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The Key to Building a Soil that Can Suppress Pathogens Naturally

By
Cassidy Million | Ph.D., Director of Ag Science, Heliae Agriculture
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Figure 1. Disease suppressive soils are defined as soils that naturally defend against pathogenic diseases before the disease attacks a growing crop, or the pathogen can infect the crop but the disease declines with successive cropping.

When it comes to managing soilborne plant diseases, methods for reducing or eliminating the impact of pathogens have heavily relied on selecting a resistant plant variety and the use of pesticides for protection. However, the soil, and specifically the biological component consisting of microbes, can be another tool growers can leverage to help protect their crops from soilborne diseases.

Healthy soil and a healthy crop can lead to reduced disease incidence, an idea that is becoming more widely accepted across the agriculture industry. Many questions arise with this concept: How are soil health and soilborne plant diseases related? Can soil health result in less plant disease increase pressure? What are the mechanisms behind this phenomenon? In this article, we will discuss and address these questions and take a deep dive into a phenomenon known as disease suppressive soil (DSS). 

Microbes Are the Key 

The activity of soil microbes or communities of microbes is what gives rise to a soil’s disease suppressive properties. As you can imagine, the soil is a complex system, and understanding the mechanisms that contribute to DSS is extremely intricate. Imagine the interactions between beneficial and pathogenic microbes as a battlefield belowground. In the case of DSS, the beneficial microbes win. Just like on a battlefield, many tactics, strategies and tools are needed to overcome your opponent; and the same is true for soil microbes. Multiple mechanisms from increasing soil and plant health to directly impacting pathogenic microbes are used by soil microbes in the battle between beneficial and pathogenic. 

Soil microbes play a key role in soil health and soil quality, which in return affect the degree of disease suppression in the soil. Soil microbes have multiple ways in which they suppress disease in the soil, including the improvement of plant health, inducing natural defense response in the host plant, secreting enzymes and antibiotics, and through microbial competition.

Defining Suppressive Soils and Why They Matter

DSS are defined as soils that naturally defend against pathogenic diseases before the disease attacks a growing crop, or the pathogen can infect the crop but the disease declines with successive cropping (Figure 1). Soils can do this through reducing the establishment or growth of pathogenic fungi or bacteria due to the makeup of their microbiome (the communities of fungi and bacteria that are living in the soil.) Soils can keep disease development at a minimum even in the presence of a susceptible host and disease-causing pathogen. 

Figure 1. Disease suppressive soils are defined as soils that naturally defend against pathogenic diseases before the disease attacks a growing crop, or the pathogen can infect the crop but the disease declines with successive cropping.

DSS can be naturally occurring but may also be developed over time through cropping practices. To help clarify, we can divide suppressive soils into two categories: general suppression and specific suppression (Figure 2). 

Figure 2. Disease suppressive soils can be naturally occurring but may also be developed over time through cropping practices. They can be categorized by general suppression and specific suppression.

General suppression gives protection towards multiple pathogenic microbes. The increase in the abundance (number of microbes) and diversity (which microbes are present) of soil microbes is key in general suppression. These soil microbes out-compete pathogenic microbes, which in turn creates general suppression. Agricultural practices can impact this type of suppression with practices such as soil sterilization reducing it and good soil health practices enhancing it. 

Specific suppression can occur naturally (long-standing) or be induced. Long-standing specific suppression is associated with a specific species of microbes or group of microbes and is naturally found in the soil without the presence of a plant. Induced specific suppression can be produced through monoculture of a crop, growing susceptible crops or by mixing small amounts of a suppressive soil into a conducive soil. 

So, why should we care about DSS? As we move forward in modern agriculture, restriction of fumigants and pesticides as well as the lack of available resistant cultivars will only make these soilborne diseases harder to manage. DSS are another tool growers can use to help manage difficult-to-control soilborne diseases.

Soil Microbial Battleground

Beneficial soil microbes have a direct impact on their pathogenic counterparts, but on the battlefield, the beneficial soil microbes can outcompete their pathogenic counterparts, leading to greater numbers of beneficial soil microbes. Underground, beneficial soil microbes use up the exudates and nutrients created and released by the host plant, blocking the pathogenic microbes from accessing and utilizing the resources needed to survive. 

During the battle, the soil microbes can display hyperparasitism, where they infect pathogenic microbes present in the soil that pose a threat to the crops around them. Plus, the microbes can cause additional impact on their pathogenic microbial counterparts by secreting antibiotic-like enzymes and toxins in a process called antibiosis. Antibiosis is a competitive tactic that kills other pathogenic microbes and increases their ability to battle and defend themselves and the host plant from pathogens. 

The health of the host plant above the ground is impacted by the soil health under the ground. The structure of the soil plays a direct role in plant health in terms of water infiltration, retention, soil compaction and access to nutrients. Soil microbes are to key to improving soil structure through increased soil aggregation, and aid in the release of mineral nutrients through a conversion process called nutrient mineralization. Soil microbes secrete exudates that can trigger disease-resistant responses for host plants. All these factors aid in improved plant health and performance. 

Soil Health Impact 

In the soil microbe-pathogen battleground, what exactly are these microbes battling for? For the pathogenic microbes, they are battling to infect the host plant. To understand the complexity and how soil health and microbes play a role in plant disease and DSS, we need to review the disease triangle (Figure 3). For disease to occur, there must be three components (representative of the sides of a triangle):

Susceptible host

Conducive environment

Virulent pathogen

If one of these sides is removed, plant disease is not possible. The environment side of the triangle is where we need to focus as this is the side that includes soil health and holds the key to creating disease suppressive soils.

Figure 3. Disease triangle. For disease to occur, there must be three components, representative of the sides of a triangle. The environment side of the triangle shows the impact of soil health and microbes on the ability for disease to occur.

 

The physical and chemical properties of the soil, including pH, soil organic carbon and nutrients, provide the habitat for microbial activity. Crop management practices such as tillage, irrigation, fertilization, the addition of green manures and weed management can directly impact the soil environment and microbial activity in the soil. Many of the factors mentioned here directly impact microbial activity and communities and are the key to soil health. Many of the factors mentioned here directly impact microbial activity and communities and are the key to soil health. As we know, there are tools and inputs to help improve soil health, meaning that if we manipulate the environment (the soil) and remove one side of the disease triangle, we can reduce plant disease infection. Therefore, improving soil health can lead to general DSS (Figure 4). 

Figure 4, How soil health is related to general disease suppression and the agricultural practices that impact soil health leading to general disease suppression.

 

What about specific disease suppression? If we recall, long-term monocropping can be used to achieve specific suppression in some systems, which is contradictive of soil health practices. The literature has noted the ability to achieve specific disease suppression by using compost and green manures. However, increasing soil health and increasing microbial communities is the best practice to achieve and improve general suppression and increase crop productivity. 

Managing the Soil to Achieve Disease Suppressive Soil 

Soil microbes are key to DSS, and soil health directly impacts the makeup and activity of soil microbes. DSS can be negatively or positively impacted by cropping systems and management practices. Soil management practices that positively influence DSS include crop rotation, intercropping, minimum tillage practices, fertility or organic inputs such as manures and composts. In addition, adding liable carbon sources as a food source for microbes will increase the abundance and diversity of soil microbes, which is critical in general suppression. Crop rotation is an important factor; rotating to a non-host can aid in reducing soilborne diseases and positively impact soil microbial diversity. What we know to date is that suppressive soils are usually mediated through soil microbial community shifts overtime, so adopting practices and amendments that increase soil health and organic matter and increase the diversity and abundance of microbial activity in your soils will aid in suppressing soilborne diseases.

Battle for a Sustainable Solution to Soilborne Diseases   

DSS can be a tool for soilborne disease management. Adapting soil health practices and increasing the soil microbial activity is a sustainable agricultural practice that will be key in the coming years. Building and maintaining suppressive soils can provide a solid foundation for crops to thrive. We’ve discussed the key to DSS (microbes) and contribution to a soil’s disease suppressive properties. Focusing on the improvement of soil health and natural defense response belowground is a good first step toward the achievement of healthy crops that are protected against soilborne disease.

Resources

Disease Suppressive Soils: New Insights from the Soil Microbiome- https://doi.org/10.1094/PHYTO-03-17-0111-RVW

Disease-Suppressive Soils—Beyond Food Production: a Critical Review- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7953945/ 

Current Insights into the Role of Rhizosphere Bacteria in Disease Suppressive Soils- https://doi.org/10.3389/fmicb.2017.02529 

Plant Health Management: Pathogen Suppressive Soils- https://doi.org/10.1016/B978-0-444-52512-3.00182-0

Fine-Tuning with Soil Health; Soilborne Disease?- https://csanr.wsu.edu/fine-tuning-soilborne-disease/

Developing Disease-Suppressive Soil Through Agronomic Management- https://www.researchgate.net/publication/292011599_Developing_Disease-Suppressive_Soil_Through_Agronomic_Management 

Phosphate vs Phosphite: Part One

By
JW Lemons
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Hands with leaves
Though phosphite is not the best nutritional source of phosphorus, it can serve as a carrier and catalyst for quick nutrition supplements and responses to nutrient deficiencies.

Phosphite has been a controversial topic for years. Its use and benefits are argued in hundreds of research papers across the world’s scientific communities. Is it a fertilizer, a biostimulant or a fungicide? These questions are discussed in multiple university research results. I believe that if we look carefully, we can conclude phosphite serves all three functions. As with everything we do with chemicals and nutrition, we need to be aware of possible negative effects. We also need to determine how we are using the phosphite materials and the results we are seeking.

First, we need to understand and differentiate between phosphate and phosphite. There is often a lot of confusion here.

It Starts with Phosphorus
Phosphorus (P) is an essential major macronutrient. It is required by all living organisms. It is a limiting nutrient that controls growth in many ecosystems. P in living systems occurs mainly in the form of inorganic phosphate as well as phosphate esters. The formation and breakdown of phosphate esters under the control of kinases and phosphatases regulates the temporal protein activity and is responsible for the generation, distribution and utilization of free energy throughout the cell in several metabolic pathways. P is a structural component of the phospholipid bilayer membranes and genetic material including DNA and RNA. In nature, most P exists in its completely oxidized state (valence of +5) as a phosphate anion (PO43-, Pi), phosphate-containing minerals and organic phosphate esters. Pi compounds are the only form of P utilized by the plants for their nutrition. Modern agriculture is currently dependent on the continuous input of Pi fertilizers, produced by the mining of rock Pi. Approximately 80% of the mined P is used to manufacture Pi fertilizer.


The Difference is An Atom
Phosphite (Phi) is a reduced form of Pi with one less oxygen atom (P valence of +3). Phi compounds have been widely used in agriculture as fungicides for controlling several plant diseases caused by oomycete pathogens including Phytophthora spp. Although Phi can be absorbed by the plant cells through the Pi transporters, plants cannot metabolize Phi, which limits its use as a fertilizer. Phi can only be metabolized naturally by certain bacteria with an enzyme called Phi dehydrogenase (PtxD), which oxidizes phosphite into phosphate, a form that can be utilized for various cellular functions, and it does not provide P nutrition to the plants. The supply of Phi attenuates the Pi starvation responses (PSRs) in plants and inhibits the growth in Pi-starved plants. The addition of Phi to plants experiencing deficiencies of P confuses the plant’s internal signal that triggers the P deficiency responses. Several studies show that too much Phi application has detrimental effects on the growth and development of various plants. Therefore, Phi compounds should not be used as a major source of P fertilizers in agriculture.


Phi is more soluble than Pi and has a smaller structured molecule, so it is more readily absorbed by plant tissue. When added with other nutrients, it can serve as an excellent carrier. For example, a Phi mix containing phosphorus and potassium, calcium or magnesium would be absorbed readily and carry the needed nutrient into the plant. This has been demonstrated in numerous trials based on Verdesian Life Sciences’ line of NutriPhite nutrient products. Because of the proven biostimulant properties of Phi and the fungicidal benefits, we have the possibility of getting multiple benefits applied with a single application.

Phosphite Not a ‘Traditional’ Fertilizer
So, though Phi is not the best nutritional source of P, it can serve as a carrier and catalyst for quick nutrition supplements and responses to nutrient deficiencies. Soil applications of Phi usually do not replace P in the soil; however, plantings the following year show the plants do better where Phi had been applied the previous season. Interest in using Phi as part of a total production package is increasing, especially for some high-value crops. Phi fertilizers, if not formulated correctly, have significant potential to be phytotoxic and induce adverse reactions with other materials in the spray tank such as microelements and pesticides. When choosing your Phi source, it would be wise to seek a stabilized formula as it is proven not to bind with other tank mix materials. Chemical bonds will create another compound that even if not phytotoxic can be completely useless and unattainable by the plant. The salt-out effect can be a potential to clog nozzles and filters and could result in a waste of dollars spent on the Phi and/or other nutrients and chemicals. By binding these up, we are losing all or most of the efficacy of an expensive input in our crop management. All fertilizers, especially Phi, should be used in close consultation with a crop consultant to meet desired production goals.


In California, it is important to recognize that research has shown foliar applications of Phi can replace Pi in citrus and avocado crops suffering from P deficiency. Phi conversion to Pi may be attributed to slow chemical oxidation or by oxidizing bacteria and fungi that have been found living on the leaves of these two crops.

In part two of this article series, we will visit the biostimulant role of Phi, which is possibly even more important than the fungicidal properties of this very diverse and very useful tool to consultants and growers.

Scouting is Important Component in Weed Control Strategies

By
Cecilia Parsons
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Post-harvest is a good time to assess effectiveness of weed management programs (photo by M. Katz.)


Postharvest scouting for weeds in tree nut orchards presents an opportunity to evaluate this year’s orchard floor management program and see which weed species have invaded or spread, where weed populations are highest and severity of infestation. Field margins, ditch banks and irrigation canals should be included in the scouting.

Scouting allows you to adjust control practices for the following year and select the best options for the weed species of concern. It can also help with selection of the best cultivation method for the weed stage.

Correct identification of weeds at all stages of growth is especially valuable, said John Roncaroni, UCCE weed specialist emeritus. Grass species identification can be difficult, he said, but it is important to determine because some species like ryegrasses are more resistant to glyphosate while summer grasses are not. Knowing the weed can also help determine if mechanical or cultural control methods would be useful. Weeds can look similar but have very different management requirements.

The UCCE publication Sacramento Valley Orchard Source recommends scouting a couple of times during the winter to catch weeds when they are young and more easily controlled. Records of weed infestations should be kept and mapped to show where weeds are a persistent problem.

Keeping records of weed infestations from year to year can help show trends and which management tactics are working.

Another recommendation is to look for different weeds in different management zones. Tree row checking may show effectiveness of previous herbicide applications. The ground cover in row middles can show if perennial seedlings are increasing. Orchard borders and row ends are places to look for new weeds.

Almond Board of California reports that as of as of 2021, there were 13 weed species commonly found in almond orchards with confirmed cases of herbicide resistance biotypes in California. Due to the increasing numbers of herbicide resistant weed species, early control of escaped weeds can reduce cost of an annual orchard floor management program. An example in Sacramento Valley Orchard Source notes that spot treating two acres of glyphosate resistant palmer amaranth with a tank mix of Glufosinate and Gramoxone is more affordable than trying to control this weed in an entire 50-acre block.

Citrus Rootstock Choices Determine Growth and Productivity

By
Cecilia Parsons
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Bud union of a 7 year old tree of DaisySL mandarin on C146 rootstock at Lindcove Research and Extension Center near Exeter.  This is an experimental rootstock that has shown some promise for HLB tolerance in Florida trials (photo by M. Roose.)


Citrus tree growth, health and productivity are influenced by the rootstock chosen. In a Citrus Research Board webinar, Dr. Mike Roose, professor emeritus of genetics at UC Riverside, outlined the newest challenge for citrus rootstocks: heat and drought tolerance.

Choosing a rootstock involves balancing objectives, strengths and weaknesses. Roose advised choosing a rootstock with characteristics matched to the location and objectives. With an existing planting with a known rootstock, management should be done to avoid known stresses to the rootstock. For example, Phytophthora control may be necessary for many rootstocks. Soil fumigation may be needed if there is low tolerance to nematodes. Spray regimes for Asian citrus psyllid may control aphids that vector Tristeza.

Salinity, calcareous soils and drought pose serious challenges to rootstocks.

Roose noted that there are little comprehensive data on drought resistance of citrus rootstocks and the topic is complex because drought is location specific. Major citrus producing countries have diverse climate, soils vary in water holding capacity and cultural practices such as mulching, irrigation and berms vary widely.

Roose said drought tolerance is a complex trait that is important at different growth stages and involves multiple adaptations. Those can maximize extraction of water from soil while minimizing loss from leaves. Deep roots and altered leaf morphology, including thicker cuticle, narrow leaves and changes in stomata, are also mechanisms that aid in drought tolerance.

Iron chlorosis is a condition that occurs on more alkaline soils (pH 7.5 to 8.5) because availability of iron is reduced. Water with higher pH can also enhance soil problems. A typical iron chlorosis symptom in citrus is interveinal chlorosis. A trial of Tango rootstocks from 2010 to 2020 showed differences in tolerance among five different rootstocks. Pomeroy trifoliate and Rich 16-6 had high tree losses while trees on Swingle, Carrizo, c35 and Volk had few chlorosis symptoms.

Roose said citrus rootstocks differ in tolerance to salinity and to specific ions present. Citrus shows yield decline when soil salinity reaches 1.7 dS/m with about 16 percent yield loss per additional 1 dS/m. If soil salinity is 4 dS/m, then 2 .0 inches of water per foot of rootzone are required to reduce salinity to 1.5 dS/m.

High transpiration increases the uptake of chlorine and other ions from soil. Leaf chlorosis levels are inversely related to water use efficiency and membrane transporters are involved in Cl uptake. Overall tree tolerance is also influenced by the scion.

Rootstock choice can also affect tree vigor in the absence of diseases, soil and water stresses, climate, compatibility with scion and genetics when zygotic seedlings are used as rootstocks.

Mealybugs Pose Threat to Grape Quality

By
Cecilia Parsons
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Vine mealybug (VMB), which is spreading throughout grape growing regions in California, has six to seven generations a year (photo courtesy Lodi Woodbridge Winegrape Commission.)


Mealybug management is an important part in controlling grapevine leafroll disease.

At a Fresno State grapevine viruses symposium, Kent Daane, UC Berkeley extension specialist, said systemic insecticides are slow to kill mealybugs, while mealybugs’ ability to transmit the disease occurs at a fast rate.

A case study published in Frontiers in Microbiology noted that control of mealybug and grapevine leafroll disease is among the top priorities in the U.S. wine grape production industry. It is most prevalent in cool climate regions where fruit on infected vines has delayed maturity that results in lower brix, affecting value of the crop.

There are three biological components to grapevine leafroll disease: a complex of viruses, grapevine host plants and species of mealybugs and soft scales that transmit the virus. Symptoms appear in the fall when red grape cultivars display leaf reddening. In white cultivars, there is slight leaf chlorosis. Both red and white cultivars develop downward rolling of leaf margins and phloem disruption.

Vine mealybug (VMB), which is spreading throughout grape growing regions in California, has six to seven generations a year and all stages of overlapping generations are found on canes, clusters and leaves and under bark on trunks and cordons.

Daane said traps are effective in determining if vine mealybug is present in a vineyard. Grape growers can be surprised to find this pest in vineyards since infestations can be difficult to spot. The highest flight rates for VMB are later in the season close to harvest.

Traps can be a good starting point for a concerted regional effort in control of VMB. Asking neighbors to also trap and compare trap counts can help with control decisions.

Grape mealybug is a native pest that has a large complex of natural enemies and is often under good biological control.

Current management strategies for vine and grape mealybug include insecticides, mating disruption, biological control and management of some ant species. The Argentine ant in particular farms the mealybugs and is aggressive in California vineyards, causing a struggle in maintaining a low mealybug population.

Conclusions from studies of mealybug control in major grape-growing regions of the world show a combination of approaches are needed for grapevine leafroll disease. It must be managed on a large scale and a long-term management strategy is necessary. Infected vines or blocks will continue to be a source of infection and the disease will spread in the presence mealybug vectors.

Having a source of certified uninfected propagation materials is also a component of avoiding grapevine viruses.

New Virus Strain of BCTV in Colusa County

By
Cecilia Parsons
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Beet curly top virus affected tomato plant. A new strain of this disease has been confirmed in Colusa County. It is vectored by the beet leafhopper (photo by Amber Vinchesi-Vahl,)


An unusual strain of beet curly top virus (BCTV) has been confirmed in Colusa County this growing season. This new outbreak follows an outbreak last year, possibly due to the drier conditions which resulted in the vector of the disease, beet leafhopper, migrating into processing tomato fields earlier than usual.

UCCE Colusa County Farm Advisor Amber Vinchesi-Vahl said that BCTV is not normally a serious issue in processing tomatoes in the northern tomato growing regions. She noted that the confirmed virus strain was different than the strain found in Fresno and Kern counties.

Some damage to fruit and plants occurs when beet leafhopper nymphs and adults feed in tomato fields, but the main issue is vectoring BCTV.

Vinchesi-Vahl , in the Vegetable Crops newsletter, said that other insects cannot transmit the virus to tomatoes. The virus is not transmitted or spread by seed, touch or machinery.

Curly top-infected plants turn yellow and stop growing. Leaves roll upward and turn purplish. Leaves and stems become stiff. Spring plantings are the most susceptible as beet leafhopper migrates from overwintering host plants when they become dry. The west side of the San Joaquin Valley is where most infections occur.

Vinchesi-Vahl said that beet leafhoppers do not complete their life cycle in a tomato field. They migrate in, feed and then move on to preferred hosts, including sugarbeets. For this reason, it is not likely that beet leafhoppers will be visible in field inspections.

When beet leafhoppers migrate out of the field, that ends the virus transmission. The infected plants, particularly the earliest infected, will die. There will be a mix of plants with different stages of the virus resulting from multiple flights of beet leafhopper into the field.

Managing surrounding weed hosts around tomato fields may be helpful in reducing sources of the virus. The UC IPM Guidelines say that insecticides applied to infested fields to control beet leafhopper and reduce the spread of the curly top pathogen may prevent some infield spread, although infected plants will not recover. In areas that are at annual risk of beet leafhopper infestations, application of a systematic insecticide may have some impact. Beet leafhopper populations are greatest in years with rainfall that promotes growth of its weed hosts in the foothills.

Growers are asked to be on the lookout for BCTV in tomato fields and to contact Vinchesi-Vahl at acvinchesi@ucanr.edu if high incidence is found.

Resistance-Breaking Virus Strain Found in Sutter County Tomatoes

By
Cecilia Parsons
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Symptoms of tomato spotted wilt virus include extensive distortion, yellowing and necrosis of leaves. Plant infections should be confirmed with molecular testing. Locations of infected plants should be documented to allow follow up next year (photos courtesy A. Vinchesi-Vahl.)


A resistance-breaking strain of tomato spotted wilt virus (TSWV) was detected in May in a northern Sutter County processing tomato field, according to the UCCE Vegetable Crops Newsletter.

UCCE Vegetable Crops Advisor Amber Vinchesi-Vahl recommends monitoring fields for TSWV, particularly those planted with resistant cultivars. Fields where more than 3% of plants are showing symptoms are suggestive of the presence of the resistance-breaking strain. Symptoms include extensive distortion, yellowing and necrosis of leaves. Plant infections should be confirmed with molecular testing. Locations should be documented to allow follow-up next year.

Growers who suspect TSWV in resistant tomato varieties should contact Vinchesi-Vahl or deliver samples to the Colusa or Yuba-Sutter UCCE offices.

Symptoms of tomato spotted wilt virus include extensive distortion, yellowing and necrosis of leaves. Plant infections should be confirmed with molecular testing. Locations of infected plants should be documented to allow follow up next year (photos courtesy A. Vinchesi-Vahl.)

Incidence of tomato spotted wilt virus has been increasing in California processing tomatoes, Vinchesi-Vahl reports. It is spread from plant to plant by thrips, mainly western flower thrips. Tomato spotted wilt virus can only be acquired by immature thrips. Infected adults can transmit TSWV throughout their 30- to 45-day lifespan.

Symptoms for TSWV depend on stage of growth. In early vegetative stages, bronzing and wilting occur, and the entire plant may look off-color and have a crumpled appearance. At later stages, purpling and leaf curling are evident. Bumpy fruit with ring spots is seen in bearing plants. Symptoms can be similar to those caused by curly top and alfalfa mosaic, but if necrotic spots and rings develop on leaves, followed by necrosis and dieback of entire leaves and shoots, TSWV is indicated.

Effective management targets thrips and the virus. Growers should use transplants from greenhouses that monitor for thrips and inspect plants for signs of TSWV. Growers should avoid planting near established crops with confirmed TSWV infection. Thrips should be managed early in the season with chemical control as monitoring or degree day predictions indicate. Tomato fields with high populations of thrips and resistance-breaking strains of TSWV should be sprayed for thrips one to two weeks prior to harvest to reduce the spread of the adults to nearby fields. Weed control in and around the field is also recommended.

Best practices after the growing season include removal of old tomato plants and other host crops on a regional level. Weed control in adjacent fallow fields can reduce sources of infection.

For additional information on testing for TSWV or developing thrips management strategies, contact UC Davis’ Robert Gilbertson, rlgilbertson@ucdavis.edu.

Young Tango Trees Targeted by Citrus Leafminer

By
Cecilia Parsons
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Adult leafminers cause no damage and live only one to two weeks. After mating, the female lays single eggs on the underside of leaves (left photo by David Rosen, right photo by Jack Kelley Clark, both courtesy UC Statewide IPM Program.)


Protection of Tango trees from citrus leafminer for the first three to four years after planting is warranted, UC research has found, and monitoring for larval mining activity should
be done to determine the timing and frequency of insecticide treatments.

Matt Daugherty, UCCE entomology specialist at UC Riverside, and Beth Grafton-Cardwell, UC emeritus entomology specialist, noted in their citrus leafminer study that monitoring is critical as the citrus leafminer populations vary during the season and tend to decline with tree age.

Citrus leafminer, Phyllocnistis citrella, is a tiny moth that was detected in the U.S. in 1990 and found in California’s Central Valley citrus growing regions in 2006. It was not considered a serious pest in mature citrus, but studies found that infestations in the rapidly growing Tango acreage could affect tree growth and, later, crop yields.

Daugherty said that anything that impacts growth of new flush in citrus will also impact citrus leafminer populations. Optimal temperatures for citrus leafminer development are between 70 and 85 degrees F with greater than 60% humidity.

Adult leafminers cause no damage and live only one to two weeks. After mating, the female lays single eggs on the underside of leaves (left photo by David Rosen, right photo by Jack Kelley Clark, both courtesy UC Statewide IPM Program.)

Visual surveys for this pest in young trees should take place in the spring and summer months. UC IPM guidelines note that citrus leafminer has four life stages: egg, larva, pupa and the adult moth. Adults cause no damage and live only one to two weeks. After mating, the female lays single eggs on the underside of leaves.

Eggs hatch about one week after being laid. The larvae begin feeding in the leaf and produce tiny, nearly invisible mines. When the larva emerges from the mine, it rolls the edge of the leaf over, causing a curling of the leaf. Inside that curled leaf edge, the leafminer becomes a pupa.

Daugherty and Grafton-Cardwell evaluated three insecticide treatment regimens to reduce citrus leafminer densities compared to untreated trees to determine if growth and development of Tango mandarin trees were affected during the first four
years after planting.

They found that the number of leaves that were suitable for egg laying by citrus leafminer fluctuated, with the lowest numbers occurring during the summer heat. Both the amount of tender leaf flush per shoot and the citrus leafminer populations per leaf declined during the three years of the study, reducing the number of applications of insecticides needed as trees matured.

Systemic imidacloprid combined with multiple foliar insecticides significantly improved the yield of trees in years three and four when they first came into bearing.

Individual insecticide applications reduced leafminer density for two to three weeks, including the systemic Admire Pro® applied at seven fluid ounces per acre.

Leaf Sampling Protocols For Tree Nuts

By
Cecilia Parsons
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Accurate leaf analysis is difficult if the right parts of the plant are not submitted. Trees selected should be of similar age, variety, rootstock and vigor (photo courtesy Jenna Overmyer, Precision Agri Lab.)


Foliar tissue analysis determines essential or toxic levels of nutrients in plants. The analysis is used to detect tree response to a fertilizer program and to determine if there are nutrient deficiencies or toxicities that need to be corrected.

When taking leaf samples to determine your orchard’s nutritional needs, it is important that the sample sent to the laboratory not only be a good representation of the orchard, but also be the right part of the plant.

Scott Fichtner of Precision Agri Lab in Madera said that analysis of leaves from a tree nut orchard can let you know where to focus nutritional efforts and adjust for nutritional deficiencies or toxicities. Leaf sample analysis may also be necessary to justify application of nitrogen.

Accurate analysis is difficult if the right parts of the plant are not submitted. Trees selected should be of similar age, variety, rootstock and vigor. Do not take leaf samples from a tree that appears weak in comparison to others in the orchard, Fichtner said. For example, leaves that are water deficient or have been damaged by spider mites should not be included in the sample. Their nutrient levels will be lower compared to healthy leaves.

Leaf tissue samples can be collected throughout the growing season; however, the least change in concentration occurs from late June to July. The UC guidelines are generally correlated to July leaf tissue samples.

Samples from almond trees taken March through April should be the most recently matured leaves from the base of the spur. Samples in May should be the most recently matured leaf from the tip of the spur. Samples taken from June through October should be the terminal leaf on the spur. Pistachio and walnut samples taken April to October should be terminal leaflets.

Fichtner said that to achieve a representative sample from a block of trees, leaves should be pulled from each of the four quadrants of an individual tree. Sample 20 to 25 trees in a block to achieve a composite sample of 80 to 100 leaves. Samples should be placed in a paper bag for delivery or shipment to the lab and protected from temperature extremes.

Proper sampling is an integral and vital part of foliar analysis. A common issue with leaf sampling, Fichtner said, is that the person tasked with pulling the samples has not received training. Precision Agri Lab has training videos available for pulling leaf samples.

Transmission Routes for Red Blotch Studied

By
Cecilia Parsons
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Adult threecornered alfalfa hopper can be found in vineyard cover crops in the spring (photo by Jack Kelly Clark, courtesy UC Statewide IPM Program.)


As researchers began to study Grapevine red blotch virus in grape vineyards and what causes this virus to spread, they began looking at possible transmission by pest insects.

Houston Wilson, UCCE specialist (Dept. of Entomology, UC Riverside) at the Kearney Agricultural Research and Extension Center, said that at this point, the only known insect vector was the threecornered alfalfa hopper (TCAH). This green, robust, wedge-shaped insect has piercing sucking mouthparts. While it can be found in vineyards, it strongly prefers legumes and is commonly reported as a pest of soybean, peanuts and alfalfa.

While conducting a study of the ecology and phenology of the threecornered alfalfa hopper, Wilson and his research team found that even though it can be found in vineyards and is capable of feeding on vines, this insect cannot actually complete its life cycle on vines. Instead, leguminous ground covers in the vineyard can support populations.

It is possible that TCAH can transmit the virus that causes Grapevine red blotch, a disease that affects grape quality, but Wilson said management of vineyard cover crops could provide cultural control. Adult TCAH can be found in vineyard cover crops in the spring. That is where they mate and lay eggs, and the immature nymphs then complete their development on leguminous ground covers. Nymphs typically mature into the adult stage just around the same time that the vegetation dries down, he said, which then triggers the adults to move up into the grapevine canopies where they potentially can feed and spread the virus if they have previously fed on red blotch-infected vines. By mowing and discing vineyard ground covers before the immature TCAH complete their development, Wilson said that fewer TCAH adults will make it up into vineyard canopies to feed on vines.

“Adult TCAH appear in ground covers in the early spring (March) and can be sampled with sweep nets. As those adult populations decline, it is likely that TCAH are mostly in the egg or nymph stage through April. It is during this time that the elimination of ground covers could have a negative impact on their populations since the nymphs are unable to migrate up to the vine canopy” Wilson said.

Wilson and his team are studying other possible insect vectors of Grapevine red blotch as well. One of these is the sharp-nosed leaf hopper, which appears to reproduce in vineyards and can pick up the virus, studies have shown, but Wilson said there is no definitive proof yet that this insect spreads the virus. Those studies are currently underway.

Grapevine red blotch has been in California vineyards since at least the 1950s, but grapevines were not tested for the disease prior to 2012. That was when growers grew concerned about transmission routes. Grapevine red blotch virus is limited to cultivated and wild grapevines. It is possible that in addition to insect vectors, the virus was also introduced in vineyards through grapevine propagation, Wilson said.

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