Soil chemical analysis is the cornerstone of an effective nutrient management program. Without a reliable soil test, significant miscalculations in fertilizer recommendations can occur, leading to drastic effects on profitability and the environment as a result of under and overfertilization and soil amendment recommendations.
Despite the large number of analytical commercial laboratories serving California agriculture, deciding which laboratory to send a sample to can be a daunting task. Unfortunately, there are no public data reporting the accuracy of the analysis performed by agricultural laboratories, and there isn’t a “true” certification program in the U.S. Although a lab may participate in a proficiency program such as the Agricultural Laboratory Proficiency (ALP) program or the North American Proficiency Testing (NAPT), these programs are not mandatory, nor do they certify lab quality. Therefore, laboratories are chosen based on “word of mouth” and prices, which can vary significantly.
Because of the absence of data, growers, farm managers, consultants, environmentalists and even researchers are left without a reliable means by which to select a testing laboratory. A study was conducted in 2019 to assess the performance of soil testing laboratories.
Accuracy and Precision Assessment
Four reference soil samples from the ALP program were submitted to eight commercial Ag laboratories in the Western U.S. (seven in California and one in Idaho) for typical fertility analyses. In most cases, fertility “package analyses” offered by the laboratories were chosen in order to optimize project funds. The same four reference soil samples were resubmitted two more times totaling three rounds, approximately three months apart each round, in order to assess the precision of each laboratory.
Methods and materials used for the laboratory accuracy assessment (photo courtesy A. Biscaro.)
Standard reference soil samples were selected from the ALP program archives, each previously analyzed by a minimum of 30 credible laboratories, in triplicate for each soil sample (totaling 90+ analyses per reference soil.) The median and median absolute deviation (MAD) of these 90+ analyses per reference soil were used to assess the accuracy and precision of the eight laboratories assessed in this study (method based on ALP consensus statistics.) While the accuracy assessment is focused on contrasting each analysis with the ALP medians, the precision assessment is focused on the variability of the analyses across the three rounds (same reference soils analyzed at different times.) Sample IDs were modified and submitted to each laboratory so they wouldn’t be aware of the objectives of the study. Names of laboratories are not disclosed to follow university policy; laboratories are referred as #1 to #8 for discussion purposes.
Each reference soil was analyzed for NO3, P, extractable K, Na, Ca and Mg, SO4-S, electrical conductivity (ECe), Cl, Ca, Mg, Na and B, pH and five micronutrients. Some labs provided additional analyses in their fertility package, such as soil organic matter, estimated and measured CEC and others, however, these were not used in this study since they were not performed by all laboratories. Nineteen analyses performed on four reference soils by eight laboratories three times equals a total of 1,824 analyses, or 228 per laboratory. While that is a rich dataset, trying to create a performance rank for the laboratories across all analysis types is quite challenging since there are multiple types of soil analyses, extraction methods and units. For that reason, performance standards used by the ALP program were applied to this project in order to assess the accuracy and precision of the analysis performed by the laboratories. Eight analyses were chosen for this assessment: Olsen P, extractable K, Ca and Mg, ECe, pH, sodium adsorption ratio (SAR) and DTPA Zn. For the purpose of accuracy assessment, each of these analyses were attributed a pass or failure score, totaling 96 scores per laboratory (8 analysis types, 4 reference soils and 3 rounds). The precision assessment was based on the relative standard deviation of each analysis across the 3 rounds.
Although all labs presented certain inaccuracy and imprecision, some stood out. Laboratories #2 and #8 were consistently inaccurate and imprecise, while laboratories #1 and #7 were the most accurate and precise. Laboratory #8 in particular presented the poorest performance for both accuracy and precision. Laboratories #3, #4, #5 and #6 presented varying accuracy and precision. These patterns of accuracy and precision are illustrated in the three graphs below.
Laboratory Results
Figure 1 illustrates the results for saturated paste pH for soil C. Listed is the pH median and the Median Absolute Deviation (MAD), with results for each lab and each round. Labs #1 and #7 were the most accurate over all rounds. Labs #2 and #8 had high bias, and lab #2 was imprecise (inconsistent). Due to funding limitations, only two rounds of samples were submitted to lab #7.
Figure 1. Soil pH analysis by the saturated paste method performed by eight commercial laboratories for soil C.
Results for exchangeable potassium analysis by ammonium acetate for soil C (Figure 2) illustrates a common occurrence of accuracy levels observed across most reference soils used in this study. Generally, labs #1 and #7 consistently reported results near the median.
Figure 2. Exchangeable potassium analysis by ammonium acetate extract performed by eight commercial laboratories for soil C (SRS-1604).
Results for Zn extractable by DTPA for soil A (Figure 3) show a general trend of all eight labs reporting higher Zn values relative to the median for this standard reference soil of 0.9 ppm. Labs #1, #4, #5 and #7 generally reported equivalent Zn concentrations for each round. Labs #2, #3, #6 and #8 were inconsistent across the three rounds. Lab #6 in particular reported values that varied by 300% across the three rounds.
Figure 3. Zinc analysis by the DTPA method performed by eight commercial laboratories for soil A (SRS-1809).
Need for Consistency
Besides the accuracy and precision parameters assessed in this study, it seems like consistency is an overall challenge for the lab industry: consistency of methods used for certain analyses, of how methods and units reported, and of the interpretation of the results (e.g. graphs illustrating sufficiency and deficiency ranges). Although it is the responsibility of the client to verify the methods used and request the most pertinent information for their application, many growers and farm managers are not familiar with the intricacies of soil analyses and nutrient management. Hence, providing an electrical conductivity analysis in 1:1 or 1:2 extraction instead of the standard saturated paste extract (ECe) can lead to misleading conclusions and inappropriate management decisions since the literature for most salinity thresholds for crop yields were defined with the saturated paste extract method. Another observation in regard to the analysis type is about the phosphorus extraction method used for soils with different pH, where some labs used the Olsen extraction method for soils with pH below 6.0, and others utilized the Bray P1 method.
Soil testing lab users in California and in the Western U.S. could benefit tremendously from a certification program designed to certify the accuracy and precision of all labs on a regular basis. Please feel free to contact the lead author directly for more detailed information about this study at (805) 645-1465.
The author wishes to acknowledges the following contributors to this article: Robert Miller, ALP Program Director, former Extension Soil Specialist UC Davis; Dirk Holstege, Director, UC Davis Analytical Laboratory (retired); UCCE Advisor Steve Orlof, Siskiyou County (in memorian); Tim Hartz, UCCE Vegetable Crops Specialist, UC-ANR (retired); Ben Faber, UCCE Advisor, Ventura County; Anthony Luna, UCCE Advisor, Ventura County; and Eryn Wingate, Agronomist, Tri-Tech Ag Products.
Control a second-generation autonomous UV-C application platform being evaluated at the Appalachian Fruit Research Station in Kearneysville, W.Va. This research plot was established to study the natural infestation of strawberry plants by pestiferous insects and mites in pesticide- and UV-C treated plots as well as in control plots (no UV-C and pesticides.) The robot has been programmed to capture images for phenotyping of strawberry plants. The person in the foreground is controlling the UAV flying over the plot (photos courtesy F. Takeda.)
This article summarizes the recent work of a multi-disciplinary research team on strawberry disease and arthropod pest management using a robotic Ultraviolet-C (UV-C) irradiation machine. The team’s desire to develop novel approaches for more sustainable strawberry culture (e.g. non-chemical and safer use of ultraviolet light) was based on the following factors: Reliance on fungicides for management of diseases (e.g. development of resistance to more synthetic pesticides, phytotoxicity, environmental issues, application residues, etc.,) make chemical management of Botrytis gray mold of strawberry often challenging and costly, especially when the harvesting period exceeds six months; Pestiferous insects and mites are developing resistance to synthetic pesticides, increasing demands by consumers for pesticide-free berries; Need for automation to reduce labor input in the field; Need for systematic approaches for disease and pest management that are less intrusive to workers in the field, harvesting and myriads of seasonal cultural operations.
Sustainable Approach Through UV-C
The idea of using UV light to control fungal diseases of strawberry fruit came from a discussion between Drs. Janisiewicz and Takeda in 2010 about problems with use of fungicides and needs for alternative control methods. UV light includes electromagnetic radiation with wavelengths between 100 and 400 nanometers (nm), which is shorter than that of visible light but longer than X-rays. UV radiation is present in sunlight, but almost all the UV light produced by the sun is filtered out by the ozone in the upper atmosphere. UV light can also be produced by specialized lights such as mercury-vapor lamps and tanning lamps, LEDs and specific wavelength excimer lamps. UV radiation is subdivided into three general classes based on their wavelengths (i.e. UV-A ranges from 315 to 400 nm, UV-B ranges from 280 to 315 nm, and UV-C ranges from 100 to 280 nm.)
Research with UV-A and UV-B has shown that insect behavior could be altered by modulating UV irradiation levels. Research on UV-B showed that a nightly ~3-hour exposure time was necessary to achieve adequate control of diseases such as powdery mildew on grapes and strawberry. UV-C irradiation is used to kill microorganisms in different situations including sterilization of air in hospitals, water treatments, sterilizing laboratory benches and treatment of meat and poultry products. The FDA has determined that UV light can be used to surface sterilize food products, and considers UV light to be safe when precautions are taken by the user.
The use of UV light in crop production, however, has been limited because the doses required to kill plant pathogens usually can cause damage such as leaf burn and defoliation. The goal of the team’s research program has been to develop UV-C treatment methods that have high efficacy for disease and pest control without damaging plants and are cost effective.
What is known is that UV-C irradiation kills microorganisms and arthropod pests by damaging their DNA. What other agricultural researchers had not considered in their treatment protocol was the fact that microorganisms and mites have a special mechanism for repairing the damaged DNA. This occurs when light is present, e.g. the repair process is “light activated”. Thus, there is a simple question: What happens if the microbes are irradiated with UV-C at night and they are kept in the dark for certain periods of time? The reasoning for this approach was a) That the dark period following UV-C irradiation will prevent the activation of this DNA repair mechanism that needs sunlight to become active and b) Without the ability to repair the damage to DNA, the organism would not be able to replicate and further infect plant, eventually causing the microbes to die. The night application of UV-C light for control of pathogens and pests allowed the team to use much lower doses of UV-C light that was still effective and, more importantly, was below the threshold that causes damaging effects on the strawberry plant.
Robotic Application Methods for Pest and Disease
In the May/June 2018 issue of Progressive Crop Consultant, the team reported on the use of germicidal UV-C irradiation for the control of major fungal diseases of strawberries (e.g. gray mold, anthracnose and powdery mildew.) Currently, the team has prototype autonomous platforms applying UV-C light to in-soil strawberry plants at night. As the research on diseases progressed, the project expanded to include management of insects and mites and development of an autonomous vehicle to apply UV-C treatments. The team is aware that research on using UV-B and UV-C irradiation to control diseases and arthropod pests on various crops had been conducted for more than two decades in various laboratories around the world, including several in the U.S., the Netherlands, Belgium, Brazil, Canada, China, Japan, Norway and the United Kingdom. Their research consistently reported plant damage described as burns, leaf curls and defoliation after irradiating the plants with doses required to control diseases and arthropod pests. Recently, some of these laboratories have transitioned to using UV-C lamps in their research.
The effect of treating strawberry plants with UV-C light on subsequent infection by anthracnose fungus (Colletotrichum gloeosporioides 162). Two agar plugs with fungal mycelia were placed on each leaflet. The top row has three detached leaflets from UV-treated plants. Note that very narrow band around each plug with fungal inoculum has turned yellow after nine days. The bottom row shows leaflets taken from control plants (not exposed to UV-C irradiation.)
Our research approach has focused on new ways of using UV-C light to kill strawberry pathogens and arthropod pests (insects and mites) without damaging the strawberry plant. Initial studies clearly demonstrated that a dark period of 2 to 4 hours (depending on which pathogen was being controlled) immediately after a UV-C irradiation of one minute or less prevented microbes from repairing DNA damage (e.g. repair mechanism requires daylight) caused by UV-C irradiation and prevented the disease to develop on fruit and leaves. This method of UV-C irradiation at night resulted in lowering the effective doses needed to kill the microbes that cause strawberry gray mold, powdery mildew and anthracnose. The reduced doses required to kill plant pathogens caused no apparent damage to strawberry plants. Further analyses revealed there was no damage to chlorophyll, no reduction in photosynthetic activity, no loss in pollen viability (ability of pollen tube to grow through the style) and development of fruit, and no reduction in fruit phenolic content as determined by metabolomic analysis.
Another aspect of the team’s research program is to study other direct effects of UV-C light on plants. Since research was being conducted on diseases, the team asked, “Can UV-C light make the strawberry plant more resistant to fungal infection?”
It is known that when a plant is perturbed by abnormal environmental or biotic factors such as insect feeding, it responds by producing more phenolic and other chemical compounds such as jasmonic acid to make itself resist infection by a pathogen or an attack from insects and mites. For this test, the team harvested leaves from strawberry plants that had been treated with brief (15 to 60 second) nightly applications of UV-C irradiation for about a month and then inoculated each leaflet with two agar plugs with actively growing fungus-causing anthracnose (Colletotrichum). Over a period of 7 to 10 days, the team observed the infection (black and yellow lesion) develop on the leaves.
12A close-up of an autonomous vehicle which has a UV-C light array mounted on its frame operating in a commercial strawberry field. For the purpose of photography, the robot was operated shortly after sunset.
On the leaves taken from UV-C irradiated plants, the dark area was confined to the agar plug, while on the leaflets taken from untreated plants, the lesion radiated out from the plugs and caused large areas of the leaflet to turn black and yellow. The team’s research has shown that multiple levels of biological effects occur on disease-causing pathogens as well as make plants exposed to night-time UV-C light treatments more resistant to fungal infection.
The team also showed that night-time UV-C treatments were effective against insects and mites. A detailed study with plants artificially infested with ~100 two-spotted spider mites (TSSM) revealed that nightly 60-second UV-C irradiation of plants for six weeks had reduced mite populations to below a commercial treatment threshold of five mites per mid-canopy leaflet, while mite population on untreated strawberry plants exploded to over 300 mites per leaf. Tomato plants artificially infested with greenhouse whitefly (GWF) were irradiated for just 16 seconds nightly. The results showed a decline in adult, nymph and egg populations on UV-C treated plants within two weeks.
The team also conducted studies on spotted wing drosophila (SWD). In that study, store-bought strawberries were artificially infested with adult female SWD and then were subjected to a pulsed UV-C light treatment. Over time, 12 SWD on average emerged from each untreated control fruits compared to only 1 SWD among 40 UV-C treated fruits. A better way to use UV-C irradiation is to prevent fungal pathogens from infecting susceptible plants, pestiferous insects and mites from infesting the crops, and prevent egg laying in the fruit. The team has plans to refine the UV-C light regimen, quality and application technology to make the UV-C technology more practical for commercial berry producers. Currently, the team is exploring ways to use UV-C irradiation more effectively and to further reduce the time required to treat each plant so that the robot can travel over a larger area each day.
Things to Consider
Substantial costs can be incurred through application of synthetic pesticides, scouting or monitoring, especially if the annual crop is being harvested over six months. UV-C irradiation provides the opportunity to reduce chemical and labor inputs for management of T. urticae, gray mold, anthracnose, powdery mildew and pestiferous insects in the production of specialty crops. In the team’s research, the engineering aspects of the robot technology and optimizing the UV-C system to make the technology economically competitive with conventional chemical control approaches are being refined.
Further field-based validation is needed to confirm the efficacy of the system on a broader scale to compare with conventional management practices (e.g. pesticide application). Prototype autonomous mobile UV-C irradiation units that could traverse multiple strawberry rows have been developed and evaluated under field conditions. To date, the team has used strawberry plants and their fruit as a model system, but this UV-C technology may be widely applicable to other specialty crops.
Future Directions
Currently, the team is evaluating ways for reducing the duration and frequency of exposure, comparison of daytime and night-time applications and addressing the need for deeper penetration of UV-C light into the plant canopy. Mites prefer to colonize and
lay eggs on the undersides of leaves and in the lower portions of canopies. Therefore, UV-C penetration is critical. The team’s study has shown that UV-C ir radiation reaching the lowest portions of the strawberry plants was reduced by ∼45% compared to that received by the upper surface of leaves at the upper canopy. This reduction in intensity may have played a role in supporting the survivorship of mites recorded in the lower canopy. Japanese researchers have reported that reflective mulches enhance UV light penetration to the undersides of the leaves. Also, arthropod management with UV-C light should be examined in conjunction with our technology to increase management efficacy and bring pest populations below the economic threshold levels.
The positive results obtained from night-time application and relatively low doses of UV-C irradiation for controlling fungal pathogens and arthropod pests to date indicate the potential of UV-C treatments to reduce disease incidence and alter insect behavior impacting their herbivory and egg laying. The team will continue to explore the efficacy of the UV-C irradiation system for management of pestiferous mites and insects on other specialty crop plants. To make UV-C technology commercially useful for open-field strawberry growing systems and nurseries, a prototype robot for applying UV-C was developed and field-tested in 2019 and 2020. USDA is now collaborating with TRIC Robotics (tricrobotics.com), which has highly skilled robot engineers, manufacturing capacity, and provides on-the-go solutions for autonomous UV-C application for specialty crops. The robotic platform is agricultural equipment built to control pests and diseases. As such, UV-C application equipment will be certified by EPA as a pesticide device. The videos of their robot applying UV-C treatments in the field during night time and day time can be accessed on YouTube by searching the key words “TRIC Autonomous UV-C Treatment.”
Now, it is not so much a question of whether UV-C light treatments can be applied effectively to field-grown specialty crops, but how soon application equipment and UV-C technology will be available to large and small growers. The team is currently conducting R&D work to develop a commercially viable UV-C delivery system that is affordable, user friendly, generally compatible with current production practices, as effective in controlling diseases and pests as currently used pesticides and reduces the overall costs of disease and pest control practices.
It is important to develop a platform that is useful and affordable to small-acre strawberry growers. The Northeast has over 200 certified organic farms with five acres or less of strawberries. To fund this aspect of the UV-C research project, the team received a grant in 2020 from the USDA Northeast Sustainable Agriculture Research and Education (SARE) program to improve control of diseases and arthropod pests of strawberry. Also, the robotic UV-C application system is being developed for large-scale strawberry growers and nurseries in California and elsewhere. Cooperating growers in California have been identified to conduct large pilot studies that will begin in early 2021.
Summary
The management of disease-causing fungal pathogens and arthropod pests is a complex task for specialty crop producers, particularly given the occurrence of latent infection by Botrytis cinerea and the highly perishable nature of fruits and berries, and the need for a complete control of SWD in berry fields. This requires comprehensive knowledge of disease etiology, host-plant biology and the physical environment determining the severity of infection and pest infestation.
Failure to manage Botrytis cinerea at bloom will have an impact on subsequent stages of fruit development, especially when the fruit reaches harvest maturity. Future research will certainly focus on multi-pronged strategies instead of searching for a single “silver bullet” answer to produce effective, long-term solutions for specialty crop growers. These solutions will surely include genetic improvements, the integration of night-time UV-C applications for a range of cropping systems along with use of “clean” nursery plants, good sanitation practices, new chemicals, and knowing how environmental conditions affect the biology of pathogens and pests to reduce disease pressures and pest infestation.
Additional information on UV-C light technology can be requested from F. Takeda (fumi.takeda@usda.gov). Further discussion on the UV-C technology can be found in the additional resources section.
Additional Resources
Janisiewicz, W. J., Takeda, F., Glenn, D. M., Camp, M. J., and Jurick, W. M. II. 2016. Dark period following UV-C treatment enhances killing of Botrytis cinerea conidia and controls gray mold of strawberries. Phytopathology 106: 386-394. https://doi.org/10.1094/PHYTO-09-15-0240-R
Janisiewicz, W. J., Takeda, F., Jurick II, W. M., Nichols, B. and Glenn, S.M. 2016. Use of low-dose UV-C irradiation to control powdery mildew caused by Podosphaera aphanis on strawberry plants. Can. J. Plant Pathol. 38:430-439. https://www.tandfonline.com/doi/full/10.1080/07060661.2016.1263807
Takeda, F., Janisiewicz, W.J., Smith, B.J., Nichols, B. 2019. A new approach for strawberry disease control. Eur. J. Horticult. Sci. 84:3-13. https://www.pubhort.org/ejhs/84/1/1/index.htm
Short, B., W. Janisiewicz, F. Takeda, and L. Leskey. 20/18. UV-C irradiation as a management tool for Tetranychus urticae on strawberries. Pest Management Science 74(11):2419-2423. https://doi.org/10.1002/ps.5045
Sun, J., W. J. Janisiewicz, F. Takeda, B. Evans, W.M. Jurick, M. Zhang, L. Yue, and P. Chen. 2020. Effect of nighttime UV-C irradiation of strawberry plants on phenolic content of fruit: Targeted and non-targeted metabolomic analysis. J. Berry Res. 10(3):365-380. https://www.doi.org/10.3233/JBR-190482
Figure 5. Fusarium wilt commonly travels from field to field on farming equipment, particularly harvesters (A), which retain large amounts of plant debris (B) (photos by C. Swett.)
Fusarium oxysporum f. sp. lycopersici (Fol) race 3 causes Fusarium wilt, a disease currently affecting most tomato-producing counties in California. Fol is divided into groups called races based on the ability to overcome plant resistances. Fol race 3 is the most recent race, which broke resistance to race 2. Fol race 3 was long restricted to the Sutter Basin, but began spreading in the early 2000s and is now present in every county with large-scale tomato production, making this one of the greatest economic threats to the industry.
Growers are seeking solutions for this damaging soil-borne disease. In this article, an issue overview as well as the latest information on Fusarium wilt race 3 spread, control and prevention is provided. The focus is on current research that is shining a light on new prospects for management of Fusarium wilt in tomato.
Key Characteristics
Bright yellow foliage on one or several shoots on an otherwise normal plant are the earliest symptoms, starting as early as 45 days after planting but typically occurring at about 60 days. The one-sided yellowing of a branch or whole plant can help distinguish this disease from other wilt pathogens (e.g. Verticillium) and other causes of chlorotic conditions (e.g. nutrient disorders) (Figure 1A). From time of initial symptoms to harvest, disease symptoms progress from shoot yellowing to branch death which leads to partial or entire canopy collapse (Figure 1B). Fruit in this exposed canopy develop sunburn and may rot. Another important diagnostic feature of Fusarium wilt of tomato is the presence of chocolate-brown vascular discoloration in the plant stem (Figure 1C). Vascular discoloration is also a symptom of Verticillium wilt, which can lead to misdiagnosis. At advanced stages of Fusarium wilt, the general canopy collapse is similar to other tomato diseases such as southern blight, bacterial canker and Fusarium crown rots. Because of the potential for misdiagnosis of Fusarium wilt, even by experienced scouters, it is prudent to submit plant samples to a diagnostics laboratory prior to making management decisions.
Figure 1. Symptoms of Fusarium wilt in tomato plants, shown here as a shoot with bright yellow and dying foliage (also known as “yellow flagging”) on an otherwise healthy plant (A), collapse of the vine (B) and chocolate-brown discoloration inside a stem (C) (photos by K. Paugh.)
Survival and Spread
Fusarium wilt race 3 occurs across the Central Valley from the Sacramento Valley region (Colusa, Sutter, Solano, Yolo and Sacramento) to the central San Joaquin Valley (San Joaquin, Stanislaus, Merced, Fresno and Kings) and, most recently, to the southern end of the San Joaquin Valley (Kern) (Figure 2). The pathogen is thought to move locally from field to field by hitching a ride in the soil and plant debris that cling to farm equipment. Hence, the increased movement of farm equipment across processing tomato regions may have facilitated spread of this disease.
Figure 2. California counties with documented cases of Fusarium wilt race 3, as highlighted in red.
Once present in the field, Fol can persist in dead tissue in the soil. Ongoing studies suggest that this pathogen can persist for at least two years in infested tomato tissue after incorporation. In addition, although Fol can only cause disease in tomatoes, it can infect many different non-tomato crops, including melons, pepper and sunflower, without causing any symptoms and survive in non-tomato crop residue in soil. Thus, Fol can feasibly be introduced into a field that has never had tomato, propagate on these silently-infected crops and cause severe losses in the first year the field is planted to tomato. Of note, there are also many Fusarium wilt diseases of rotation crops (e.g. Fusarium wilt of melon, watermelon and lettuce,) but these Fusarium wilts are all caused by completely different pathogens. Therefore, if you have Fusarium wilt of melon in your field, this does not mean you will get Fusarium wilt in your tomatoes.
Management
Overview of IPM for Fusarium wilt
The most effective tool for Fusarium wilt management is preventing pathogen introduction. If introduced into a field, the disease can usually be successfully managed with resistant cultivars (F3 cultivars), although there are some caveats in F3 efficacy. If F3 cultivars are not available for management, pathogen-tolerant cultivars and early-season chemical management options are also available. Crop rotation can help reduce pathogen pressure and reduce risk that an F3 resistance-breaking race will emerge (race 4).
Effective management of Fusarium wilt requires accurate diagnosis. As noted above, there are many diseases that look like Fusarium wilt, and at present there is no way to differentiate these diseases in the field. Before developing a Fusarium wilt management plan, it is critical to submit samples for analysis by a diagnostic lab. Ideally, growers could have their soils tested for Fusarium wilt prior to planting. Although there are no commercial soil testing services available, there are preliminary soil testing tools under development at UC Davis (contact C. Swett for more information.)
Management with genetic resistance
Fol race 3 resistant cultivars, called F3 cultivars, typically develop no disease and are an excellent management tool. The tomato industry has worked hard to overcome challenges in F3 cultivar quality, yield and seed availability. In addition, certain F2 cultivars are “tolerant” of Fusarium wilt race 3 in that their yield does not appear to be significantly impacted in infested fields. Fusarium wilt tolerance is not a listed trait for existing commercial cultivars, but this information is available through seed dealers.
In some cases, F3 cultivars develop Fusarium wilt due to Fol race 3. This is typically attributed to either the presence of off-types (when incidence is below 2%) or environmental stresses (when incidence is higher.) Abiotic and biotic stresses appear to play a role in influencing stability of resistance, and recent studies have demonstrated that salt stress can compromise F3 resistance, leading to Fusarium wilt development in up to 30% of F3 plants in a field. While the role of various stresses in mediating F3 resistance is not well-characterized, research in this area is ongoing. Management of these stresses can help maintain the efficacy of host resistance against Fol race 3.
Chemical control pre- and post-planting
Fusarium wilt race 3 is notoriously difficult to control once established in soil. Although host resistance is the gold standard for management, F3 cultivars are not always available. Chemical management may function as a short-term alternative. Recent studies have shown promising results for pre-plant fumigation with K-Pam HL (AMVAC Corporation) at 30 gal/A or higher (maximum rate of 60 gal/A) for optimal efficacy (Figure 3, see page 6).
Figure 3. Results for a 2019 small plot trial at the UC Davis Plant Pathology research farm on the efficacy of the drip-applied fumigant, K-Pam HL, against Fusarium wilt of tomato.
Crop rotation
Crop rotation is a common recommendation for management of host-specific pathogens like Fusarium wilt because non-host crops suppress pathogen propagation and survival. However, the efficacy of this method relies on the inability of the pathogen to infect rotation crops. It has been discovered that Fol race 3 can infect many crops without causing symptoms, which may reduce the effectiveness of crop rotation. Several rotation crops were found to be poor hosts and were suppressive to pathogen build up in soils; these include cotton, bean crops (i.e. garbanzo, fava, lima and green bean), grass crops, including wheat and potentially corn and rice (poor hosts, not field tested), and onion (Figure 4, see page 6). These appear to be good crops to grow right after tomato and the year before planting to tomato. Pathogen-enhancing crops should be avoided when possible; these include pepper, melons, pumpkins, and sunflower.
Figure 4. Preliminary results for the effect of rotation crops on development of Fusarium wilt (FW) in tomato. Plots at the UC Davis Plant Pathology research farm were previously planted to a summer or winter rotation crop (or tomato) or left in chemical or unmanaged fallow (=weeds) in summer 2019 (A) or winter 2019-2020 (B), respectively
No free rides for pathogens
The most effective tool for Fusarium wilt management is preventing pathogen introduction. On farms where Fusarium wilt is not present, this is best achieved by only using equipment that remains within the farm. However, as this is not an option for most producers, a second option is to use a sanitation regime for shared equipment. There is limited information on which equipment is the most important to target for sanitation, but Fusarium has been found at high levels on harvesters which retain large amounts of plant debris (Figure 5). An assessment of critical control points on harvesters indicates that areas which only have contact with fruit have lower levels of microbes, whereas the chipper and other areas which come into contact with whole plant material have higher levels. Robust analysis of effective sanitation methods is lacking, but preliminary data indicates some efficacy of current practices such as physical removal of contaminants using scrapers, combined with chemical and steam treatment.
Further management options understudy
Compost amendments are commonly used for soil fertility management, and recent studies at UC Davis suggest that composts may also suppress Fusarium wilt in soil. Pathogen-infested tomato residue decomposes more rapidly in soil with long-term inputs of poultry manure compost. Preliminary studies also indicate that cover crops such as hairy vetch may be suppressive to Fusarium wilt, as has been seen for Fusarium wilt of watermelon, but results are inconsistent.
Where to Go Next
There are no documented cases of Fol race 4 in California. However, given the history of this pathogen, it is almost inevitable that a new race (race 4) will emerge that overcomes race 3 resistance. As a result, race 4 monitoring continues to be a top priority across the state.
F3 resistance can be compromised by stress, and Fol race 3 has been documented causing disease in multiple F3 tomato fields every year. Knowing which types of plant stress affect host resistance would help growers prioritize management strategies. Long-term studies of Fol race 3 survival in soil are important for knowing optimal durations for rotating out of tomato.
There are several common rotation crops whose risk status is still unknown, including safflower, alfalfa, potato and hemp. Furthermore, there is clearly a need for soil testing tools for Fol race 3. The UC Davis Vegetable Pathology program is working to develop more rapid molecular tools for both soil detection and diagnosis in plants. Industry innovations in effective sanitation, such as full room steam or fumigation sanitation stations, could provide a breakthrough in slowing spread of Fusarium wilt as well as other soil-borne pathogens.
While the aforementioned management approaches have primarily focused on Fusarium wilt alone, in reality, Fusarium wilt also occurs as part of disease complexes. Developing strategies that minimize damage wrought by these soil-borne complexes is a critical next step in achieving healthy tomato production in California.
Studies could demonstrate benefits of compost for alfalfa as well as the environment (photo by Marni Katz.)
Joint research by UC Davis and UC Cooperative Extension will look at the impact compost applications on young alfalfa fields can contribute to soil health. Sites will include first year alfalfa fields in San Joaquin and Yolo counties.
Michelle Leinfelder-Miles, Delta crops resource management advisor and Rachel Long, field crop integrated pest management advisor in Yolo, Solano and Sacramento counties, along with UC Davis researchers Kate Scow and Radomir Schmidt were awarded funding for this project from CDFA’s Healthy Soils Program. The project will demonstrate compost application to alfalfa fields to improve soil structure and fertility and use practices designed to reduce greenhouse gas emissions and store carbon.
Although many dairies do spread manure on their alfalfa fields, compost is not typically applied during the course of an alfalfa field’s productive life. Leinfelder-Miles said alfalfa production practices along with heavy clay soils could impair soil physical conditions.
“Physical characteristics of soil can be degraded over time due to high-traffic alfalfa production practices that contribute to soil compaction and poor water infiltration,” Leinfelder-Miles said. Soil health characteristics include fertility, water holding capacity, long term productivity and sustained organic matter content.
This project began with soil sampling at both sites to determine the current status of soil conditions. Compost applications were due to commence this month as the fields were entering winter dormancy. In collaboration with Westside Spreading LLC, the compost applications will be made in the fields. Two treatment rates and a control will be done in each field. Compost will be applied at 3 tons per acre and 6 tons per acre and soil changes will be monitored and compared with the control plots. The compost to be applied is derived from green waste.
According to UCCE’s Alfalfa and Forage News, alfalfa has the ability to take up large amounts of nitrogen and phosphorus from the soil, two components of concern with organic wastes. With more than half million acres of alfalfa grown in California, proving use of green waste compost to improve soil health could open an organic waste management stream and divert green waste from landfills.
Compost will be applied annually in the fall (2020, 2021, 2022), Leinfelder-Miles said. Soil will be sampled ahead of compost application and then a final soil sample in 2023. They will conduct monthly greenhouse gas monitoring. Alfalfa yields over the course of the study will be monitored.
This model shows the robotic pressure chamber under development by UC Merced and UC Riverside researchers (photo courtesy photo courtesy of UC Merced.)
A robotic pressure chamber that can harvest its own leaf samples and test them on site is being developed by UC Merced Computer Science and Engineering Professor Stefano Carpin, Environmental Engineering professor Joshua Viers and UC Riverside professors Konstantinos Karydis and Amit K. Roy-Chowdhury. These instructors received a $1 million grant from the USDA through the National Science Foundation’s National Robotics Initiative.
Having accurate field data that is updated frequently can help growers plan irrigation frequency and conserve water.
‘If we’re going to use precision agriculture, we need the most accurate information gathering systems we can make,” Carpin said.
One current water status measuring technique calls for leaf sample collection and using a pressure chamber. Karydis explained that mix up of samples, different properties of leaves and time elapsed between pulling the samples and testing them can produce inaccurate data that may negate any water savings. In addition, using hand held instruments in the field can be time and labor intensive and require special training to ensure accuracy of the data.
Carpin has worked with researchers at UC Davis and Berkeley to create RAPID the Robot-Assisted Precision Irrigation Delivery system which travels along rows, adjusting irrigation flows according to sensor data that tells the robot precisely what is needed for each plant.
The same base robot as RAPID will be used in this study, but it will be equipped with GPS and a pressure chamber being designed in Riverside and paired with drones that can survey the field and direct the robot to areas of interest.
The project has four phases: development of the chamber, developing machine vision so the robot can ‘see’ the water coming from the leaf stems; coordinating multiple robots in the air and on the ground and evaluation.
Researchers plan to have the first set of automated pressure chamber prototypes fabricated by Spring 2021 and will evaluate their performance and refine designs in controlled settings over Spring and Summer 2021. They expect to have a completed set up by Winter 2022 and begin controlled field testing.
When all of the components have been designed, the designs and code will be open source and all the data collected during the project will be made available to the scientific community.
The project was initiated after Carpin and Viers spoke with growers about the challenges of growing almonds and grapes. Karydis and Roy-Chowdhury also reported similar conversations with citrus and avocado growers in the Riverside area.
“California agriculture presents a challenge in terms of scalability,” Carpin said, “but this is an exciting collaboration because we’ll get to develop a system that will work on different kinds of crops.”
Septoria produces dark, pitted blotches on infected fruit which are much more conspicuous after fruit develop their mature color (photo courtesy USDA.)
New fungicides that provide a new level of efficacy, along with integrated management practices can help California citrus growers deliver high-quality fruit to valuable export markets.
In a Citrus Research Board webinar, UC Riverside plant pathologist Jim Adaskaveg cited pre- and post-harvest diseases that infect citrus fruit when environmental conditions including rainfall favor spread of the fungal pathogens.
“All pre harvest diseases need high rainfall to develop and cause economic damage,” Adaskaveg said. Most post-harvest diseases have their origins in fruit injuries that allow entry of pathogens, he added.
Two citrus fungal diseases that will keep citrus fruit out of the export market are Septoria spot and Phytopthora. Infections of Septoria spot begin with injury to the peel. An ice mark due to a freeze event will leave small irregular pitted, shallow lesion on fruit and an entry point for a pathogen. As the infection advances, the lesions become dark and extend into the albedo- the white part of the peel. The disease does not affect the juice quality, but the appearance of the fruit will limit marketing opportunities. S citri, the pathogen that causes Septoria spot, is a quarantine pest in South Korea, a major market for California citrus. Adaskaveg said the Korean market for citrus is maintained through compliance with quarantine measures, following GAPs and fruit certification in the NAVEK program (a joint UC/citrus industry program).
Phytopthora, is a soilborne disease that can affect tree roots, trunk and the fruit. This complex disease can cause tree decline, root rot, brown rot and yield reduction A pre-harvest infection of brown rot is most likely to be found on fruit growing on the lower one-third of the tree as water splashing from rain or irrigation can spread the pathogen up into the tree.
Orientation of the orchard, tree density and skirting are cultural practices that can help keep infections low pre-harvest. Complementary strategies include pre-harvest fungicide treatments and implementing handling procedures to minimize injury. Planting tolerant rootstocks in areas where Phytopthora infections are common is recommended.
Adaskaveg said new fungicides registered for use in citrus are highly effective in reducing Phytopthora soil populations and root rot as well as improving yields. Two of the new formulations are Orondis and Presidio.
These new fungicide each have a different mode of action and field studies have shown that the active ingredient in these formulations were detected in root, stem and leaf tissues of treated citrus seedlings indicating systemic uptake.
The new fungicides are expected to reduce dependence on fumigation of orchard sites and provide resistance management for sustainable control of Phytopthora.
Cal DPR’s reevaluation of neonicotinoid insecticides as a pollinator risk will likely lead to new regulations around the use of this class of chemistry.
Following an adverse effect report with imidacloprid, California Department of Pesticide Regulation (CDPR) is reevaluating use of neonicotinoid pesticides to develop mitigation measures to protect pollinators.
As a result of the reevaluation, which included issuance of a pollinator risk determination, CDPR determined that additional mitigation measures are needed to protect pollinators from the use of neonicotinoids in agricultural crops and is developing mitigation measures in the form of regulations.
During two webinars, CDPR shared information on their reevaluation process and gathered feedback on the proposed pollinator protection mitigation measures for the use of nitroguanidine-substituted neonicotinoids in agricultural crops. The original comment period has been extended. Comments and feedback on the proposal will now be accepted until the end of day on October 30, 2020. Comments can be made by e-mail to neonics@cdpr.ca.gov or by leaving a voicemail message at 916-445-0003.
CDPR is looking for feedback on extent of mitigation, organization and clarity, ratings and timings, efficacy against pests, impacts for critical uses and alternative approaches.
CDPR proposals include development of regulations to mitigate risks to bees and pesticide residue and honeybee toxicity studies. Other considerations include current pest management practices, critical pest issues, resistance management and level of pollinator exposure. The agency is taking a multi-level approach determining crops that are highly attractive to bees, crops that are moderately attractive to bees and crops that are not attracted to bees.
For more information on neonicotinoid reevaluation or view the current semiannual report summarizing reevaluation status visit the Cal DPR Reevaluation Program page. To provide comments click this DPR email link.
CDPR reports they will continue to meet with stakeholders. Draft regulations are anticipated to be posted by end of the year.
UCCE farm advisor Amber Vinchesi-Vahl said the parasitic weed branched broomrape is likely to establish and spread in California due to the similarity to the species’ native climate (photo courtesy UC Weed Science blog.)
A parasitic weed, branched broomrape, has recently reemerged in California processing tomato fields. The weed uses a modified root called haustorium to fuse into a host plant root and extract nutrients and water.
The UC Weed Science newsletter reported branched broomrape is causing concern among tomato growers as infestations in other tomato growing regions have shown vulnerability of the crop. This weed seems likely to establish and spread in California due to the similarity to the species’ native climate. Limited crop rotation and a wide range of hosts, including carrot, sunflower and safflower, may also contribute to spread of this weed. It can be spread via machinery or irrigation water, and the tiny seed is long lived in the soil, allowing it to persist in the absence of host plants. The major portion of the parasitic weed’s life span is underground, making it inaccessible to cultivation or contact herbicides.
UCCE vegetable crop advisor Amber Vinchesi-Vahl said that there are currently no herbicides registered in California for tomatoes to control branched broomrape.
The short-term goal is to minimize spread of broomrape, Vinchesi-Vahl said. Next steps will be to develop mitigation measures.
There was a severe infestation of branched broomrape in the Sacramento Valley in 1959, and fumigation with methyl bromide was used to kill the soil seedbank. Eradication efforts from 1973 to 1982 involved intensive field surveys and fumigation of infested fields. Reintroduction or recurrence from long dormant seed in the soil and subsequent spread have been speculated as cause of reemergence.
Branched broomrape is classified in California as an ‘A’ pest, an organism of known economic importance, and is subject to enforced action including eradication, quarantine, regulation, containment, rejection or other holding action. At this time, discovery of a branched broomrape infestation in a commercial processing tomato field will result in a hold order and crop destruction without harvest.
Infestations of the Q-listed Egyptian broomrape in California is also causing concern in the processing tomato industry. The Q listing is a temporary ‘A’ classification pending determination of permanent rating by the state.
Counties reporting branched broomrape detections include Colusa, Sacramento, San Benito, Santa Clara, San Joaquin, Ventura and Yolo.
Studies in Israel and Turkey showed that extreme infestation levels of branched broomrape could cause yield losses as high as 70%. Chile has reported 80% crop losses due to branched broomrape infestation in tomato fields.
Spotted lantern fly is an invasive species that has not yet been found alive in California, but several dead lantern flies have been found in aircraft in California (photo by USDA-APHIS.)
The threat posed by many invasive insect species to California agriculture is accelerating, reports UC Riverside’s Mark Hoddle, who believes proactive biological control could be the key to averting disaster.
Non-native invertebrates (e.g. insects, mites, mollusks, etc.) are establishing in California at the rate of nearly 10 per year. Trade and tourism are major drivers in movement of exotic invasive species worldwide, Hoddle said, and live plant imports are a major conduit for pest and pathogen movement into and throughout the U.S. Prior to 1989, Hoddle said, California acquired around six new exotic arthropod species a year. For the time period 1989-2010, that number increased to nearly ten new exotics detected each year.
The group of insect invasive species most likely to invade California and threaten agriculture include sap feeders such as aphids, psyllids, mealybugs and whiteflies.
Should California have been better prepared for the Asian citrus psyllid? Hoddle asked in an Ask The Expert UC webinar. With the knowledge that ACP and the bacterium known to cause Huanglongbing were in Florida, Texas and Mexico, the approach by California was reactive rather than proactive, he said. ACP spread rapidly once in California and large populations resulted. When dealing with a predictable invasion threat, Hoddle proposed a more disruptive approach to managing invasive species, getting ahead of an obvious problem before it happens.
For example, it took nearly 10 years to run a biocontrol program targeting ACP in California. This program didn’t start until the invasion of California by ACP was well underway; it was a reactive approach. During this time, colleagues in Pakistan were contacted for foreign exploration efforts, and time was needed to collect, identify, screen and raise and mass-release host-specific parasitoids that can suppress ACP populations in California, Hoddle said.
A proactive approach would involve identifying and collecting natural enemies, maintaining colonies in quarantine and running host specificity and host range tests, and prepare a report for USDA-APHIS review in advance of the anticipated invasion of the target pest. Detection of the first established non-eradicable pest population would initiate the biocontrol program against the target pest. This proactive response would save years of time and allow a rapid, almost immediate, response to managing the pest with biocontrol agents.
The concept of proactive biocontrol is simple: Have natural enemies ready before the anticipated pest invasion occurs. Had the parasitoid for ACP been identified and ready, much economic and environmental impact could have been avoided. For example, there has been about a 70% decline in ACP numbers after the biocontrol program started. Could this suppression have been achieved earlier and the spread of ACP slowed if the ACP biocontrol program had been proactive instead of reactive?
Spotted lantern fly is a good target for proactive biocontrol, Hoddle said. This prolific and highly polyphagous insect is expected to arrive and establish in California and poses a threat to vineyard and tree nut crops. Spotted lantern fly infestations in Pennsylvania have killed vineyards, removing sap and leaving the vines vulnerable to winter cold. In South Korea, where spotted lantern fly has become established, it is a known pest in walnuts.
Hoddle recommends being proactive about identifying potential invasive species and developing a management plan before they arrive. A proactive program targeting spotted lantern fly is currently underway in quarantine at UC Riverside where the safety of an egg parasitoid imported from China (the native range of SLF) by the USDA is currently being assessed.
Cultural practices can be key to long-term orchard health. Disease tolerant rootstocks, adequate soil drainage and planting direction make a difference in orchard health and performance (photo by Franz Niederholzer.)
While the concept of integrated pest management (IPM) is not new, current economic considerations in almond production make these practices even more important.
UCCE Orchard Systems Advisor Franz Niederholzer said the goal of IPM is long-term pest management with economic sustainability.
“It’s a buyer’s market now, and buyers are increasingly asking about sustainable production practices.” Niederholzer said. “IPM is a key part of sustainable management.
“This is why it is even more important to re-consider and review IPM practices. The goal is to end the season in the ‘black’, with the best chance of staying there for years to come.”
IPM is a pest management strategy that first uses cultural and biological control practices followed, where needed, by careful, measured, strategic response to known pest pressure. The cultural & biological control practices work in the background, providing a safety net if active treatment does not deliver the desired pest control.
Planting decisions are key to long-term orchard health. Disease tolerant rootstocks, adequate soil drainage and planting direction make a difference in orchard health and performance.
Key practices for a strong IPM program during a growing season include: 1) Monitoring pest populations regularly, basing timely spray treatments on economic thresholds (where known) and 2) Careful spraying with materials selected to control the specific pest(s) and limit harm to non-target organisms.
Targeting diseases, Niederholzer said Sacramento Valley almond growers have less flexibility in fungicide programs due to higher disease risk under wetter conditions common in the northernmost almond growing region of the state. The best disease control is usually achieved with fungicides applied before rainfall. Focus on effective disease control early in the growing season when good spray coverage is easier to achieve and before symptoms appear. Information developed by UC plant pathologists (Adaskaveg, Gubler, Michailides) listing fungicides (conventional and organic,) their efficacy and key use timings is available at UC IPM.
Insect control with IPM can be challenging, especially for pests with limited biological control options including navel orangeworm (NOW) and leaffooted bugs. Cultural control practices including orchard sanitation and harvest timing are key to NOW control in a solid IPM program.
“You can’t spray your way out of long-term problems. Cultural and biological control practices in an IPM program are critical to effective pest control and delivering a product the market wants,” Niederholzer said.
