Esca, Botryospaeria and Phomopsis diebacks, but Eskalen said that at least 60 different fungal species have been identified in grapevines.

Delayed pruning and pruning wound protectants are two prevention routes identified in research, but a survey revealed that the majority of growers use neither preventative practice.

Antagonistic microorganisms already live in the plant tissue, Eskalen said, but they may become depleted. He and his research team are studying how to deliver the beneficial microorganisms back to plants, both in the nursery and in established vineyards.

Eskalen said there is evidence that these beneficial microorganisms not only increase the host plant’s defense mechanism, but also improve the health of the plant and potentially increase yield. Inside their host plant, the naturally occurring beneficial microbes secrete secondary metabolites that inhibit the growth of the harmful fungal organisms. Eskalen said they have identified several of the beneficial microbes and are now focusing on methods of delivery back into host plants.

Biological control can be an important tool in controlling grapevine trunk diseases, Eskalen said. The fungal pathogens that cause diseases each have a different mode to enter a plant and cause disease. No single fungicide can prevent all of those pathogens.

He also pointed out that naturally occurring microbes might be lost over time in vineyards where no other plants exist. Cover crops in vineyards foster more diverse microbe populations. Those populations will differ due to climate, soil type and other environmental factors. Eskalen said his research team is sampling vineyards in different areas of California to get a ‘big picture’ of beneficial microbe populations.

With this information, cultural practices can be adopted to encourage growth of the beneficial microorganisms. Increasing their levels in a plant, Eskalen said, will not only help with disease defense, but also improve overall health and yields.

Mass production and delivery of beneficial organisms is the goal of this research. Trials are introducing the microbes into cuttings in the nursery prior to grafting. In mature vineyards, vine injection and soil application are under study.

Citrus Breeding Efforts Aimed at HLB Resistance

By
Cecilia Parsons
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Citrus fruit from a hybrid HLB resistant tree. The fruits are 6×6 centimeters with 46% juice. According to Dr.Chandrika Ramadugu, most of the hybrid trees bear lime- or lemon-like fruit (photos by C. Ramadugu.)

UC Riverside scientists are working on breeding new varieties of citrus that will be resistant to citrus greening disease.

Citrus greening disease or Huanglongbing (HLB) is a bacterial disease that has killed citrus orchards worldwide. It has been detected in California citrus, but not in commercial citrus production. The disease is vectored by Asian citrus psyllid.

UCR researchers believe a sustainable solution to preserving the citrus industry is to develop varieties that carry natural resistance to HLB. The UCR research team, including Dr. Chandrika Ramadugu, has been awarded funding by the National Institute of Food and Agriculture to pursue breeding work. The research team includes collaborators from Texas A&M, University of Florida, Washington State University and USDA.

Resistant varieties of citrus have been identified, but the challenge is to use them to generate hybrids that will have the flavor consumers prefer along with resistance. The plan is to generate many hybrids and screen them for suitability for the citrus marketplace.

The Australian finger lime is one of six micro citrus varieties from Australia and is being used to create the hybrid varieties. It carries resistance to HLB, but few other attributes to fit the citrus market. The finger limes are about three inches long and roughly the size of an average person’s index finger, but fruit from juvenile trees can be less than one inch long.

The UCR team is currently studying the genetic makeup of the hybrids that have already been produced. Analyzing the new plants’ DNA will show if they carry enough disease resistance along with marketable qualities. Dr. Ramadugu said currently most of the hybrids have lime- or lemon-like fruits. The research team is still in the process of breeding other types of citrus.

One of the main challenges in this process is the length of time it takes before the hybrid citrus varieties bear fruit. With the help of UCR plant cell biology professor Sean Cutler, the team is hoping to accelerate the time it takes for the hybrid plants to bear fruit in the greenhouse. Clones of the best plants will be grown in Florida and Texas field trials.

Other approaches in the HLB fight at UCR include altering soil and root bacteria to improve plant immunity, and using an antibacterial peptide to clear HLB from an infected tree.

UCCE Looks at Whole Orchard Recycling in Walnuts

By
Cecilia Parsons
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Whole-orchard recycling site being prepared for planting new walnut orchard. Process involves pulling the old orchard, grinding the trees into woodchips, spreading the chips and incorporating them into the soil (photo by L. Milliron.)

Work has begun to determine if whole-orchard recycling (WOR) can be as successful in walnut orchards as it has been in almonds.

In a UCCE Virtual Walnut Series, UCCE orchard systems advisor Luke Milliron detailed WOR efforts in two walnut orchard trials.

Whole-orchard recycling involves tree removal and chipping, then spreading the wood chips over the orchard footprint and incorporating them into the soil. Burning restrictions and loss of cogeneration plants that would pay for wood chips spurred research in WOR over the past 10 years. UCCE farm advisor Brent Holtz has been studying WOR in almonds, and his research and field trials show both soil and tree benefits in replanted orchards.

Documented benefits include increased soil organic matter and carbon, increased soil nutrients and increased soil microbial diversity. There was no evidence of increased replant disease and no interference in pre-plant fumigation. There were also water use related improvements and increased orchard productivity.

Milliron shared that while his first trial, which begun in 2018, had some challenges with chip spreading, results of soil and leaf analysis were encouraging. Root lesion nematode levels in the soil were low, indicating successful fumigation. Leaf analysis showed no differences in nitrogen. Potassium and boron levels were slightly higher in trees grown on chipped ground. There were no growth differences between second leaf trees in chipped and non-chipped ground.

Due to the challenges of this trial, Milliron said the total tonnage of chips incorporated could not be determined.

A second WOR walnut trial in collaboration with Cliff Beumel at Agrimillora California is underway.

In this recent trial, Milliron said the dry chip tonnage came out to 91 tons per acre or 136 tons wet. In almond, chips are typically spread about two inches thick. In the walnut trial, the chips were spread at closer to three to four inches, and Milliron said they tried to put chips back at the same rate that trees were removed.

One of the “carrots” for growers is inclusion of WOR in CDFA’s Healthy Soils program designed to promote carbon sequestration and reduce greenhouse gas.

Some of the qualifications in the CDFA program include: trees must be at least ten years of age; orchards must be chipped and incorporated in place; chips must be evenly distributed throughout the orchard and incorporated into the soil to at least six inches in depth.

A list of whole orchard recycling providers can be found at orchardrecycling.ucdavis.edu/california-orchard-recycling-resources.

Mulching Trials in Mandarins Saved Water

By
Cecilia Parsons
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Mulch is applied around base of Satsuma mandarin. Trials conducted in this foothill mandarin production region showed soil moisture must be monitored when applying mulch. Growers will have to adjust irrigation scheduling to avoid over watering (photo by C. Fake.)

Keeping orchard floors bare is standard practice in California citrus production, but a Placer and Nevada County study on mulching mandarin trees is breaking ground for alternative management.

As the study enters its fifth year, UCCE farm advisor Cindy Fake noted some of the key findings. Mulched orchards have lower soil temperatures during the hot summer months and soil moisture is rarely depleted. Excessive fruit drop is mitigated and herbicide use has declined.

Parts of this foothill mandarin growing region east of Sacramento have much poorer soils than other growing regions. There are also hillside plantings where erosion is an issue, Fake said. Summer heat can be intense, and the trend toward longer dry conditions adds stress to the trees. Frost damage at the elevation of this growing region is rare and use of microsprinklers provides frost protection.

The mulching trials were initiated to help mandarin growers maintain and improve tree health and resilience in the face of climate change.

A downside of the mulch applications is the time and labor it takes to mix and apply.

Growers participating in the trials applied mulch composed of 50% horse manure and 50% wood chips, as both materials were readily available in the area. Mulch was applied under the canopies of the trees at depths ranging from four to six inches. The depth was reduced to four inches after two very wet years. Mulch was applied in the spring, Fake said, and it is critical to apply to moist soil. If it is applied to dry soil, it will take three to four major irrigations to move water through the mulch into the soil.

The trials also showed that when applying mulch to an existing orchard, soil moisture must be monitored. Growers used to applying the normal amount of water to their trees will have to adjust their scheduling, Fake said, as about 30% less water is needed.

The trial showed that moisture levels under the mulched tree were consistent and the profile is able to retain moisture with minimal depletion at a 6-inch to 12-inch depth. Most moisture depletion is occurring in the top six inches of soil and is lower than the control. Moisture levels in the control depleted at a more rapid rate throughout the soil to a depth of 12 inches.

By applying mulch in the spring, Fake said it would decompose over the next six months and not be a food safety issue at harvest. She also recommends mulching new trees to help maintain moisture for root growth.

More information on the mulch trial can be found at

Placer/Nevada Foothill Farming website. There is also an information sheet on how to use mulch on the UC ANR website.

Husk Fly Threatens Walnut Quality

By
Cecilia Parsons
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Adult walnut husk fly. These insect pests emerge from the soil during the summer months and lay eggs inside the walnut husk. UC IPM specialist Jhalendra Rijal said early season WHF infestation can also cause shriveled kernels (photo by Jack Kelly Clark, courtesy UC Statewide IPM Program.)

All walnut varieties are susceptible to damage from walnut husk fly (WHF). WHF are distinguished from other fruit fly varieties by their yellow coloring at the base of their wings and by the dark triangular band on their wing tips. Size is similar.

Damage to developing walnuts occurs after the female husk fly lays eggs inside the walnut husk. Feeding by larvae results in a soft and dark husk inside.

UCCE IPM advisor Jhalendra Rijal presented information on the biology, monitoring and control of this walnut pest for the West Coast Nut walnut conference.

Rijal said that WHF damage appears different than sunburn damage on walnut husks. Sunburn will cause a dry appearance, typically in the sun-exposed side of the nut, while nuts infested by WHF will appear soft and moist at the feeding site. Larval feeding inside the hull causes staining on shells and makes husks difficult to remove after harvest. Early season infestation can cause shriveled kernels.

WHF produce one generation per year. Adults emerge from soil in the summer months. Female WHF will feed on some sugar and nitrogen sources out in the orchard including aphids or bird feces for a week or so prior to laying eggs. Larvae hatch from eggs, feed on the husk and drop to the ground to pupate under the soil over winter.

Rijal said monitoring of adult WHF emergence is important to time insecticide applications for control. Yellow sticky traps with the attractant ammonium carbonate should be hung on the north side of the tree as high as possible and shaded by foliage. Check these traps two to three times per week to help to determine if WHF is emerging and laying eggs.

Rijal advised using a lens to identify female WHF in the traps. They are larger than the males and have light-colored segments on legs. Crushing their abdomen will reveal if they are carrying ‘rice-grain’ looking eggs.

Insecticide applications may be needed as soon as the first female WHF with eggs is found, depending on the orchard damage history, Rijal said.

The control materials target adult WHF and spray applications should be made before large numbers of egg-carrying females are found. The most effective control insecticides can be found at on the UC IPM web site.

Multiple applications may be necessary to cover the entire susceptible period as adult flies emerge over two to three months in the summer.

Rijal said research is needed to determine if cultural control (disking orchard soils to break life cycle) is possible or use of potential biocontrol agents such as insect pathogenic fungi or nematodes is needed to kill the larvae or pupae when they are in the soil. There is not natural biological control known for WHF.

Mechanized Pruning on the Rise in Vineyards

By
Cecilia Parsons
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A field demonstration of mechanized vineyard pruning at the Kearney Agriculture Research and Education Center. Transitioning existing vineyards to a single high wire system has been most successful for mechanized pruning, reports Fresno County UCCE farm advisor George Zhuang (photo courtesy G. Zhuang.)

Mechanical pruning operations continue to increase in San Joaquin Valley wine grape vineyards as a way to save on labor costs. Dormant pruning, suckering and leaf removal in vineyards can all be done mechanically.

George Zhuang, UCCE viticulture farm advisor in Fresno County, in a presentation at the San Joaquin Valley virtual Grape Symposium, said existing trellis systems in vineyards can be converted to mechanical pruning systems and retain production and fruit quality.

The most common trellis systems in San Joaquin Valley wine grape vineyards have been two-wire bilateral cordon, “California sprawl,” and quadrilateral, and both can be adapted to mechanized pruning, Zhuang said. The single high wire system is the standard system for mechanization.

Recent trials have looked at differences in cordon height and how that affects vineyard production. Transitioning existing vineyards to a single high wire system has been most successful for mechanized pruning operations. This trellis system has a single high wire at 62 to 66 inches in height, is single canopy, non-shoot positioned and has around 35% exposed leaf area. Production is at 18 to 24 months and yields are 11 to 24 tons per acre with 7×10-foot spacing.

Zhuang said the other trellis system in use in vineyards where mechanized pruning is done is the quadrilateral. This system has a divided canopy and a higher percentage of exposed leaf area. Production depends on variety and spacing of vines.

Setting up the box size is important in mechanized pruning. The spur height sets the height of the bearing surface. Precision pruning is four inches, while 6- and 8-inch set ups may require some hand pruning to keep from overloading the vines as the bearing surface increases.

Two Fresno county trials were done to compare single wire height in trellis systems for winegrapes. Vines were planted in 2017 and hand pruned the first year of production in 2019. In 2020, vines were mechanically pruned. Heights tested were 68-inch cordon and 52-inch, the classic “California Sprawl” height. First yields were comparable in both systems. Sugar (measured by Brix) was increased in the high wire system due to more leaf area, Zhuang said. Water use was lower in the higher wire system.

He said the trial would continue to determine if there are significant differences as the vines mature.

Availability of mechanical pruning custom operators may be a limiting factor going forward. The machinery is a significant investment more suitable for larger acreages. Smaller growers would need to hire a custom operator. Zhuang said he has seen efforts by winegrape growers to design and build their own pruning machinery.

Garlic Clove Rot Studies in Central California Continue

By
Cecilia Parsons
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At least four species of garlic bulb rots have been recovered from samples submitted to UCCE researchers by garlic growers. Fusarium proliferatum has been the most common, being recovered from all storage facilities and farms (photo courtesy T. Turini.)

Lesions on garlic cloves, which affect crop quality, have been associated with several species of Fusarium, said Tom Turini, UCCE vegetable crops farm advisor. Similar garlic diseases caused by this soilborne fungus were reported in other production areas, but recent studies investigating this issue in Central California until now have not been conducted.

The Fresno County 2019 crop report noted that 24,180 acres of garlic were harvested that year, making garlic one of the higher acreage vegetable crops in the county. In addition, the 2018-19 CDFA Agricultural Statistics Review documents that more than 80% of the garlic produced in the state was in Fresno County based on gross crop value.

Turini is working with UC Davis plant pathology specialist Cassandra Swett, who is receiving samples collected from fields and those submitted by growers and processors. In 2020, Swett ran tests to identify the Fusarium species associated with the clove lesions. The work planned for this growing season should provide a robust set of detailed information regarding the identity of the fungus responsible for the crop damage.

Swett reported that at least four species of garlic bulb rots have been recovered from samples submitted. Fusarium proliferatum has been the most common, being recovered from all storage facilities and farms. F. oxysporum and F. falciforme are also common, present in about 50% of farms or facilities. These three species have been confirmed to be capable of causing a clove rot in preliminary trials.

  1. proliferatum is the only species found occurring alone, suggesting it might be the primary driver of bulb rot. A fourth species, F. brachygibbosum, was also found and is being tested for pathogenicity. Neither this species nor F. falciforme have previously been described as garlic pathogens. Swett said that with multiple species causing garlic bulb rot, it will be critical to identify strategies that can co-manage all species. Understanding when and how infections are occurring can help with identifying critical target periods for control.

Garlic bulb rots are not a new issue in garlic production, Turini said. Fusarium was previously associated with garlic rot in the Fresno production area and the fungus was considered ubiquitous. Current observations are that the issue is more common, and the character of the symptoms is not identical to those described decades ago. The work in progress will provide insight into what fungi are involved in the disease and other details critical in taking reasonable steps to mitigate the damage.

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