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The Botrytis Gray Mold Fungus: Pervasive Pathogen, Formidable Foe

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Introduction and Significance

When evaluating the impact and importance of a plant pathogen, one could consult various metrics to make such an assessment. One could consider the values of the affected crops, the acreages planted, the geographic distribution (how widespread is it?) of the pathogen, the mode of pathogen attack (does it affect leaves, flowers, both?), the persistence and staying-power of the organism, and the difficulty in controlling the pathogen. The Botrytis fungus, causal agent of gray mold and other related diseases, is one of the few plant pathogens that could arguably be placed at or near the top of a key pathogen list based on all of these criteria. Field professionals likely are familiar with the challenges of gray mold for the crops they oversee. However, what may be overlooked is the impact of Botrytis throughout a broad spectrum of many agricultural commodities and settings. Botrytis is an unusually dangerous threat due to its ability to infect dozens of crops, uncommon versatility as a microorganism, and propensity to change genetically in adapting to fungicide chemistry.

Table 1. Examples of Vegetative, Flower, and Fruit diseases caused by Botrytis on diverse crops.
*In contrast to most other crops diseases on onion are not primarily caused by Botrytis cinerea but by other Botrytis species.

Broad Spectrum Impact on Crops Worldwide

Botrytis is a highly ranked plant pathogen due to its broad host range that includes hundreds of plants. Such hosts are in almost all commodity groups: annuals and perennials, herbaceous and woody plants, food and ornamental crops, vegetable and fruit and field crops. Included within this diverse list are dozens of high value vegetable, fruit, and ornamental commodities (examples are listed in Table 1 and seen in Photos 1 to 4), for which gray mold can inflict sizeable economic losses. This broad spectrum of activity can also be described based on the type of plant tissue affected. Depending on the crop host, Botrytis can infect the vegetative portions (stems, petioles, leaves), flowering parts (buds, sepals, petals, reproductive tissues), and fruit (immature and ripe phases).

The Botrytis impact on all these crops and commodity groups is further compounded by two factors. First, the gray mold fungus can cause significant disease both before and after harvest. Pre-harvest disease occurs when Botrytis is active on developing plants in the field and greenhouse, resulting in blight, decay, and rot that reduces crop quality and harvestability. In addition, for many crops the Botrytis fungus can be a contaminant on or even inside the harvested plant commodity. Once these contaminated commodities are stored, the Botrytis fungus can become activated under certain postharvest conditions and cause rots and decay in storage (Photo 5). Secondly, we note that Botrytis is found throughout most agricultural and horticultural production regions in the world. In the USA, there are only a few states where an official report of Botrytis is lacking. Likewise, Botrytis is found throughout the world and is reported to cause disease on a huge number of crops and plants.

On pea, Botrytis can cause a pre-harvest disease but is perhaps more important as a postharvest problem, as seen by these lesions on stored pea pods.

Versatility as a Microorganism

A notable feature about Botrytis is the organism’s ability to function in different modes or survival strategies. This diversity of biological activity is not commonly seen in plant pathogens and points to the extreme versatility of Botrytis as a microorganism. Pathogen: We are most aware of Botrytis as an aggressive, difficult to control primary pathogen of plants. From an agricultural point of view, this is the most prominent role for Botrytis. Saprophyte: In a different mode, Botrytis does not even need a living plant. As a saprophyte, the gray mold fungus can grow, thrive, and reproduce on senescent, dying, and dead organic plant tissue. Spores that land on decaying matter can readily germinate and colonize such substrates. Secondary invader: If a plant is injured from weather extremes, damaged from mishaps in the field, or has symptoms caused by other pathogens, spores of Botrytis can drop onto such compromised plant tissues and aggressively grow on and overwhelm the injured plant. In such situations Botrytis did not initially cause the problem but is a secondary invader that can make the overall problem worse.

Dormant sleeper: Botrytis can be a sneaky adversary. This pathogen can infect and penetrate plant tissues but remain dormant within the protection of its host. Later, when the conditions change, the weather warms, or the plant tissues mature and grow, the once dormant Botrytis wakes up and begins to colonize the host, resulting in gray mold disease. Such dormant infections are called latent or “quiescent” infections. Opportunist: Botrytis is an opportunist because it can switch from being a harmless saprophyte, growing on dead tissue that growers do not care about, to an aggressive pathogen causing problems. Numerous examples exist of this switching. The Botrytis starts by colonizing dead parts of plants, such as old flower parts; if these Botrytis-laden dead pieces are in contact with healthy tissue, the Botrytis is able to bridge over from the dead to the living, resulting in a primary disease problem (Photo 6).

Gray mold is also versatile in how it survives and is moved around in the agricultural environment. Airborne spores: The gray color of the fungus, as it appears on infected plants, indicates Botrytis is producing millions of spores. These spore masses (Photo 7) are readily spread long distances by winds, splashing water, and physical contact. Sclerotia: Under certain conditions Botrytis can produce a survival structure, the sclerotium, which is a hard, black, oblong to spherical structure that can be up to ½ inch long. Sclerotia can withstand dry, warm, or cold conditions and can survive inside dead crop debris or buried in the soil; under conducive conditions these sclerotia can germinate and produce mycelium that infects the host. Sclerotia can form within the hollow stems of plant hosts and be carried with the plant if these stems are moved to other locations. Sclerotia can become mixed in with seed and become a seedborne contaminant. Embedded mycelium: In another survival mode, the hyphal strands of Botrytis can penetrate and be buried inside the living flowers, buds, or stems of plant hosts. This strategy allows the gray mold fungus to be protected from harsh environmental conditions, giving it an opportunity to wait for more favorable situations.

Botrytis can cause a brown decay on orange fruit by first colonizing dead flower parts that stick to the fruit; here the old blossom has been moved aside to show the developing brown lesion.

Genetic Plasticity and Loss of Fungicide Efficacy

Botrytis is notorious for becoming resistant (insensitive) to fungicides because of its high genetic variability and adaptability, profuse production of spores, and multiple cycles of spore production. Molecular recombination, mixing of genes between strains, and mutations provide the raw genetic material for resistance to develop. When fungicides are applied numerous times to a susceptible crop, the presence of the chemical challenges Botrytis and can result in the selection of individual strains that are no longer affected by that chemistry. Fungicides with single-site modes of action are especially at risk for inducing resistance in Botrytis. Worldwide, resistant isolates of Botrytis have been confirmed for all of the single-site fungicide categories used to manage gray mold (Table 2). Even more alarming are the research findings that show individual isolates of Botrytis can possess resistance to multiple fungicide classes; there are even isolates that are shown to be resistant to seven different fungicides, each of which has a different mode of action.

Table 2. Fungicide classes for use against Botrytis and for which resistance has been reported. *FRAC = Fungicide Resistance Action Committee

A Note About Botrytis Species

This article is addressing the broad topic of gray mold caused by Botrytis, and for most crops the pathogen species is Botrytis cinerea. However, examining the DNA of Botrytis from different crops in different parts of the world, powerful molecular tools and innovative techniques are detecting multiple, diverse genetic signatures which indicate that B. cinerea is not the only gray mold species out there. For example, a series of studies documents that strawberry can be infected by one or more of the following Botrytis species: B. cinerea, B. fragariae, B. caroliniana, B. mali, B. pseudocinerea. Such findings can have implications for the farmer and the field. On grapes in France, for example, the B. pseudocinerea species is more active in the early spring, while B. cinerea is active in both spring and fall. There are indications that a higher percentage of B. pseudocinerea isolates are resistant to some fungicides than B. cinerea isolates. So in the future it might be critical to know exactly which species is causing gray mold, since different species may require slightly different management approaches.

Management of Gray Mold

Controlling gray mold diseases requires the implementation of IPM practices. Fungicides: Judicious and strategic use of fungicides remains the primary means of managing gray mold. Multiple applications usually are needed, throughout the season, using diverse products having different modes of action. Fungicides with single-site modes of action are especially at risk for inducing resistance in Botrytis, so multi-site products should be integrated into spray programs. Sanitation: Sanitation measures, such as the removal of dead leaves and diseased fruit, appear to only slightly decrease gray mold incidence and cannot replace reliance on fungicide programs. Given the logistical difficulty and expense of such measures, these sanitation steps are not practical for most commercial growers but could be a consideration in certain circumstances, such as for greenhouse crop production.

Botrytis produces masses of airborne spores that readily land on host crops.

Modifying the environment: Because Botrytis is dependent on free moisture and high humidity, reducing such factors can help reduce disease severity. The venting out of moist air in a greenhouse is one example of such environmental modifications. Use of drip irrigation is a field equivalent of reducing water on foliage and can reduce gray mold severity. Canopy modification (pruning to remove leaves and laterals and increase air flow and light penetration) can help with bunch rot control in grapes. Resistant or tolerant cultivars: For most crops there are no cultivars that are genetically resistant to Botrytis. However, some cultivars may experience less gray mold for other reasons. For example, some strawberry cultivars suffer less gray mold due to the upright growth habit of leaves and flowers. Use such cultivars if available. Reduce damage: Reducing damage to crops in the field will reduce the opportunities for Botrytis to invade as a secondary decay organism. Proper harvesting and postharvest handling of fruit are critical to reducing fruit injury and lowering the impact from gray mold. Storing fruit at low temperatures is also necessary to retard gray mold and slow down the aging and senescing of fruit.

Kasugamycin for Managing Walnut Blight

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How does kasugamycin-copper or -mancozeb mixtures compare to copper-mancozeb?

Kasugamycin (tradename Kasumin) was registered in 2018 for managing walnut blight and bacterial canker and blast on sweet cherry. The bactericide was already federally registered for fire blight on pome fruit, but in 2018, registration for this disease was also approved in California. Kasugamycin is a unique bactericide because it is not used in animal or human medicine. Environmental monitoring studies have shown that it does not select for human bacterial pathogen resistance with uses in plant agriculture. Furthermore, kasugamycin has its own Fungicide Resistance Action Committee (FRAC) Code 24 or mode of action that is different from other registered plant agricultural bactericides like streptomycin (FRAC Code 25) and oxytetracycline (FRAC Code 41). Kasugamycin meets new toxicology standards for pollinating insects (e.g., honey bees), it has a low animal toxicity with a “Caution” rating and a 12 h re-entry time on the label. As with any cautionary pesticide, mixers and applicators need to have standard personal protective equipment (PPE) when handling the bactericide.

Copper is classified as FRAC Code M1 for the first element historically used for fungal and bacterial disease control. Copper affects many physiological pathways in plant pathogens and is classified as having a multi-site (M) mode of action. Not many bactericides have been developed for managing plant bacterial diseases, and fewer have been registered. Thus, there has been a great dependency on copper. Because of the multi-site classification, many agriculturalists thought that plant pathogens would not develop resistance to copper. Unfortunately, after many years of usage, bacterial pathogens such as the walnut blight pathogen, Xanthomonas arboricola pv. juglandis (Xaj), have developed resistance to copper. This is a direct result of overuse of one active ingredient (i.e., copper) and being limited with the lack of bactericides available to apply modern approaches to resistance management such as rotating between active ingredients with different modes of action and limiting the total number of applications of any one mode of action per season as part of following “RULES” (http://ipm.ucanr.edu/PDF/PMG/fungicideefficacytiming.pdf). Over-usage of any one active ingredient, such as copper, can create other environmental issues including soil contamination, orchard water-runoff, higher concentrations in watersheds, and potential crop and non-crop phytotoxicity especially in perennial crop systems.

After the industry used copper exclusively for approximately 50 years (1930s to 1980s), copper-maneb (e.g., Manex) mixtures were first identified for use on walnut in 1992 and emergency registrations ensued for 22 years before a full registration was obtained for the related compound mancozeb in 2014. The walnut industry and University of California (UC) researchers knew that more alternatives were needed, otherwise someday the pathogen would develop resistance to copper-mancozeb. Because copper resistance had already developed, this selection pressure is maintained and resistance levels are increasing even when mancozeb is used in the mixture, because copper has been the only tank mix option. In effect, resistance management is not being effectively practiced since copper-resistance already exists and the use of mancozeb (M3) is selecting for resistant strains of the bacterial pathogen to the mancozeb mode of action. Having only one treatment (i.e., mancozeb) available to manage a disease not only can limit crop production each season but could economically devastate the entire industry by making harvests sporadic and inconsistent, lowering crop quality, and preventing profitability. Growers and the entire walnut industry consider walnut blight a threat to the industry and their livelihood.

Why do we need kasugamycin for managing walnut blight?

There is a great need to develop other modes of action for managing bacterial diseases including walnut blight that can be integrated into management programs. Kasugamycin was identified, developed, and registered for the purpose of resistance management, reducing over-usage of any one mode of action, and sustaining the walnut industry of California. The aminoglycoside bactericide has a unique mode of action (FRAC Code 24) as stated above and can be used in combination with copper or mancozeb. When kasugamycin is used in combination with mancozeb, resistance management is being practiced since resistance has not been found in Xaj pathogen populations to either mode of action.

Use on Walnuts

Kasugamycin is labeled as Kasumin for managing walnut blight at 64 fl oz/A in a minimum of 100 gal water/A for ground application. The full 64 fl oz per acre labeled rate for kasugamycin should always be used. Adjuvants that are stickers may also be used, whereas spreaders and penetrants should be avoided. Reduced spray volumes may be utilized for small trees provided that the volume of water is sufficient to provide good coverage of treated foliage. Applications should be initiated when conditions favor disease development. This is the same timing as for copper-mancozeb. In orchards with a history of the disease and when high rainfall is forecasted, applications should be initiated at 20-40 percent catkin expansion. Under less favorable conditions for disease (i.e., low rainfall forecasts and minimal dews), applications should start at 20-40 percent pistillate flower expansion (also known as the “prayer stage”). The preharvest interval is 100 days or approximately mid- to late June depending on the walnut cultivar harvest date. The minimal re-application interval is seven days. The current labeled uses of Kasumin allows for two applications or 128 fl oz of product per season with a label change for up to four (256 fl oz) per season planned later this year. Still, only two consecutive applications will be allowed without rotating to other modes of action. Alternate row applications, applications in orchards that are being fertilized with animal waste/manure, or animal grazing in orchards treated with Kasumin are not allowed. The first restriction is to prevent selection of resistant isolates of the target pathogen, Xaj; whereas, the latter two restrictions are to ensure that the selection of non-target, human-pathogen bacteria is prevented.

For walnut blight management, the best way to use the bactericide is in combination with mancozeb or copper. Application management strategies for a four- or five-spray mixture, rotation program include, but are not limited to, the following:

A) Copper/mancozeb—kasugamycin/mancozeb—kasugamycin/copper—copper/mancozeb

B)  Copper/mancozeb—kasugamycin/mancozeb—copper/mancozeb—kasugamycin/copper — copper/mancozeb

How do kasugamycin treatments compare to copper-mancozeb treatments in managing disease?

The research used to develop kasugamycin was based on a 7- to 10-day re-application interval. The reason for this was that Kasumin is locally systemic or translaminar and thus, is less likely to be re-distributed. With new growth increasing the canopy volume weekly in the spring as walnut trees come out of dormancy, multiple and frequent applications are necessary. Kasugamycin-mancozeb mixtures applied in our research trials were often the most effective of all treatments evaluated.

Radial streaks of 16 isolates of Xaj on each plate exposed to different toxicants. Top image: Copper 50 ppm (fixed concentration). Spiral gradient plates with the highest concentration towards the center and lowest concentration at the edge of the plate. Middle image: Kasugamycin (gradient range 0.5 to 64.9 ppm); and Bottom image: Kasugamycin + mancozeb (concentration gradients). Lack of growth towards the center of each plate indicates inhibition. No inhibition for copper at 50 ppm whereas inhibition concentrations averaged 20 and 5 ppm for kasugamycin and the kasugamycin – mancozeb mixture, respectively.

In general, bactericides have a short residual life of a few days to a week or two. In toxicology in-vitro testing, Xaj is only moderately sensitive to kasugamycin with a mid-range to high minimum inhibitory concentration (MIC) value. When kasugamycin is mixed with mancozeb, the MIC of the mixture is approximately 5 parts per million (ppm). Kasugamycin is applied at 64 fl oz per 100 gal or 100 ppm. Thus, the labeled rate of kasugamycin-mancozeb mixtures are approximately 20X of the MIC value for Xaj. Because of the short residual activity and a moderate buffering residue (20X), the rotation of bactericide mixtures containing kasugamycin described above need to be applied in 7- to 10-day intervals.

Kasugamycin and Resistance.

Resistance is a relative term indicating a change in sensitivity to an inhibitory compound. A moderately high MIC for a bactericide does not mean that the pathogen is resistant. We have conducted baseline studies with kasugamycin, kasugamycin-copper, and kasugamycin-mancozeb for Xaj with MIC values of 20, 8.3, 5.3 ppm, respectively. This was done before the bactericide was registered in California to determine any change in sensitivity after registration and commercial usage. To date, resistance has not been found and isolates evaluated are all within the baseline distributions.  Still, with a single site mode of action compound such as kasugamycin, there is a risk for selecting resistant sub-populations of the pathogen especially when resistance management strategies are not employed. This is the reason why we developed the mixture-rotation programs suggested above.

Efficacy of treatments for managing walnut blight. Treatments applied using an air blast sprayer (100 gal/A). The walnut blight pathogen was sensitive to copper. Disease incidence is the number of diseased nuts per 100 nuts evaluated. Four single – tree replications were used for each treatment. Bars followed by the same letter are not significantly different.

Conclusions

The integration of bactericides with different modes of action and application strategies of rotations of mixtures of bactericides with different modes of action with forecasting tools such as XanthoCast (http://www.agtelemetry.com/) should provide the stewardship necessary for having the tools available for managing walnut blight for years to come. The hope with the Kasumin registration is to provide resistance management and prevent or reduce the risk of resistance to copper-mancozeb while new approaches can be developed and integrated to protect both of these compounds. Walnut blight is the most serious disease impacting growers in California and multiple tools like kasugamycin, copper, and mancozeb need to be available to maintain a successful industry.

Biostimulants and Grape Production

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Biostimulants are a broad category of biological products used in crop production to enhance and/or improve conventional nutrition programs. The term “biostimulant” was officially defined in the Agricultural Improvement Act (aka Farm Bill) of 2018 as:
“[Plant biostimulants are] a substance or micro-organism that, when applied to seeds, plants, or the rhizosphere, stimulates natural processes to enhance or benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, or crop quality and yield.”

However, on March 25, 2019, the US Environmental Protection Agency (EPA) released a report titled, “Draft Guidance for Plant Regulator Label Claims, Including Plant Biostimulants” to better understand manufacture label claims for plant growth regulators and biostimulants. In it, the EPA defined biostimulants in much the same way as found in the Farm Bill, except that EPA’s definition refers to improving soil as a possible outcome rather than crop quality or yield.

The EPA is deciding if and how biostimulants should be regulated. If manufacturers make claims that are similar to plant growth regulators, which are subject to regulations found in the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), then they would require registration with the EPA. Some see this as an opportunity to raise the bar on biostimulant products, and reduce outrageous claims not supported by replicated field research. Others are less optimistic about more regulation and the potential for increased costs on useful products or the complete loss of product categories.

Biostimulant Categories

Biostimulants fall into three general categories 1) acids (such as fulvic or humic), 2) microbials (such as beneficial fungi or rhizobium), and 3) extracts or secondary metabolites (such as polyphenols or botanicals). However, there are other types of products, such as nitrogenous compounds or proteins, which don’t fit neatly into the primary categories (Heacox 2018). Acid based products can be applied as foliars, through irrigation systems, or directly to the soil. Depending on application, they have been shown to reduce plant stress, increase root growth and/or improve soil health. Microbial products are primarily fungi or bacteria that help improve nutrient uptake either directly or by improving soil conditions for the plant. Some microbial products may need an incubation period prior to use, which requires planning if large acres will be covered. Extracts can also be applied as foliars or through irrigation systems. They have been found to improve soil conditions for roots or microbes that are able to make elements more available.

Biostimulants vs Fertilizers

It is important to remember that biostimulants are not fertilizers. Inorganic fertilizers are mineral salts that consist of single or multi-nutrient constituents in varying ratios (i.e. calcium ammonium nitrate=CAN17). In contrast, organic fertilizers are plant and/or animal derived products that also have varying ratios of elements. Both types of fertilizers are regulated with a focus on quality and quantity guaranteed by manufactures. Biostimulants are biological products that improve crop growth through a variety of methods (i.e. reduce plant stress, improve nutrient uptake). They may have some low levels of nutrient value, but that is not their primary benefit to crops. Biostimulant activity is not fully understood but it is thought that they act indirectly to improve crop health by increasing soil microbe activity, or through the additions of acids, plant hormones, or metabolites that react with the biological processes.
Biostimulant research is ongoing and has increased substantially since 2010 to help demonstrate their impact and activity on plant growth. Dr. Russell Sharp of Plater Bio, who spoke with AgriBusiness Global, said that in 2010 a combination of new technologies, increasing interest from investors, and lower growth in traditional pesticide and fertilizer sales, led to a greater interest in biostimulants (Pucci 2018). Given the number and diversity of biostimulants, performance claims about what can be achieved when applied to a crop vary widely. Some evidence suggests biostimulants may reduce plant stress by improving soil environmental conditions when there is a water deficit, high disease pressure, non-optimal pH, or salinity levels in the soil that might otherwise reduce plant health or growth. Under these conditions, biostimulants are thought to increase nutrient uptake and yield, and may even improve fruit quality. Some research has found microbial products solubilize essential nutrients to increase their availability to the crop and enhance drought tolerance by stimulating root growth (Calvo et al. 2014). Still, while some work has shown that adding microbes to the soil benefits crops, other research shows less positive results. One study found establishment of arbuscular mycorrhizal fungal inoculants was highly variable at best and did not significantly improve crop growth even when they were present (Hilton 2019). Limited conclusive data suggests growers should view biostimulants as products that enhance the efficiency of fertilizers so that less is required during the season.

Considerable research has focused on biostimulant use in annual crops, but less research exists for permanent crops such as grapes. Biostimulant grape research has mostly been with foliar applications. Foliar applications pose the benefit of entering the plant and potentially reacting more rapidly with the biological processes than if they were applied to the soil. Foliar applied biostimulants that have shown benefits to grapes include chitosan, which improved postharvest grey mold infections equally as well as synthetic fungicide applications (Romanazzi et al. 2006). Chitosan was also shown to protect against downy mildew (Romanazzi et al. 2016), which is a devastating disease that impacts foliage and fruit. Some studies have shown improved anthocyanin concentrations, which are an important component of grape and wine color. Foliar seaweed applications increased levels of anthocyanins and phenolics (Frioni et al. 2018), both important characteristics of wine. Another study showed that methyl jasmonate and yeast increased anthocyanins in Tempranillo grapes and wine when applied foliarly (Portu et al. 2016). Methyl jasmonate is a plant growth regulator and an elicitor, a type of organic biostimulant that can induce the synthesis of phenolic compounds, which then triggers defense reactions (Gutierrez et al. 2019). Methyl jasmonate is one of the most effective elicitors, but its use can be cost prohibitive.

Although biostimulants have been available for some time, and researched since the mid-seventies, more research is needed before conclusions are drawn on perennial crops. The multitude of products manufactured under the biostimulant umbrella, and their unique impacts on the numerous US perennial crops grown in different climates, necessitates multiple years of research to better understand their benefits.

On-Farm Research Trials

Growers interested in biostimulant products are encouraged to test them in their own vineyards. They should work closely with a Certified Crop Advisor (CCA), Pest Control Advisor (PCA) or university extension advisor to identify what plant health problem needs to be solved (i.e. improved nutrient uptake). On-farm trials should be designed so they can be repeated over multiple years and help determine if they improve production and solve the problem of interest. When possible, a trial site that reduces variables that may impact results should be chosen. For example, if improved nutrient assimilation is the goal, a trial site that has a consistent soil type would produce the best results by eliminating soil as a variable. Clay verses sandy soils retain nutrients differently and will impact plant nutrient and water uptake. Select products that claim to solve or improve a problem that has been experienced at a location over several years. Do not attempt to evaluate too many different products at once since it will make trial results more difficult to interpret. A “grower standard” is important to include so comparisons can be made against the experimental biostimulant regime. Collect data on the plant characteristics that you expect to see a change. For example, if the products being tested claim to improve yield or fruit quality, take fruit samples from each test block and compare them. If product claims are to improve plant nutrient absorption, collect leaves and/or petioles and have them analyzed by a commercial analytical lab. However, when collecting samples for data analysis, it’s important to be aware of edge or perimeter effects. Plants near edges of a plot tend to grow differently than plants in the middle of blocks that have competition for water or sunlight, and this can confound results. When possible, implement a replicated on-farm trial so that you have multiple locations to review treatments. If results from a replicated trial are consistent, that is a good indication that the biostimulants are the cause.

Contact a local CCA, PCA or university extension advisor to help design the trial, decide what data needs to be collected and interpret the results so the best information is gathered from an on-farm research trial.

More Information

To learn more about the use of biostimulants you can visit the Biological Products Industry Alliance (BPIA) website: https://www.bpia.org/ BPIA is an organization with membership from manufactures of various biostimulant products. Their focus is “advancing sustainability through biological solutions”, working with regulators to improve product registration and distribution and to educate producers on products and their best use for different crop production systems.

References

Calvo P, Nelson L, Kloepper JW. 2014. Agricultural uses of plant biostimulants. Plant Soil 383(1-2):3-41. https://doi.org/10.1007/s11104-014-2131-8
Frioni T, Sabbatini P, Tombesi S, et al. 2018. Effects of a biostimulant derived from the brown seaweed Ascophyllum nodosum on ripening dynamics and fruit quality of grapevines. Scientia Horticulturae. 232:97-106. https://doi.org/10.1016/j.scienta.2017.12.054
Gutiérrez-Gamboa G, Romanazzi G, Garde-Cerdán T, Pérez-Álvarez EP. 2019. A review of the use of biostimulants in the vineyard for improved grape and wine quality: effects on prevention of grapevine diseases. J Sci Food Agric. 99(3):1001-9. https://doi.org/10.1002/jsfa.9353
Heacox L. 2018. Biostimulants gaining ground. CropLife. https://www.croplife.com/special-reports/biologicals/biostimulants-gaining-ground/
Hilton S. 2019. Are biofertlizers actually effective? Team-Trade. https://blog.teamtrade.cz/are-biofertilizers-actually-effective/
Portu J, López R, Baroja E, et al. 2016. Improvement of grape and wine phenolic content by foliar application to grapevine of three different elicitors: methyl jasmonate, chitosan, and yeast extract. Food Chem. 201:213-221. https://doi.org/10.1016/j.foodchem.2016.01.086
Pucci J. 2018. What’s really behind the biostimulant boom. AgriBusiness Global.
https://www.croplife.com/crop-inputs/micronutrients/whats-really-behind-the-biostimulant-boom/
Romanazzi G, Nigro F, Ippolito A, et al. 2006. Effects of pre and postharvest chitosan treatments to control storage grey mold of table grapes. J. Food Sci. 67: 1862-1867. https://doi.org/10.1111/j.1365-2621.2002.tb08737.x
Romanazzi G, Mancini V, Feliziani E, et al. 2016. Impact of alternative fungicides on grape downy mildew control and vine growth and development. Plant Dis. 100(4):739-748. https://doi.org/10.1094/PDIS-05-15-0564-RE

Management of White Rot of Onions and Garlic and Recent Research

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White rot is caused by the fungus, Sclerotium cepivorum, which survives for decades in the soil as poppy seed-sized resting structures. If soil temperatures are between 50o and 75oF, compounds produced by onions and garlic trigger the fungus to break dormancy. Infection results in a soft rot of the garlic head or onion bulb, which will produce a white fluffy growth and then the poppy seed-like resting structures. Very few resting structures (only two in about a pint of soil) can result in losses in these crops. Thousands of resting structures may be produced on each diseased plant. Therefore, the levels of this pathogen in the soil can increase very rapidly in fields with a susceptible crop, which is limited to onions, garlic and a few relatives. The resting structures are spread within a field with tillage equipment and are moved to other fields with anything that moves soil. It can also be moved into new areas on garlic planting material.

For many years, the primary approach to white rot management was avoidance of infested fields. However, there are now more than 21,000 acres known to be infested with this pathogen and it is in areas where garlic and onions are important crops so it would limit production of these crops to completely avoid infested fields.

Sanitation

Sanitation is an important approach to limit spread. Cleaning equipment between fields will not only reduce risk of movement of the white rot pathogen through a production area but also limit risk of other soilborne diseases. Planting white rot-free garlic planting material is critical in keeping the pathogen out of fields that are not infested.

Disease Management
Several approaches to managing this disease hold promise. Metam applications can reduce soil inoculum levels but soil preparation and moisture conditions are critical in optimizing efficacy. Use of materials that emit compounds like those produced in the roots of onions and garlic to trigger germination of the resting structures in the absence of a host and starve out holds promise. However, additional work is needed to refine this approach for more reliable results than what has been observed experimentally. Research efforts now are focused on quantification and increasing concentrations of active volatile compounds in onion and garlic containing materials, identifying the levels needed to trigger germination and the specifics of effective approaches in applying these materials in the field.
Some fungicides applied in the trench where the planting material is dropped has consistently reduced incidence and severity of white rot. Fungicides applied through drip irrigation systems were not effective. Three years of studies were conducted in which fungicides were applied through drip irrigation systems with tubing either on the surface shallowly buried or buried at six inches and the treated plots were always the same as the untreated control.

Dry straw colored leaves and black appearance of the below-ground tissue of garlic characterizes infections occurring at early stages of crop development.

Commercial Field Evaluation
During the 2016-17 and 2017-18 production seasons, fungicides were evaluated in a commercial field naturally infested with white rot in Fresno County. Fungicides in three conventional cate

gories were tested and in 2017-18, a non-living fermentation product with reported systemic acquired resistance activity was also included (Table 1, see page 13.) On 20 November 2016 and 11 November 2017, California late garlic was hand transplanted following the treatment of a 6-inch band of the trench into which the garlic cloves were dropped. The plots were rated on a scale from 0 to 10 with 0 being symptomless and 10 being collapsed. At maturity, 17 feet of each plot were hand dug and weighed, and per acre production was calculated. Data was subjected to Analysis of Variance and means were separated with Least Significant Difference P=0.05.

Under the conditions of these studies, most treatments had lower levels of above-ground symptoms than the untreated control. In the 2016-17 study, disease was severe and there were also significant differences in yields among treatments (Table 2, see page 14). The treatment with the highest yield was 95 percent higher (more than four tons per acre) than the untreated control. Disease pressure was lower in 2017-18 and there were no yield differences among treatments (Table 3). Tebuzol, Fontelis, A19649 and Rhyme 14 fluid oz/a consistently had lower levels of disease than the untreated control. Cannonball with SP2700 and Velum One demonstrated efficacy in the season that these materials were included in the test.

Fungicides are not intended to be the only approach to management of this disease and are not likely to provide commercially acceptable levels of control as the soil inoculum levels continue to increase. In addition, risk of the pathogen becoming resistant to the fungicides increases with increased and repeated use. Fungicides in three different groups have now consistently demonstrated efficacy and trials are underway evaluating different approaches to management of this production issue.

The research mentioned in this article was supported by the California Garlic and Onion Research Advisory Board and by industry donors.

Tools, Tactics, and Strategies for Managing Postharvest Decay of Apple Fruit

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Introduction: Apple Production, Storage and Rots
Apples have an estimated annual farm gate value of nearly $4 billion dollars in the United States, with downstream revenues exceeding $15 billion (US Apple Association). Apples are stored for extended periods of time (up to six months at 1°C in air, and for one year maximum in controlled atmosphere) to preserve their quality and provide fruit yearlong to meet customer demands. During storage, fungal rots can cause significant amounts of decay resulting in product losses, reduced quality, and lower economic returns for producers. The three most problematic rot causing fungi in the United States are Botrytis cinerea (gray mold), Penicillium spp. (blue mold), and Colletotrichum spp. (bitter rot) (Figures 1A-C). Both gray mold and bitter rot occur in the field and during storage. However, Penicillium expansum and other Penicillium spp., are found exclusively in storage and are also economically important (Xiao and Boal, 2009). The focus of this article will be on the blue mold fungus, but the information contained here is applicable to other fungal rot pathogens as well.

Figure 1: Top three most common postharvest diseases of apple in the United States. A. apples surrounded by a blue mold-infected fruit in a bin from commercial cold storage. The disease is typified by blue-green colored conidia that form on the surface of soft-watery decay that is easily separated from the healthy portion of the fruit. B. Apple with gray mold symptoms typified by light gray colored mycelium and copious amounts of black hardened sclerotia on the surface of the fruit. C. Apple fruit in the field showing typical bitter rot symptoms caused by Colletotrichum spp. that also occur during storage. Note concentric rings of spores and spore-producing structures that are formed as the decayed area develops over time.

Blue Mold Biology
A survey of postharvest diseases in Washington State revealed that blue mold accounted for 28 percent of decay in storage (Kim and Xiao, 2008). Blue mold is characterized by a soft, watery rot that is light brown in color accompanied by the appearance of blue-green colored conidia on the fruit surface that develops at advanced stages of decay. P. expansum and other Penicillium spp. do not directly infect fruits, as they require wounds often caused by stem punctures and severe bruises that occur before, during, and after harvest (Figure 1A). Blue mold spreads by spores that are produced terminally in chains on the surface of whorled conidiophores (Figures 2A and B, see page 6). Stem punctures, during harvest and handling, provide places for rot to occur, but the fungus can also enter natural openings like lenticels, open calyx/sinus, and stem pull areas. Penicillium spp. also produce mycotoxins, such as patulin, citrinin, and penicillic acid, that pose potential human health risks when blue mold infected fruit are used to make juices and other processed products (Figures 3A-C). However, of the three mycotoxins, only patulin is regulated by the Food and Drug Administration (FDA) and European Food Safety Authority (EFSA) with a maximal allowable limit of 50 parts per billion (50 µg/kg-1) and 10 µg/kg-1 for babies and young infant products (European Union (EU), 2006).

Figure 2: Spore producing structures and conidia shown via scanning electron micrographs. A. Scanning electron microscopy (SEM) micrograph of conidia terminally produced in chains on a whorled conidiophore. B. SEM of conidia produced in chains which are typical dispersal units for Penicillium species.

Decay Management

Postharvest fungicides

Long term storage, coupled with lack of host resistance in commercial apple fruit cultivars, provides limited options but to rely on fungicides to manage postharvest decay of apple fruit (Rosenberger, 2012). Application of postharvest fungicides depends on the stage of product handling, and is typically made by bin drenching before storage, sorting line sprays, dips in flume water, together with fruit waxing, or by thermofogging storage rooms (Figures 4A-C, see page 9). There are four postharvest fungicides (Academy, Mertect, Penbotec, Scholar) registered and being used in the United States for apple fruits to manage postharvest decay. Both Scholar (active ingredient, fludioxonil) and Penbotec (active ingredient, pyrimethanil) were labeled for postharvest use in 2004 (Xiao and Boal, 2009). Recent reports indicate reduced efficacy of these materials that have resulted in increased blue and gray mold decay in commercially stored apple fruit in Pennsylvania and Washington State (Amiri et al., 2017, Yan et al., 2014; Gaskins et al., 2015). Thiabendazole (TBZ active ingredient, Mertect®), was labeled for blue mold management in 1968 and is applied primarily as a drench. Consequently, repeated long term use of TBZ has resulted in resistant Penicillium spp. for multiple apple growing regions of the United States (e.g. New York, Pennsylvania, Maryland) and in British Columbia (Rosenberger et al., 1990; Sholberg and Haag, 1996; Jurick II personal observations). A new product (under the name Academy) was introduced in 2016, containing two single site mode of action fungicides, fludioxonil & difenoconazole, to manage postharvest decay on apples.

Sanitation

Research studies have shown that fruit bins harbor fungal spores that serve as a source of inoculum to cause rot. Implementing physical (steam) or chemical (peroxyacetic acid, quaternary amines, etc.) sanitation methods in the packinghouse to reduce inoculum levels, reduces the incidence of fruit decay. While not commonly a stand-alone tool for postharvest rot management, bin sanitation complements existing chemical controls and helps to ensure their efficacy as resistant populations are kept in check (Rosenberger, 2012; Sholberg 2004; Hansen et al., 2010). Most inoculum of Penicillium spp. comes from the orchard soil and leaf litter as this pathogen can survive very effectively as a saprophyte (Lennox et al., 2003). Therefore, bins contaminated with soil and litter introduce this fungus into the packinghouse environment which is the source for the blue mold epidemic (Figure 5A-C, see page 10). Treating bins with steam, cleaning packinghouse walls and floors with quaternary amines or peroxyacetic acids, and maintaining proper chlorine levels in sizing flumes are all important measures that can have a positive impact in managing decay (Lennox et al., 2003). Hence, studies involving quantification of blue mold spores on bin surfaces in the Pacific Northwest region concluded that bin sanitation should be a component in an integrated decay management plan (Sanderson, 2000).

Fungicide resistance monitoring

Monitoring fungicide resistance in rot fungi is critical to maintain the efficacy of single-site mode of action fungicides. This is based on developing both, conventional and molecular-based methods, to detect fungicide-resistant pathogen populations. Routine monitoring can detect shifts in baseline sensitivity of pathogens and prevent postharvest fruit losses due to fungicide resistance. Site-specific or single site mode of action chemistries have been introduced that disrupt a metabolic process, and are more prone to develop resistance. Baseline information is routinely derived from a representative pathogen population before an active ingredient is introduced to market (Russell 2004). This allows researchers to establish a Minimum Inhibitory Concentration (MIC) or discriminatory dose for a specific active ingredient generated from an “unexposed” pathogen population based on mean and the range of EC50 values. The EC50 is defined as the concentration of fungicide that reduces fungal growth by 50 percent compared to growth on non-amended media (Secor and Rivera, 2012).

Research by two collaborating groups in Washington State and Maryland have independently determined a mean EC50 for unexposed, difenoconazole-sensitive Penicillium spp. populations (Ali and Amiri, 2018; Jurick et al., 2018). Mean EC50 values for 130 P. expansum isolates from Washington State was reported to be 0.17 ppm and was 0.16 ppm for 97 Penicillium spp. isolates largely obtained from Maryland and Pennsylvania. A discriminatory dose for monitoring difenoconazole resistance should be 1 ppm or higher and up to 5 ppm to detect truly resistant isolates. Baseline data, and corresponding discriminatory doses for MIC phenotyping, have been vital in monitoring fungicide resistance in Penicillium spp. populations and identifying fungicide-resistant blue and gray mold fungi (Li and Xiao 2008; Yan et al., 2014). Our laboratory has utilized published discriminatory doses for phenotyping fungicide resistant blue and gray mold pathogens with 0.5 ppm fludioxonil, 10.0 ppm Thiabendazole and 1.0 ppm pyrimethanil in agar-based Petri plates amended with technical grade chemicals (Li and Xiao 2008).

Figure 3: Chemical structures of mycotoxins known to be produced by Penicillium species during apple fruit decay. A. patulin, B. citrinin, and C. penicillic acid. Of these three compounds, only patulin is regulated by the FDA and EU. Images courtesy of PubChem.

Recent Scientific Breakthroughs and Their Applications
The first genetic blueprint of the blue mold fungus was accomplished using Next Generation Sequencing Technology in the Penicillium expansum strain R19 (Yu et al., 2014). This isolate was obtained from decayed apple fruit in Pennsylvania commercial storage and shown to be highly aggressive when inoculated onto healthy apples. The P. expansum genome sequence is critical to understanding how this fungus develops resistance to various postharvest fungicides and has provided new clues about various infection strategies used by the fungus to decay apple fruit. Using sophisticated computer software analysis programs, Yu et al. determined that P. expansum R19 has 62 different secondary metabolic gene clusters and toxin biosynthetic pathways including one for patulin production. Hence, the fungus can produce a wide variety of chemicals/toxins/small molecules that may provide new uses in medicine and biotechnology as most have yet to be characterized. By sequencing and comparing different Penicillium spp. strains, genes involved in apple fruit decay, toxin production, and sexual recombination have been discovered (Julca et al., 2016; Wu et al, 2018; Yu et al., 2014). Even though the blue mold fungus has the genetic capacity to undergo sex, a definitive sexual stage for this fungus has not been observed in the laboratory or in nature. The practical impact of this discovery is that the fungus has the potential for genetic recombination, which can allow for movement of genes controlling decay, toxin secretion and/or fungicide resistance between different strains of the blue mold fungus. Hence, recombination could result in more fit strains capable of resisting multiple modes of action chemicals, and or become more aggressive resulting in increased control failures during storage.

Translating fundamental scientific information on the blue mold fungus is important and is envisioned to gain deeper insights into the genetic toolbox used by Penicillium spp. to cause decay in apple fruit. Once elucidated, the fungal tools that it uses can be exploited to develop controls that block decay from developing on apple fruit during storage. This is a similar approach to what is being done in cancer research to discover new drug targets to fight tumorigenesis, block metastatic development, and aid in early detection. Uncovering the genetic mechanisms of fungicide resistance will help tailor specific classes of new chemicals and natural products that provide durable control with lower likelihood for developing resistance. These discoveries will also enable the development of new detection tools that can be used not only by scientists, but producers as well, which would enable more timely detection of fungicide resistant isolates. Utilizing the latest molecular approaches such as CRISPR/Cas9-mediated genome editing, RNA interference, and single gene deletion strategies can be integrated to attack the fungus at multiple points that are critical for it to cause decay. These new tools for decay management, detection of fungicide resistant strains, and next generation chemistries to control postharvest rot will be a welcomed addition to the producers’ management scheme resulting in less decay, maintaining fruit quality over an extended period, and providing safer apple products devoid of mycotoxins. These breakthroughs will have wide impact and benefit our stakeholders and customers in industry, the scientific community, and the public at large.

Figure 4. Various methods for postharvest fungicide application. A. truck drench, B. bin drench, C. line spray wax.

Conclusions & Summary
Postharvest decay management currently follows an integrated pest management strategy that focuses on the pathogen including sensible fungicide application, implementation of sanitation methods in the packinghouse, and periodic fungicide resistance monitoring efforts. Current knowledge concerning utilization of different postharvest chemistries based on FRAC codes (Fungicide Resistance Action Committee) suggests that rotation is critical to maintain product efficacy. However, more research must be conducted to determine the impact of chemistry type on selection of fungicide resistant rot isolates, the frequency of rotation (e.g. yearly vs. throughout the season) and impact of preharvest fungicides to predispose fungi in the field for developing resistance in storage. In the meantime, cold chain management, bin sanitation and proper fungicide use will help keep decay at acceptable levels until new controls and methods are developed, refined, and implemented. For more information on specific postharvest decay control measures, please check out online resources provided by Cornell University, Penn State, and Washington State University Extension programs via the world-wide web.

Figure 5. Wooden commercial apple storage bins with A. copious leaf litter and B. visible fungal mycelium on the bin and fruit surfaces. C. blue mold fungi sporulating on the surface of a wooden storage bin. These photos emphasize the need to sanitize bin surfaces and remove infected fruit and leaves. Figure 4A and 5C courtesy of Dr Wojciech J. Janisiewicz—USDA-ARS Appalachian Fruit Research Station

Acknowledgements
Dr. Wayne M. Jurick II is the research plant pathologist and lead scientist on the project entitled “Development of Novel Tools to Manage Fungal Plant Pathogens that Cause Postharvest Decay of Pome Fruit to Reduce Food Waste” which is funded by United States Department of Agriculture (USDA)-Agricultural Research Service (ARS) National Programs 303 Plant Diseases. Financial support was also made possible through multiple competitive grants awarded to WMJII from the State Horticultural Association of Pennsylvania.

References
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Amiri, A., Mulvaney, K.A., and Pandit, L.K. 2017. First Report of Penicillium expansum Isolates With Low Levels of Resistance to Fludioxonil From Commercial Apple Packinghouses in Washington State. Plant Disease. 101:5.
Gaskins, V.L., Vico, I., Yu, J., and Jurick II, W.M. 2015. First report of Penicillium expansum isolates with reduced sensitivity to fludioxonil from a commercial packinghouse in Pennsylvania. Plant Dis. 99:1182.
Hansen, J., Xiao, C.L., and Kupferman, G. (2010). Bin Sanitation: an effective was to reduce codling moth and fungal decay spores. WSU publication, 1-3
Jurick II, W.M., Macarisin, O., Gaskins, V.L., Janisiewicz, W.J., Peter, K.A., and Cox, K.D. 2018. Baseline Sensitivity of Penicillium spp. to Difenoconazole. Plant Disease. 103:331-337.
Julca, I., Droby, S., Sela, N., Marcet-Houben, M., & Gabaldón, T. (2015). Contrasting genomic diversity in two closely related postharvest pathogens: Penicillium digitatum and Penicillium expansum. Genome Biology and Evolution. 8: 218– 227.
Kim, Y.K., Xiao, C.L. 2008. Distribution and incidence of Sphaeropsis rot in apple in Washington State. Plant Disease. 92:940-946.
Lennox, C.L., Spotts, R.S., and Cervantes, L.A. (2003). Populations of Botrytis cinerea and Penicillium spp. on pear fruit, and in orchards and packinghouses, and their relationship to postharvest decay. Plant Disease 87, 639-644.
Rosenberger, D.A. 1990. Blue mold. In Compendium of Apple and Pear Diseases, A.L. Jones, and H.S. Aldwinkle, eds. (Saint Paul, Minnesota, APS Press), pp. 54-55.
Rosenberger, D.A. 2012. Sanitize apple storage rooms to minimize postharvest decays. Scaffolds Fruit J 21: 4-5.
Russell, P.E. 2004. Sensitivity baselines in fungicide resistance research and management. FRAC monograph. 3:1-54.
Sanderson, P.G. 2000. Management of decay around the world and at home. 16th annual Postharvest Conference, Yakima, Washington. Pages 1-8. http://postharvest .tfrec.wsu.edu/pgDisplay.php?article=PC2000Z.
Secor, G. and Rivera, V. 2012. Fungicide resistance assays for fungal plant pathogens. In: Plant Fungal Pathogens Methods and Protocols (M.D. Bolton and B.P.H.J. Thomma, eds). New York: Springer, pp. 385-392.
Sholberg, P.L., and Haag, P.D. 1996. Incidence of postharvest pathogens of stored apples in British Columbia. Canadian Journal of Plant Pathology. 18: 81-85.
Sholberg, P.L. 2004. Bin and storage room sanitation. Washington Tree Fruit Postharvest Conference.
Wu, G., Jurick II, W.M., Lichtner, F.J., Peng, H., Yin, G., Gaskins, V.L., Yin, y., Hua, S., Peter, K.A., Bennett, J.W. 2018. Whole-genome comparisons of Penicillium spp. reveals secondary metabolic gene clusters and candidate genes associated with fungal aggressiveness during apple fruit decay. PeerJ. 1-17. 7:e6170. doi.org/10.7717/peerj.6170
Xiao, C.L., and Boal, R.J. 2009. Residual activity of fludioxonil and pyrimethanil against Penicillium expansum on apple fruit. Plant Disease 93:1003-1008.
Yan, H., Gaskins, V.L., Vico, I., Luo, Y., and Jurick II, W.M. 2014. First Report of Penicillium expansum isolates resistant to pyrimethanil from stored apple fruit in Pennsylvania. Plant Dis. 98:7.
Yan, H., Gaskins, V.L., Lou, Y., Kim, Y.K., and Jurick II, W.M. First Report of Pyrimethanil Resistance in Botrytis cinerea from Stored Apples in Pennsylvania. Plant Dis. 98:7. 2014.
Yu, J., Jurick II, W.M., Cao, H.Y., Yin, Y., Gaskins, V.L., Losada, L., Zafar, N., Kim, M., Bennett, J.W., and Nierman, W. (2014). Draft genome sequence of Penicillium expansum R19, which causes postharvest decay of apple fruit. Genome Announcements 2, e00635-00614

Angled Shoot Projection (SASP) Trellis Design

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We have a small vineyard consisting of mostly French and a few Spanish varietals planted on deep sand, sandy loam, and river rock clay soils. The deep sand soil creates vines that are balanced in growth and grapes that produce wines with a mineral touch. In contrast, the sandy loam soil creates vines that are overgrown with grapes that are excellent as long as the vine growth is controlled in order to keep the vines balanced. The vines were planted in the river rock clay area a few years ago.

The area where the vineyard is planted is a micro-climate within Region 4 (warm growing area) with fall wine grape ripening season in the 90’s during the day and 50’s at night, excellent for slow and balanced ripening.

Since I take care of all vineyard and cellar requirements, I am always looking for designs and procedures that decrease time, work, and number of steps for completion. Everything is consciously engineered and tested for simplicity, repeatable results, and ease of care.

VSP

When the vines were first planted in 2007, I naturally assumed that Vertical Shoot Projection (VSP) was “the” way to trellis the vines because of its popularity and my ignorance of trellising designs. Because the rows are oriented east-west for esthetic reasons, special considerations were required for sun protection on the south side of the vines.

I discovered that it was very difficult to get grapes of full physiological maturity balanced with the right brix to make premium wines, so I began looking at the trellis design wondering if there was a better way to achieve my goal of premium grapes without the extensive leaf and cane thinning and hedging. As I looked more intently at the VSP design, I decided there was a better way to trellis the grapes for this area; one that enabled easier vine maintenance without multiplying issues, like the ever-prominent powdery mildew.

With the VSP trellis, I had to grow the southside of the vine canes longer to protect the fruit from premature raisining because of the intense sunlight; but that created a perfect environment for powdery mildew because of the “umbrella-like” structure that resulted. Essentially, when the vines were watered by the drip irrigation, the moisture turned to humidity that rose up and hung in the fruiting area encouraging mildew growth while the multiple layers of canes and leaves prevented the mildew sprays from reaching the fruit. I then pushed the vine canes up to get some airflow in the fruiting zone; but there was still a serious humidity problem in the fruiting area.

After a few seasons, I decided to find a different trellising design that would eliminate the problems that VSP created. I analyzed the issues with VSP and made a list to be addressed by a different design.

The VSP trellis design relies on the canes projecting vertically, but in warm growing environments with intense sunlight, there is a need for shading of the fruit to prevent premature raisining; but the number of canes required for protection also served as an effective protection from the mildew spray reaching the fruiting zone, while also preventing the sun from penetrating the multiple layers of leaves to created color in the grapes. This technique of allowing the canes to flop over on the vine is known in this area as “California Sprawl” and it shades the fruit with many layers of leaves, thus preventing adequate air movement to help prevent powdery mildew. Additionally, having canes over 4 feet long, the green matter of the vines was exceeding the green matter-to-fruit ratio for growing premium quality grapes. The ratios for growing premium quality fruit are generally known to be 15 leaves per bunch and six to eight bunches per vine; but that is for vines grown in a cooler environment, which does not provide adequate protection in Region 4. Consequently, I have been working on creating the appropriate ratios for growing wine grapes in Region 4; but the long canes required to protect the fruit was creating a higher level of pyrazines in my fruit and thus flavors of bell-pepper in my Cabernet wines. Essentially, by protecting the fruit from too much sun with the VSP trellis design, there were additional issues of not enough sun to achieve physiological maturity in the grapes, preventing mildew sprays from reaching the grapes for their protection, and off flavors in the Cabernet wines.

Figure 2: Syrah on SASP Trellis. All photos courtesy of Steve Shoemaker.

Spur Pruned

Since VSP trellised vines are spur pruned, it was always a fight between what I wanted the vines to do in terms of growth and what the vine actually did. The issue is that the number of buds left on the spur is inversely related to the number of canes that the spur will produce in the spring, especially on mature vines. I pruned to two-buds and would end up with four to six canes from each spur, requiring extensive spring cane and leaf thinning. I then pruned to four-buds which resulted in three to four canes from each spur; and although better, it was still a real issue to get the fruiting zone cleaned up since it was only me doing all the leaf and cane thinning. Interestingly, I take care of a neighboring vineyard that is trellised on the VSP design; and each year, even though it receives leaf and cane thinning, it loses about 15-20 percent of the fruit from powdery mildew.

In my analysis, I noticed the VSP trellis design puts all the fruit in the same area just above the horizontal cordon where all the canes are protruding from and the dead leaves from senescence land and stay, thus covering the fruit. For some vineyards that have adequate and well trained help, these problems might not be an issue; but for a vineyard that has little to no help, I was cleaning all the time. I noted in that having all the fruit in one area, it created problems of cane and fruit entanglement making it harder to harvest the fruit, a higher incidence of bunch-rot, and the dead leaves laying on top of the fruit in the crux of the canes formed at the cordon assisted with additional formation of mildew. I have also found that the fruit from VSP vines had more bird damage because of the readily available canes for perching and eating the grapes.

Nutrition

Concerning the nutrition of the grapes, there is a general theory that states the closer the fruit is to the soil, the better the nutrient supply to the fruit, thus making better fruit for wine. The VSP design puts the fruit a reasonable distance up the trunk away from the nutrient source, which has the potential for decreasing the fruit quality. I have found that the physiological maturity and brix in the fruit harvested from the VSP trellis design is not as balanced as it could be, for some reason.

For example, the fruit I harvested from the vines on my neighbor’s property is on a sandy red clay mix soil and grown on the VSP trellis design. Although the soil has a huge effect on the maturity and quality of the fruit, this vintage’s fruit is very unbalanced with high pH and low Titratable/Total Acid (TA). I also noticed very little sunlight reached the fruit down in the crotch of the cane/cordon, thus creating an issue of physiological maturity, which might have contributed to the acid imbalance.

In summary, the VSP design, at least in our area, produces lower quality and unbalanced fruit, contributes to increased powdery mildew, prevents mildew sprays from reaching the fruit, allows for more bird damage, requires more time and effort to maintain, and requires the soil nutrients to travel farther to the fruit.

New Trellis Design

The goal of a new trellis design became one that allows more light and air into the vine while still protecting the grapes from sunburn and being easy to care for on a small scale. I started looking into other trellising systems that might satisfy the requirements by studying publications, like Dr. Smart’s “Sunshine into Wine” and others, to find the right design. The research spanned the world of grape trellising including designs of France, Italy, Australia, and the U.S. As the analysis proceeded, I discovered there really wasn’t a design that satisfied the identified requirements while still being easy to maintain.

Consequently, I decided to create my own design that would answer my requirements and be easily maintained. The design is essentially a “V” shape with cordons angled up sharply and is named “Shoemaker’s Angled Shoot Projection” (SASP).

SASP

The SASP trellis design has resulted in less maintenance while providing higher quality fruit, significantly less powdery mildew, less bird damage, and easier harvests.

Specifically, the SASP trellis design provides two to four leaves between the sun and fruit, thus providing the correct amount of sunlight on the grapes to achieve the 20 percent flecking recommended by Dr. Smart while preventing sunburn and premature raisining.

This trellis design allows mildew sprays to easily reach into the vine to the fruit without the need for much leaf movement or an expensive fan-style spray rig, resulting in cleaner fruit at harvest. Even though it is still necessary to spray for powdery mildew, the SASP trellis design has decreased the number of sprays by more than half.

The SASP trellis design allows the person harvesting to easily see the fruit for a faster harvest without expensive preparatory leaf cane and leaf thinning. (See above photo.)

In Figure 2, the bird netting can be seen rolled along the drip line; but there are years that I don’t get all the nets up to protect from the birds. The SASP trellis design produces fruit along the angled cordon canes hanging free and making it almost impossible for birds get to the fruit.

As an added benefit, the SASP trellis design has allowed for ‘interplanting’ of additional vines because the angled shoots extend upward and thus require less horizontal space along the support wires. Originally, the vineyard was planted with vines 5 feet apart, now because of the SASP design, I have been able to interplant vines at 2.5 feet apart which has doubled the number of vines while each one is mining the soil for its own nutrients resulting in high quality fruit on each vine.

Interestingly, the vines planted in the fertile sandy loam soil were largely overgrown creating even more of a powdery mildew problem; but now, at the closer spacing, the vines are competing with each other and the amount of green matter growth has decreased resulting in more balanced vines between the leaves and fruit weight.

In Conclusion

The SASP trellis system was created to answer identified issues in our vineyard by mixing design parameters to satisfy the requirements in one trellis design structure. SASP has resulted in more balanced vines with higher quality fruit and easier and less expensive maintenance.

Anyone is considering the SASP trellis design, the individual vineyard’s terroir, requirements, and issues should be considered prior to making the decision to use this design.

Steve Shoemaker is a former Counter-Terrorism expert who started his vineyard to help with PTSD as a result of the 7 years in warzones. He is a current student in the UC Davis Post Graduate Winemakers Certificate Program; and has about two years left to complete his PhD in Counter-Terrorism. However, his happy places are the vineyard and cellar; but sharing his art with others in the greater Clovis, California area is the most fun. He can be reached at: 3oaksvineyardclovis@gmail.com.

Mechanized Vineyard—Is it the Wave of the Future?

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Mechanization progress in a traditionally labor intensive crop is yielding improved production and quality.

Wine grape growers in California’s San Joaquin Valley and other wine grape growing regions are finding benefits in University of California Cooperative Extension (UCCE) research into mechanized dormant pruning and shoot removal. While the traditional winegrape training system can work for mechanical harvest, mechanical dormant pruning and shoot removal operations have not been as successful. The aim in further mechanization is to lower labor costs while still ensuring crop yields and quality.

Mechanical Pruning

University of California (UC) researchers Kaan Kurtural, a specialist at the UC Davis Department of Viticulture and Enology and George Zhuang, UCCE viticulture advisor in Fresno County found that introducing mechanized pruning and other vine management operations could be done in existing vineyards. Vines could be re-trained during the transition from hand pruning and they would still retain production and fruit quality.

This choice is significant for wine grape growers in the San Joaquin Valley, who produce more than half of the wine grapes in California, because in recent years they have faced increased labor costs, worker shortages and tighter profit margins.

A report from UC Davis noted mechanical pruning in wine grape vineyards reduced labor costs by 90 percent, increased grape yields and had no impact on the berry’s anthocyanin content.

One of the research sites was an eight-acre portion of a 53-acre block of 20-year-old Merlot vines in Madera County. The field study took place over three growing seasons.

The report noted that following completion of the research trial, the remainder of the vineyard was converted to the single high-wire sprawling system used in the trial block. UC researchers also reported other wine grape growers in the area are beginning to transition vineyards to the new system.

Trellis Systems

In the San Joaquin Valley, the traditional trellis system consists of vines head trained to a 38-inch tall trunk above the vineyard floor and two eight-node canes laid on a catch wire in opposite directions. There are also two eight-node canes attached to a 66-inch catch wire. This system can work for mechanical harvesting, but not dormant pruning and shoot removal and limits options for other canopy management operations.

In the trial, the vines were converted to a bilateral cordon-trained spur pruned California sprawl training system or to a bilateral cordon-trained, mechanically box pruned single high wire sprawling system. The UCCE report noted that the second system was the most successful for mechanical pruning.

A report on converting vineyards to mechanical pruning, authored by Kurtural, Andrew E. Beebe, Johann Martinez-Luscher, Zhuang, Karl T. Lund, Glenn McGourty and Larry J. Bettiga and published in HortTechnology, concluded that conversion of traditional systems to the bilateral cordon-trained mechanically box pruned single high wire sprawling system (SHMP) sustained greater yield with more clusters per vine and smaller berries without affecting the canopy microclimate.

This was due to a higher number of nodes retained after dormant pruning. Compared to the traditional and the bilateral cordon-trained spur pruned California sprawl training system, the SHMP canopies filled their allotted space earlier. The report authors said that earlier canopy growth coupled with sufficient reproductive compensating responses allowed for increased yields while reaching maturity without a decline in anthocyanin content. This system is recommended for growers in the hot Central Valley winegrape growing areas to increase sustainability of production while not sacrificing adequate berry composition.

From 2013 to 2015, labor costs per acre with the SHMP system totaled $463.05 while the hand labor during that time totaled $1,348 per acre.

Vineyard Mechanization Conversion

Kurtural also led research on vineyard mechanization conversion in a 40-acre vineyard in Napa County. The research began as a labor-saving trial, but Kurtural reported that when they began looking at the physiological aspects of how plants grow, there were benefits to fruit quality as well.

Taller canopies due to increased trunk height protect the developing winegrapes from sun damage. The taller canopies and the increased yields from mechanically pruned vines also mean that water and nutrient requirements in the vineyard can be different from those in hand-pruned vineyards.

Zhuang confirmed that interest in mechanical pruning and vineyard transition is growing, not only in the San Joaquin Valley, but on the Central Coast where sufficient labor for cultural practices is becoming difficult to find. The premium wine grape growing areas in northern California have strong traditions with hand spur pruning, but tighter labor markets may lead growers there to consider mechanization.

The vine re-training could also be feasible for raisin grape vineyards, Zhuang said, but not for table grapes due to different production needs.

Changes in Nutrient and Irrigation Management

Where mechanized pruning is done, Zhuang said, there would need to be changes in nutrient management and irrigation. Water and fertilizer requirements for mechanically pruned vines are different than those of hand pruned vines.

“Canopies will grow faster and bigger on those vines,” he said.

Mechanized pruning operations will leave many more buds, as many as double the number left after spur pruning, and that will change the plant physiology. Buds will break earlier in the growing season, Zhuang said and the vines would begin to push new growth much faster. The early growth will mean early water demands will need to be met. Demand for nutrients will be accelerated by the early growth, larger canopies and yields.

Labor

Nick Davis, ranch manager for The Wine Group, a company that farms 13,000 acres of wine grapes between Kern County and Lodi, said reducing the impacts of increased labor costs prompted the decision to move toward mechanized pruning in their vineyards.

Any new vineyard developed by the company, in the Central Valley, will be set up for mechanization, Davis said, and the goal is to become 100 percent mechanized in the future.

“We know this system works, but we will be working on managing the hedge-pruned box and not allow it to creep out,” Davis said.

Transitioning existing vineyards requires removal of cross arms and foliage wires and t-posts, but if the berry quality and the production are the same or better than vines that are hand pruned, transitioning will continue, Davis said.

Maximizing the Efficiency of Airblast Spraying

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Agricultural operations are becoming more efficient-have you noticed? Efficiency is defined as using the least amount of input to achieve the highest amount of output. And any business person, engineer or farmer knows that efficiency saves money. Still, there is one critical piece of equipment on every farm that sometimes is forgotten when we talk efficiency: the airblast sprayer.

When I think of maximizing the efficiency of an airblast application, I think of coverage. Spray coverage is the opposite of drift, and good spray coverage on the target, while minimizing off-site pesticide movement, is the goal when we take the sprayer out.

This pump pre-filter was so full of sulfur and oil that it couldn’t be easily removed. It came lose after an overnight soak with a tank cleaner. No wonder the pressure was so low-impeded flow! Photo credit: F. Niederholzer

Here, some tips for improving the efficiency of your airblast sprayer.

  1. Take care of your equipment, understand how it works. Don’t ignore the basics. Keep a clean machine. Cleaning improves the life of the sprayer, reduces the chance of cross-contamination of pesticides and crop injury, and improves spray quality. Although this is a “duh”, I often encounter sprayer problems that are due to neglect of the basics:
    • The pump pre- and post- filters should be cleaned at the end or start of every spray day.
    • Likewise, the nozzle strainers. Cleaning the filters doesn’t take much time but can make a huge difference in the application.
    • Replace the nozzles annually at least. Enough said.
    • The fan grill should be clear of leaves and debris so it can intake air.
    • Be sure that the agitation-either mechanical or hydraulic-is working properly-this ensures a uniform pesticide suspension.
    • Make sure your pressure gauge is easy to read, uses a scale that makes sense for your typical spray pressure (no need to go to 1000 psi), and check the pressure gauge against another gauge for accuracy.

 

  1. Check your calibration variables to make sure they are accurate. Calibration is an essential part of sprayer efficiency. I prefer to use the basic calibration formula, which works with any sprayer and is easy to remember:

 

Spray volume (GPA, gallons per acre) = Flow rate (GPM, gallons per minute)
Land rate (APM, acres per minute).

 

No matter what formula you choose to use to calibrate, the variables you need to measure are the same: nozzle output, tractor ground speed, and spray swath width.

 

Nozzle output (flow rate) is a function of the pressure and the type of nozzle. You can check this in the nozzle manufacturer’s catalog-most are available online. But you should also confirm by measuring the flow rate because the output can change when nozzles wear, or when the pressure differs from that listed in the catalog. I’ve found that even new nozzles can have flow rates that differ significantly from what is expected.

 

To measure the entire sprayer flow rate, follow these steps:

  • Park the sprayer on a level surface. Fill the tank with clean water up to a verifiable spot at the top of the tank—usually you can see a line at the strainer or even make a mark with a Sharpie.

  • Working with the driver, bring the PTO or engine up to operating RPMs (540) and open all the nozzles while timing with a stopwatch how long they are open. You’ll want to keep them open for a minute or two. Check to confirm the pressure while they are open (you’ll need to wear PPE, personal protective equipment because you’ll get wet!). Be sure to stop your stopwatch when the nozzles are shut off and use that time for your calculation.
    .
  • Refill the sprayer up to your line and record how much water it takes to refill. Be sure to use a bucket that has been calibrated itself to make accurate measurements. Then divide the number of gallons it took to refill by the time to get the gallons per minute.
No fancy tools needed! A simple bucket can be used to measure the sprayer output after spraying out for a measured amount of time, as demonstrated here by U.C. Cooperative Extension Farm Advisor Franz Niederholzer.

This method gives you output of the entire sprayer, all nozzles. If you want to measure individual nozzle flow rates, you will need to either make or purchase a nozzle adapter, to fit over the nozzle with a hose attached to capture the flow. We’ve made an adapter from dishwasher plumbing supplies, brass hose bibs and hose clamps. AAMS Salvarani manufacturing in Belgium is a source to purchase nozzle adapters.

Once you have the actual sprayer flow rate, confirm your tractor ground speed. Don’t rely on the tractor speedometer, these are notoriously erroneous as they are typically set with the tires at the place of manufacturing. When tire sizes are changed, as they often are once the tractor reaches the sale point, the number of rotations and corresponding speedometer mph will be affected.

To check the tractor speed, measure out at least 100 feet in the terrain you’ll be working in, note the tractor gear and setting, and time the travel. This is typically in seconds, so you’ll need to convert to distance travelled over time in minutes to get feet per minute.

Land rate is defined as the swath width in feet multiplied by the ground speed in feet per minute. Swath width in orchards and vineyards is the row spacing, in feet. From the square feet, or area sprayed, you can then do the conversions to acres sprayed per minute.

 

  1. Recheck your calibration variables by looking at spray coverage. Use water sensitive spray cards or add a visible marker like kaolin clay to the tank, to check the spray coverage once you’ve calibrated. Water sensitive spray cards used to be hard to find, but a quick check online gave me 3 results—Gemplers, Sprayer Depot and Amazon—for places to purchase.

 

Put the spray cards in the canopy where you are targeting your spray. You can use mailing tags, with a card stapled to each side, to easily hang them in the canopy. Hang several also in areas where you don’t want to see spray. Remember to flag the branches where the paper is hung for easy retrieval. Then, run your sprayer down the row and retrieve the cards after. Interpreting what you see on the paper can be a bit tricky—you want to look for about 85 dots per square centimeter (see https://sprayers101.com/confirm-coverage-with-water-sensitive-paper/ for a visual of what this looks like). Don’t get too caught up in how many dots though; the most important thing is that you want some dots but not a totally blue card, which would indicate too much spray; nor a totally yellow card, which would indicate not enough spray.

The cards can give you clues on adjustments to make to refine your calibration: nozzle position, nozzle flow rate, fan speed, and ground speed may need to be modified for the best efficiency! Plan to spend at least a morning on optimizing your sprayer efficiency, it will pay off in the end.

Water sensitive spray cards are yellow and turn blue when sprayer water drops hit. They can be attached to mailing tags for easy hanging in the canopy.

Weed Management in Vineyards

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From wine grapes to table grapes and raisins, there are several ways to prevent and manage weeds in the vineyard. Ideally, weeds are managed while they’re still small, since the crop is closer to the ground, and taller weeds can provide easy access for pests, disease, and other complications.

While industry best practices and research hasn’t changed significantly very recently, there has been one change that farm advisors now recommend to growers: spray volumes.

“The old recommendations were thirty gallons an acre,” says Kurt Hembree, Weed Management farm advisor for the University of California Cooperative Extension, “but it really needs to be about forty to fifty gallons an acre.”

Farm advisors are seeing better results for the contact herbicides at this higher volume, with better coverage and less weed regrowth, and overall a cleaner vineyard floor than at the lower volumes. Everything else, including timing and materials, remain the same.

The main component of struggling with weed control is the fact that even though herbicide labels are very specific about application timing, many growers get into the field later than they should. Sometimes pruning can take growers into the middle of winter, and by then there are rainstorms that can prevent them from getting out into the field, especially in vineyards with heavier soils.

Before growers catch themselves at odds in the winter, there are things that can be done earlier to ensure a well-implemented program. “Late summer and early fall is a great time to make sure all your equipment is working properly,” says Hembree. “Check your spray nozzles and all your machinery.”

Whichever weed management program a grower chooses, Hembree insists on sticking to it, and adhering as closely as possible to the timings. “As soon as everything is pruned and the canes come out of the field, be ready to go. Timing is the biggest issue. Like in the case of raisins, you only have a few months before the canopies touch the ground.”

Another major key for ensuring spray effectiveness is cleaning up trashy berms and keeping them clean. If there is debris at the base of your vines, there’s no telling whether or not the spray chemicals are making it into the soil and reaching the weeds, or if they’re getting stuck and then washing away.

According to Hembree, if the field is clean and the proper spray timings are followed, the rest of any weed management program will fall into place.

This is especially true for organic vineyards, as trying to rein in weeds from an organic standpoint is very challenging. This becomes trickier because available options are much more limited, but organic growers who dedicate the right equipment, manpower, and timing can do just as well.

Doing nothing—in both organic and conventional—is a recipe for disaster, and can wreak havoc on the crop, the harvest crew, and the bottom line. Using raisin grapes again as an example, tall horseweeds or prickly lettuce can overrun a vineyard, which can also bring white flies and leaf hoppers. Combine this with an angry crew that has to fight with weeds to hunt for clusters, a too-lax weed management system will have weeds seeds nestled in the folds of raisins and will, therefore, result in a contaminated crop.

“I’ve seen weeds seeds end up in the trays, and I’ve seen loads get rejected from overseas because of them,” says Hembree.

Growers who are vigilant about their weed management program will benefit greatly because of it. Both pre-plant and post-plant options are available to control weeds in vineyards.

Pre-Plant Options

Preparing the soil prior to planting a new vineyard is a great time to initiate a solid weed management regiment. For one, controlling weeds at this stage is critical because of the competition for resources that can happen if weeds are present during the planting of vines. An irrigation program that supports weed seed germination, which is then followed by tillage to uproot these new seedlings, can help greatly reduce the presence of weeds. There’s also the option of burying seeds even further beneath the ground, preventing them from sprouting at all. However, a drawback of this is that the soil shouldn’t be tilled for about four years, otherwise the seeds might be brought back to the surface and germinate.

There is also the option of laying UV-inhibiting, clear plastic between rows that are six feet wide and damp. The longer the days, the better, and this should be started no later than late August in most California regions to ensure that this process can be completed within four to six weeks, before the seasonal change.

Post-Plant Options

Most vineyards aren’t starting anew when it comes to weed management, so staving off weeds can either be a proactive endeavor, or a reactive one that can leave growers—and harvest crews—battling weeds.

Cultivation

With this method, weeds can either be uprooted or buried, with uprooting working better for larger weeds, and burial working better for smaller ones. Keeping the depth of grape roots in mind, cultivation ideally destroys weed roots to remove current plants without turning over the soil enough to allow for germination of new seeds. Established perennial weeds will need more attention in order to remove them, and growers may need a special mechanism to protect vine roots.

Flaming

When a burst of heat is applied at 130°F, the plant’s cell wall ruptures. This is most effective on non-grass plants that have fewer than two true leaves. Burning isn’t necessary, and a weed that loses its shine or retains a fingerprint when pressed has been adequately flamed. Propane-fueled flamers are the most commonly used models for weeds in the vine row, and it’s extremely important to avoid this method in windy conditions or around dry vegetation of any kind.

Herbicides

Contact herbicides are effective, and since the results are dependent upon the chemicals touching the plant, they can also damage grape leaves and vines. New flushes of weeds will require additional application, as contact herbicides don’t generally have a residual effect. Following all provided directions—from application method and timing, to protective equipment and storage—is extremely important when using any specific herbicides.

Mulch

What is weed control without the mention of mulch? While mulch can be made from a variety of materials, such as wood chips, straw and even newspaper, the ultimate goal is to completely block any light from reaching seeds, thus preventing germination. Organic mulches break down faster, so the layer will have to be thicker. Winter cover crops can be cut and then moved to be used as mulch. Though cover crops can outcompete weeds, they can also compete with the vine. Mulches in general may provide cover for some unwanted visitors as well, such as rodents that can damage vine trunks and roots.

Whichever method a grower chooses, it’s helpful to keep a weed survey of the field. These records can assist in method selection, can track changes in the field, and can help with diligently sticking to a weed management program, which is imperative for vineyard success.

Entomopathogenic Fungi Antagonizing Macrophomina phaseolina in Strawberry

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Charcoal rot, caused by Macrophomina phaseolina, is one of the important fungal diseases of strawberry in California. Macrophomina phaseolina is a soilborne fungus and has a wide host range, including alfalfa, cabbage, corn, pepper, and potato, some of which are cultivated in the strawberry production areas in California. The fungus infects the vascular system of the plant roots, obstructing the nutrient and water supply and ultimately resulting in stunted growth, wilting, and death of the plant. The fungus survives in the soil and infected plant debris as microsclerotia (resting structures made of hyphal bodies) and can persist for up to three years. Microsclerotia germinate and penetrate the root system to initiate infection. Plants are more vulnerable to fungal infection when they are experiencing environmental (extreme weather or drought conditions) and physiological (heavy fruit bearing) stress.

Soil fumigation is the primary management option for addressing charcoal rot in strawberry. Additionally, crop rotation with broccoli can reduce the risk of charcoal rot due to glucosinolates and isothiocyanates in broccoli crop residue that have fungicidal properties. Beneficial microorganisms such as Bacillus spp. and Trichoderma spp. are also considered, especially in organic strawberries, to antagonize M. phaseolina and other soilborne pathogens and provide some protection. The role of beneficial microbes in disease management or improving crop growth and health is gaining popularity in the recent years with the commercial availability of biofungicide, biostimulant, and soil amendment products. In a couple of recent strawberry field studies in Santa Maria, some of the beneficial microbial products improved fruit yield or crop health. These treatments can be administered by inoculating the transplants prior to planting, immediately after planting or periodically applying to the plants and or the soil. Adding beneficial microbes can help improve the soil microbiome especially after chemical or bio-fumigation and anaerobic soil disinfestation.

Similar to the benefits of traditionally used bacteria (e.g., Bacillus spp. and Pseudomonas spp.) and fungi (e.g., Glomus spp. and Trichoderma spp.), studies with entomopathogenic fungi (EPF) such as Beauveria bassianaIsaria fumosorosea, and Metarhizium spp. also demonstrated their role in improving water and nutrient absorption or antagonizing plant pathogens. The advantage of EPF is that they are already used for arthropod pest management in multiple crops, including strawberry; now, there are the additional benefits of promoting crop growth and antagonizing plant pathogens. In light of some promising recent studies exploring these roles, a study was conducted using potted strawberry plants to evaluate the efficacy of two California isolates of Beauveria bassiana and Metarhizium anisopliae s.l. and their application strategies against M. phaseolina.

Methodology

About six-week old strawberry plants (cultivar Albion) from a strawberry field were transplanted into 1.6-gallon pots with Miracle-Gro All Purpose Garden Soil (0.09-0.05-0.07 N-P-K) and maintained in an outdoor environment. They were regularly watered, and their health was monitored for about five months prior to the commencement of the study. Conidial suspensions of the California isolates of B. bassiana and M. anisopliae s.l. were applied one week before, after, or at the time of applying microsclerotia of M. phaseolina to the potting mix. The following treatments were evaluated in the study:

  1. Untreated control
  2. Soil inoculated with  phaseolina
  3. Soil inoculated with   bassianaone week prior to M. phaseolina inoculation
  4. Soil inoculated with   anisopliae s.l. one week prior to M. phaseolina inoculation
  5. Soil inoculated with   bassianaat the time of M. phaseolina inoculation
  6. Soil inoculated with   anisopliae s.l. at the time of M. phaseolina inoculation
  7. Soil inoculated with   bassianaone week after to M. phaseolina inoculation
  8. Soil inoculated with   anisopliae s.l. one week after to M. phaseolina inoculation

EPF were applied as 1X1010 viable conidia in 100 ml of 0.01 percent Dyne-Amic (surfactant) solution distributed around the base of the plant. To apply M. phaseolina, 5 grams of infested cornmeal-sand inoculum containing 2,500 CFU/gram was added to four 5 cm deep holes around the base of the plant. Each treatment had six pots each planted with a single strawberry plant representing a replication. Treatments were randomly arranged within each replication. The study was repeated a few days after the initiation of the first experiment.

Plant health was monitored starting from the first week after the M. phaseolina inoculation and continued for seven weeks. Plant health was rated on a scale of 0 to 5 where 0=dead and 5=very healthy and the rest of the ratings in between depending on the extent of wilting. Data from both experiments were combined and analyzed by ANOVA using Statistix software and significant means were separated using LSD test. The influence of EPF treatments applied at different times as well as the combined effect of different applications within each fungus were compared for seven weeks. Ratings for some plants that were scorched from hot summer temperatures and died abruptly were removed from the analyses.

Results

Untreated control plants maintained good health throughout the observation period varying between the rating of 4.3 and 4.9. In general, plant health declined considerably from the 5th week after M. phaseolina inoculation. Plant health appeared to be slightly better in plants treated with EPF, but there was no statistically significant difference in any except one instance. Plants treated with M. anisopliae one week prior to the application of M. phaseolina had a rating of 3.0 compared to 1.6 rating of plants inoculated with M. phaseolina alone.

When data from different treatments for each EPF were compared, both B. bassiana and M. anisopliae s.l. appeared to reduce the wilting, but the plant health rating was not significantly different from the M. phaseolina treatment without EPF.

This is the first report of the impact of EPF on M. phaseolina with some promise. The current study evaluated a single application of EPF. Additional studies under more uniform environmental conditions and with more treatment options would be useful to improve our understanding of EPF antagonizing M. phaseolina. When growers use EPF for controlling arthropod pests, they could count on additional benefits against diseases or improving general plant health.

Acknowledgements: We thank Dr. Kelly Ivors (previously at Cal Poly San Luis Obispo) for the pathogen inoculum and Dr. Stefan Jaronski, USDA-ARS, Sidney, MT for multiplying the entomopathogenic fungal inocula.

Dara, S. K. and D. Peck. 2017. Evaluating beneficial microbe-based products for their impact on strawberry plant growth, health, and fruit yield. UC ANR eJournal Strawberries and Vegetables. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=25122

Dara, S. K. and D. Peck. 2018. Evaluation of additive, soil amendment, and biostimulant products in Santa Maria strawberry. CAPCA Adviser, 21(5): 44-50.

Dara, S. K., S.S.R. Dara, and S. S. Dara. 2017. Impact of entomopathogenic fungi on the growth, development, and health of cabbage growing under water stress. Amer. J. Plant Sci. 8: 1224-1233. http://file.scirp.org/pdf/AJPS_2017051714172937.pdf

Dara, S. K., S. S. Dara, S.S.R. Dara, and T. Anderson. 2016. First report of three entomopathogenic fungi offering protection against the plant pathogen, Fusarium oxysporum f.sp. vasinfectum. UC ANR eJournal Strawberries and Vegetables. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=22199

Koike, S. T., G. T. Browne, and T. R. Gordon. 2013. UC IPM pest management guidelines: Strawberry diseases. UC ANR Publication 3468. http://ipm.ucanr.edu/PMG/r734101511.html

Partridge, D. 2003. Macrophomina phaseolina. PP728 Pathogen Profiles, Department of Plant Pathology, North Carolina State University. https://projects.ncsu.edu/cals/course/pp728/Macrophomina/macrophominia_phaseolinia.HTM

Vasebi, Y., N. Safaie, and A. Alizadeh. 2013. Biological control of soybean charcoal root rot disease using bacterial and fungal antagonists in vitro and greenhouse condition. J. Crop Prot. 2(2): 139-150.

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