Thousands of the poppy seed-like resting structures of the fungus that causes white rot are produced on infected garlic heads. All photos courtesy of Tom Turini.
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.
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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Figure 1: Syrah on SASP Trellis. All photos courtesy of Steve Shoemaker.
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.
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.
Photo credits: Lynn R. Wunderlich except where noted.
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.
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.
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.
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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.
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.
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.
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 bassiana, Isaria 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:
Untreated control
Soil inoculated with phaseolina
Soil inoculated with bassianaone week prior to M. phaseolina inoculation
Soil inoculated with anisopliae s.l. one week prior to M. phaseolina inoculation
Soil inoculated with bassianaat the time of M. phaseolina inoculation
Soil inoculated with anisopliae s.l. at the time of M. phaseolina inoculation
Soil inoculated with bassianaone week after to M. phaseolina inoculation
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
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.
Southern blight, caused by the fungus Sclerotium rolfsii, is a destructive crown rot disease that rapidly kills tomato plants. The fungus is favored by high temperatures (over 86°F), high soil moisture, dense canopies, and frequent irrigation. Southern blight survives in soil as hardened structures called sclerotia for at least five years. Each infected plant can produce tens of thousands of sclerotia and then become more widely distributed in a field with each successive field operation. Although this disease may initially only affect a few plants in the field, southern blight can be serious enough to cause significant yield loss within a season or two. With a host range of over 500 plants, this fungus can easily persist from year to year in infected crop debris.
Southern Blight Identification
Southern blight misdiagnosis is likely if it occurs in an area where it has not historically been an issue, like the Sacramento Valley. It can be easily confused with other crown rotting diseases, like Fusarium crown rot. Severely affected plants can have vascular discoloration, which may be confused with Fusarium wilt. Accurate diagnosis is critical to effective control. You can distinguish southern blight in the field based on the following diagnostic traits, one or more of which may be present. Diagnosis requires looking at the soil around the crown of the plant, in addition to the plant itself.
Small tan to reddish brown sclerotia form at the base of the plant and/or in the soil around the plant.
White fungal mycelium (thread-like strands) growing INTO the soil. No other fungus will grow extensively in the soil (Figure 1).
White fan like mycelium (thread-like) growing on the crown/affected tissues.
Plants go from healthy to dead in less than a week—much faster than most crown rots (Figure 2).
Circular disease patches. From a distance, they look like bands of dead plants.
Figure 1: Fungal mycelium growing into the soil.Figure 2 (right): Rapid plant collapse and death caused by southern blight. Photo credits: C. Swett.
If none of these characteristics are present, the best way to diagnose the disease is to put infected tissue in a plastic bag on a moist paper towel and leave at room temperature for one to two weeks. The southern blight fungus will produce distinct fan like growth within about 5-7 days (Figure 3). After about 5-14 days, it will make round white balls that turn into amber colored sclerotia (Figure 4). Both the fan growth and the sclerotia are unique to this fungus.
Figure 3: Distinct fan-like growth of southern blight.Figure 4: Sclerotia developing and turning from white to amber-colored spheres. Photo credits: C. Swett.
Southern Blight Management
Soil Moisture
Maintaining a dry surface may help reduce losses if the fungus is detected in your field. The one advantage of drip irrigation is that the soil surface can more easily be kept dry, which inhibits infection by Sclerotium rolfsii. Avoid alternating wet and dry periods—wet followed by dry episodes can be particularly conducive to disease development.
Crop Rotation
If you have detected southern blight in your field, one of the best things you can do the following year is to plant a narrow canopy crop that you can effectively manage with fungicides to prevent sclerotia from increasing.
Rotations with non-host crops are limited because of the wide host range of the pathogen. Poor-host crops such as corn and small grains (wheat, millet, oats) can help to reduce sclerotia levels in the field.Most if not all of these crops can become infected by the fungus, but either they are not good hosts and/or the environmental conditions during the growing season are not favorable for pathogen growth. For instance, wheat can be a host, but it’s typically too cold for fungal growth during the time that wheat is grown. On the other hand, rotation with highly susceptible crops such as legumes (beans, peas and hairy vetch) can greatly increase soil infestation levels. Mustard cover crops can suppress southern blight, and may be useful for organic producers, where fumigation is not an option.
Soil Treatment
Deep plowing will bury the sclerotia and prevent it from attacking plants at the soil line. Sclerotia deeper than six inches are usually parasitized by other microbes and killed over time. Of course, plowing is not an option for fields where buried drip irrigation systems are already installed.
Sclerotia near the surface of the soil can be killed when exposed to high temperatures (105-120°F) for two to four weeks during the summer months. Solarization alone is not generally considered a viable management strategy, but when soils were solarized before the addition of biological control or a fungicide, disease was reduced by 70-100 percent compared to the same biological or chemical treatment without solarization. Make sure to prepare the soil for planting before solarizing, since cultivation and the incorporation of amendments can bring buried sclerotia back to the upper soil layers.
Monitoring Southern Blight Prevalence in Colusa County
Southern blight is not usually considered to be a widespread problem in California—major impacts are generally limited to Kern County. However, in 2017, late spring rains in the Sacramento Valley led to later planting dates, followed by record high temperatures, even consecutive nights where the temperature remained above 70°F. The combination of late planting dates and record high temperatures in 2017 created unusually favorable conditions for the pathogen in northern California. In 2018, we conducted a project funded by the California Tomato Research Institute to monitor southern blight prevalence in Colusa County, which had five fields with positive southern blight diagnoses in 2017. The objective of this research was to quantify southern blight spread and impact in annual rotations in the region. Sarah Light-Area Agronomy Advisor was involved in this project in addition to the authors.
We sampled soil from eight fields, five of which were confirmed to have southern blight in 2016 or 2017, the other three thought to have southern blight based on grower and pest control adviser experience and observations. Six of the fields were in tomato in 2017, one was in tomato in 2016 and wheat in 2017, and one was planted with canary bean in 2017. We sampled the soil in May 2018 to get baseline data on early season southern blight sclerotia levels. The rotational crops in 2018 included sunflower and corn. Sunflower fields were checked twice monthly for southern blight symptoms once temperatures were over 90°F for seven consecutive days because sunflower is a known host of southern blight. Tomato fields near or adjacent to the monitored fields were also checked for southern blight symptoms twice a month. Soil was collected from the same spots in the fields in August/September 2018 to determine if there were any changes to southern blight sclerotia levels in the field. Cassandra Swett’s lab analyzed the soil samples using the methanol method (Rodriguez-Kabana et al 1980). Methanol kills most microbes, but not southern blight. Trays of soil were sealed in plastic bags so the moisture could stimulate germination of the southern blight sclerotia. The germination of sclerotia was evaluated at 3 and 7 days (Figure 5).
Figure 5: Southern blight sclerotia germination in soil samples. Note white thread-like mycelial growth.
Sclerotia were recovered from three fields in May 2018, possibly five fields but identification was unclear for two of the fields. The confidence level for identifying southern blight was whether the germinated growth in the trays produced sclerotia. Germination was observed in the two samples where identification was unclear, but no sclerotia were produced from these colonies. The end-of-season samples from August and September 2018 contained much higher volumes of soil than the May samples, and we recovered sclerotia from seven of the eight fields. For total number of sclerotia, five fields had increased sclerotia levels, two fields decreased, and one field had the same number of sclerotia in both the May and September samples. Southern blight increased in all three of the sunflower fields, which was expected since sunflower is a host crop. Corn fields were spread between increases, decreases, and no changes. It is worth noting though, that the only fields with decreased levels were corn fields. Also, fields where no sclerotia were recovered may contain southern blight that was not captured among our samples.
Sunflower is not recommended as a rotational crop because it is a southern blight host. Corn is likely a better choice for rotation.
We were able to recover southern blight sclerotia in fields throughout Colusa County and demonstrate southern blight increases over the growing season with certain rotational crops. Unlike 2017, southern blight was not a large issue for tomato growers in 2018. Because southern blight requires specific conditions for development to occur, it remains a disease that is a problem in the Sacramento Valley only when environmental conditions are ideal for development, especially for certain fields. Currently in 2019, due to late spring rains and high temperatures, we have identified southern blight in the Sacramento Valley from a few tomato and vineseed fields.
We would like to thank our grower and pest control adviser cooperators on this project. We would also like to thank the California Tomato Research Institute for funding this project.
References
Rodriguez-Kabana, R., Beute, M. K., & Backman, P. A. 1980. A method for estimating numbers of viable sclerotia of Sclerotium rolfsii in soil. Phytopathology, 70(9), 917-919.
Figure 1. The biological agent, Aspergillus flavus strain AF36, producing large sclerotia (black, spherical structures in the plate).
It was in 1991, when I received a call by the President of the former California Pistachio Commission, Karen Reinecke, asking if there was a way to get involved as a technical member of the newly then established Aflatoxin Elimination Workgroup. The goal of this Workgroup was to evaluate proposals submitted by the United States Department of Agriculture (USDA) and University researchers to the USDA Special Fund to participate toward research to eliminate the problem of aflatoxin contamination by the year 2000. We are now in the second half of 2019 and aflatoxin was not only eliminated by 2000, but indeed contamination created major problems in some years in both pistachio and almond.
Our first proposal submitted in 1992 to the USDA/ Aflatoxin Elimination Technical Committee. It was funded and research started after hiring postdoc associate Dr. Mark Doster, a University of California (UC) Davis graduate with postdoc research at the University of Cornell. Mark an expert in fungal pathology and practical plant pathology was immersed quickly in this research. We were also supported then by the California Pistachio Industry which supplemented funding to intensify the research to reduce aflatoxins and find solutions for the growers. At the same time other researchers from UC Davis focused on aflatoxin research to reduce it in almond and walnut. Later on, we expanded our aflatoxin management research and included almonds and figs.
Aflatoxins are toxic compounds produced mainly by certain molds called Aspergillus flavus and A. parasiticus when these molds grow on various susceptible crops. These molds produce toxins that are considered toxigenic. The toxins produced are called aflatoxins which are considered as the most potent naturally-produced carcinogenic compounds causing liver cancer and in acute situations deaths. Strains of Aspergillus flavus that do not produce toxin are called atoxigenic, and they act as biological control agents. When they are applied on the orchard floor displace the toxin-producing mold strains, and reduce the potential for aflatoxin contamination in various crops.
There are four major aflatoxin types: the B1 and B2 produced by both the above mentioned fungi and G1 and G2 produced only by A. parasiticus. B1 is the most toxic among the four aflatoxin types. Because of this high toxicity these compounds are regulated strictly by various governments, and in fact, the B1 is regulated separately. For instance, in the USA the tolerance for B1 is 10 ppb (parts per billion) and for all the aflatoxins (total) is 15 ppb. The European Union (EU) has even stricter tolerances, i.e., 8 ppb for B1 and 10 ppb for total aflatoxins. One can judge the seriousness of aflatoxin contamination not only from the very strict tolerances but also from the losses and additional costs associated with the re-sorting and losses associated with the dumping the contaminated product. It should be noted that when shipments exceed the threshold, the consignments are rejected and must either be reconditioned or destroyed.
Below is a historical summary how this technology developed to help our California nut crop industries and the fig industry.
In the first eight years we focused on cultural practices that affect the predisposition of the pistachio crop to aflatoxin contamination and also find out whether contaminated nuts show special characteristics that can be used to sort out these nuts at the processing plant.
Below is a list of the findings from those studies led by Drs. Doster and Michailides:
We confirmed that early split nuts (ES) contained large amounts of aflatoxins and we named these nuts the “Achilles Heel” for aflatoxin contamination.
ES by themselves explained 84 percent of the aflatoxin contamination in the samples we analyzed.
When the ES were combined with the navel orangeworm (NOW) damaged nuts explained 99 percent of the aflatoxin contamination in the samples we analyzed.
We reduced the incidence of ES by providing the trees with sufficient water during early season (May) when it is the critical time for the full size development of nut shell (water stress of trees during May leads to higher incidence of ES).
We compared the incidence of ES on Kerman under the influence of four rootstocks: UCB1 and Pioneer Gold I resulted in significantly lower ES incidence than Pistacia atlantica and Pioneer Gold II rootstocks.
In my first Aflatoxin Elimination Committee (AEC) Meeting in 1991, in Peoria, Illinois, I learned for the first time that some strains of Aspergillus flavus do not produce aflatoxins and some USDA researchers reported that a very large portion of the A. flavus population consisted of strains that do not produce any aflatoxin, called atoxigenic strains. They started using these atoxigenics as candidates for biological control of aflatoxigenic fungi. It was also discovered that the proportion of strains that produced aflatoxin included strains that produced variable amounts of aflatoxins. USDA researchers started first working with atoxigenic strains to be used as biocontrol agents to reduce aflatoxins in the various crops. One of this strains was initially selected in Arizona from samples taken from cotton fields, and the Cotton Research Council of Arizona that supported financially this research were able to get AF36 registered for use in cotton and corn in 2008. Meanwhile starting in 2002, we discovered the same strain was the most commonly encountered strain among the other atoxigenic strains in pistachio, almond, and fig orchards in California, and immediately included this strain in our aflatoxin biocontrol studies. With multiyear support of USDA funding and funding by the pistachio and fig industries in California, we were able to show that this strain is among the most common atoxigenic strains occurring in California nut crop and fig orchards, and indeed it can be found at much higher incidence in comparison with all other atoxigenic strains. For instance, during these studies 15 different groups of atoxigenic strains were determined, and each one was at a rate of less than one percent, while the AF36 atoxigenic strain (Figure 1) was found in an average of five to eight percent depending on the field and the type of the crop, and in some instances up to 12 percent of populations of the atoxigenic strains
Initially, the studies were confined in small experiments in replicated micro-plots where we showed that when the AF36 was applied once preseason, it persisted well until the next application in the following year and displaced the toxigenic A. flavus strains at a rate of 90 to 95 percent displacement. These results, the fact that this strain was native to California orchards, and it was the most common atoxigenic strain and the toxicological data developed by the USDA in Arizona were sufficient to submit to the Environmental Protection Agency (EPA) to request an experimental use permit (EUP) to test the strain commercially on a larger acreage without the requirement to follow crop destruct requirements as needed with application of experimental compounds. The EUP was approved in 2008 and 3,000 acres of pistachios were treated with the commercial product of Aspergillus flavus AF36 strain produced in the Cotton Council of Arizona facility in Phoenix, Arizona. Also, 3,000 acres of pistachio close to the AF36-treated orchards were used as untreated controls. All this acreage for the EUP was provided by the former Paramount Farming Company (now Wonderful Orchards Company). At harvest the treated and the untreated fields were sampled separately and the samples, specifically called “library samples”, were analyzed for aflatoxins. For four years we showed a significant reduction of aflatoxins (Figure 2A). The average of this reduction for the four years of the EUP was close to 40 percent. When library samples of reshakes were analyzed for aflatoxins, this reduction in one year reached to 85 percent (Figure 2B). These commercial efficacy data were sufficient to obtain registration of AF36 for use in pistachio in the states of California, Arizona, Texas, and New Mexico. After the registration of AF36 on pistachio, the Almond Board of California and the California Fig Institute (the latter has funded research during the early stages of our aflatoxin research) became interested in completing any additional research so that the AF36’s registration is expanded to include almonds and figs.
Figure 2. Reduction of aflatoxin contamination (left, main crop; right, reshakes) during the years of experimental use permit (2008 to 2011) after application of AF36 strain in commercial orchards.
It took 5 years of additional research (funded by the Production Research and Food Safety and Quality Committees of the Almond Board of California) to provide the USEPA and the California of Pesticide Regulations Department the additional data for the registration of AF36 for use on almond and figs. Meanwhile because the manufacturer of the commercial product changed the initial wheat carrier of the AF36 strain to sorghum (Figure 3) the product was registered as AF36 Prevail® in January 2017 and included all, pistachio, almond, and fig. Although the pistachio industry adapted the use of AF36 widely and from 75,000 acres treated in 2012 reached to up to 200,000 acres in 2018, the almond industry was a little hesitant in widely adopting this new technology, despite the fact that there was good efficacy in reducing aflatoxin contamination in pistachio orchards. We expect to see better efficacy when all pistachio, almond and fig orchards are treated on an areawide basis because the spores of the biocontrol can spread from field to field easily with even slight windy conditions and dust.
Figure 3. The sorghum career of the AF36 Prevail® commercial product.
Description of the Biocontrol Product
Initially the AF36 product used sterilized wheat as the carrier. Sterilized wheat seed was inoculated and incubated under certain favorable conditions for the strain to invade and grow in the entire wheat seed. More recently and after additional studies in Arizona it was determined that sorghum, which has a lower cost was as good carrier as the wheat (even better under conditions in cotton and corn fields) and the manufacturer replaced the wheat with sorghum and at the same time changed the method of inoculations. The sorghum seed now is coated with a mix of a polymer and spores of AF36 instead of waiting for the seed to be colonized by the atoxigenic mold. This was done in order to satisfy the increased demand for tons and tons of inoculum since the rate is 10 lbs per acre. Studies in California under orchard conditions indicated that the sporulation on the sorghum seed is delayed, although by the end of a week both sorghum and wheat showed similar sporulation rates. The time of production of spores and the rate of sporulation are very critical for the successful use of this biocontrol agent. It is a numbers game: we want to overload the soil with atoxigenic spores and displace the spores of the toxigenic molds. That is the way this biological control approach works in the field (see challenges at the end of this article relevant to sporulation).
Proper Way for Ground Application:
For detailed information please read the label of the product:
Michailides emphasized that as far as we know up to now there are four critical application factors that need to be taken into account for AF36 Prevail to be successful in the orchard: a) the timing of application; b) the rate (amount) per acre; c) the proper placement on the orchard floor; and d) the proper irrigation before and after the application.
Timing for pistachio, almond, and fig: ApplyAspergillus flavus AF36 to the surface of the soil under the plant canopy with a granular applicator and do not cover the AF36—colonized grain with soil. For pistachios apply the product from late May through July, for almonds from late May to early July, and for figs from early May to early June. Specifically in almonds, application should be timed around hull split. If you know when to expect hull split, you should time application about one to two weeks before. You want to have the max sporulation of the biocontrol during the hull split stage of the nuts.
The rate (amount): The proper application rate is 10 pounds per acre. A single application should be made each year.
Proper placement: AF36 Prevail should be applied within the berm area of the orchard, not at row middles, so that it will be reached by the irrigation system and minimize delivery to areas that do not get wet.
Proper irrigation: Irrigation is required directly after application. Irrigation within three days after application of Aspergillus flavus AF36 will improve efficacy. The AF36 product will not sporulate without moisture and can fail if there is too much moistureAim for soil moisture levels around 13-18 percent. Proper placement within the berm, close to the irrigation system, will ensure it is successfully activated.
“Conditioning” of the orchard floor before application: This is practice that some growers have figured out on their own. Pre-irrigating and then about two days later apply the AF36 inoculum and then apply irrigation as in (d) above. Although Michailides and his crew do not have any data to support this practice, they strongly believe the practice of pre-irrigation will speed up the sporulation of the product since the rehydration can start as soon as the product comes in contact with the pre-wetted soil.
A 45 Percent Reduction of Aflatoxins is a Reality Now
Dr. Themis Michailides, plant pathologist at the UC Davis/Kearney Agricultural Research and Extension Center and former member of the National Aflatoxin Elimination Technical Committee, and Dr. Mark Doster have devoted decades to studying aflatoxins and their work has been instrumental in development and registration of AF36 in pistachio, almond, and fig.
Until now, Michailides notes, tree nut and fig growers had no direct way to combat aflatoxin. Instead contamination has been managed primarily through preventing navel orangeworm damage. While as noted below, effective orangeworm management is still very critical and essential in reducing crop damage, AF36 offers an additional tool that has a direct impact on reducing harmful toxigenic Aspergillus mold strains and the aflatoxin they produce.
Recent Challenges with the use of Aspergillus flavus AF36
AF36 Prevail can result in more than 80 percent reduction of aflatoxin contaminated cotton and corn, but here in California, only once we reached an 85 percent reduction (Figure 3) and this was only in the second harvest (reshakes) pistachios, which have higher risk for NOW infestation and aflatoxin contamination than the pistachios of the main (first) harvest. Results of analyses of a large number of library samples obtained from treated and untreated fields of Wonderful Orchards Company, resulted in a significant reduction of up to 45 percent in aflatoxin contamination. Michailides’ lab crew is trying to explain why the efficacy of AF36 in reducing aflatoxin cannot match the one reached in cotton fields in Arizona and corn fields in Texas. The following challenges may explain partially these disadvantages of the AF36 product in California: a) inadequate soil moisture and temperatures; b) not correct timing of application; c) delayed harvest, and d) inefficient control of NOW, both contributing to increased aflatoxin contamination; e) arthropod pests of career seed; and f) other predators (rodents, birds, etc.). Recent research projects are focused on addressing all these challenges. One new development is that a new product (Afla-Guard®, manufactured by Syngenta Company) that is registered to reduce aflatoxin contamination in peanuts and corn was introduced in California for experimentation and gathering data to support registration for use in pistachio, almond and fig. If successful, this will be the second product registered in the USA which represents a different from the AF36 atoxigenic strain of Aspergillus flavus and the carrier is barley (Figure 4). Studies done by Drs. Jaime and Michailides (Kearney Agriculture Research and Extension Center) in 2017 and 2018 showed that this product sporulated faster, better, and under lower temperatures and lower soil moisture content than the Aspergillus flavus AF36 strain. Registration of this product in California is anticipated in 2020, so that growers may then have a second tool to choose to combat aflatoxin contamination in 2020.
Figure 4. Another biocontrol product (Afla-Guard®) not registered yet, using barley as the career.
NOW Management is Still Essential
Aflatoxin contamination becomes a major problem in years when damage by navel orangeworm is higher than the standard low level. Relatively studies by Drs. Michailides and Palumbo (ARS (Agricultural Research Service), USDA, Albany, CA) showed that NOW moths are heavily contaminated with spores of aflatoxigenic fungi as soon as they emerge from mummies in early spring. Also as the damage on nuts increases so is the incidence and the amounts of aflatoxins (Figure 6). Therefore, it is essential to keep up with navel orangeworm pest management practices. Growers should use all the available tools for reducing damage by NOW to supplement the mummy sanitation, which should be the first step towards aflatoxin reduction. Reduction of NOW damage can also be achieved by timely harvest, in season insecticide sprays, and winter mammy shake.
Figure 5. Correlation of the sites of feeding damage of almonds by NOW larvae and amounts of aflatoxin contamination.
Where to Find the Product, Learn More, and Application Services
To learn more about AF36 and its application, watch two short videos produced by California Pistachio Research Board. Although they were filmed in a pistachio orchard, the information is useful and accurate for almond and fig growers.
Western Milling is the distributor for AF36 in California. Growers can contact Jeff Chedester, seed business manager, at (559) 302-2593; and Agri Systems, Inc., c/o Brendan Brooks, at 559-665-2100 for product information and application. Distributors for the second product will be provided as soon as it is registered in California.
Acknowledgments:
The author thanks all his collaborators for the dedicated research, the California Pistachio Industry (California Pistachio Research Board), the Almond Board of California, and the California Fig Institute for their continuous financial support of these studies. We also thank the USDA the initial seed grants provided by the Aflatoxin Elimination Technical Committee, and California Department of Food and Agriculture (CDFA) (Grant SCB16054). Special thanks go to Wonderful Orchards for their continuous support of this research by allowing using their orchards for various experiments; also we thank Keenan, Sutton, and Nichols Farms for their support as well. In addition, we appreciate very much the support by Syngenta in 2018 and 2019.
Good quality water is extremely important for agriculture throughout the world. However, due to reduced availability of water and increasing food demands, future use of degraded waters is evident. One of the major concerns of utilizing degraded waters for irrigation is their high salt concentration.
Salinity is one of the main abiotic stresses faced by the agriculture industry. Modest increase of soil salinity level impacts both plant growth and yield by causing several physiological and biochemical changes. Based on salt tolerance level plants are classified broadly in two groups: halophytes and glycophytes. The halophytes have special mechanisms to tolerate high concentrations of salts and therefore can grow in saline environments. The majority of plants (including almonds) are glycophytes and cannot tolerate high salt concentrations and so grow in soil containing low salts. However, among glycophytes, salt tolerance level varies tremendously not only at the species level but also at the variety level within a species. This variation is directly dependent on the functional status of various molecular components that play critical roles to protect plant during salt stress.
In the initial stages of salinity exposure, a plant faces osmotic stress, resulting in ion imbalance in cells, membrane disintegration and reduced photosynthesis. In addition, osmotic stress in the root sends a signal throughout the plant causing reprograming of physiological and molecular activities to initiate defense response against salinity stress. Slowly ionic stress develops, leading to accumulation of Na+ (sodium ions) and Cl- (chloride ions) in plant tissues. High ion concentrations are not only toxic but also interfere with absorption of essential nutrients by a plant. High Na+ and Cl- levels interfere in many molecular, biochemical, metabolic and physiological processes which could also lead to unnatural senescence and cell death. For plant species that are moderately tolerant to salinity, osmotic stress may play an important role. However, for species sensitive to salinity such as almonds, low salt concentration is able to impose ionic stress that may reach to an intolerable level, whereas it may not generate osmotic stress critical for plant growth. Hence, when studying salinity stress in almonds, it is important to focus on responses related to ionic stress and the exposure to salinity should be gradual to avoid any osmotic shock. This also mimics field conditions in an almond orchard, as during spring salt moves to subsurface layers due to the rain and slowly moves to upper layers during summer, which leads to gradual increase in root zone salinity.
Due to a continuous increase in cultivated area under almonds, farmers are forced to utilize marginal lands with low quality saline water for irrigation. In almonds, rootstock plays an important role in regulating plant growth in salinity stressed environment. Hence, the development of new almond rootstocks tolerant to salinity is highly desirable. In last two decades, several studies focusing on screening almond rootstocks for salinity tolerance have been conducted and some tolerant rootstocks have been identified. However, a comprehensive approach to screen and develop new rootstocks with enhanced salinity is missing in almonds.
Impact of Salinity on Water Relations and Photosynthesis
Water uptake by a plant is drastically affected under salinity, which leads to reduced water potential, relative water content, stomatal conductance and transpiration. As plants take nutrients with water, reduced water uptake also decreases tissue concentration of essential nutrients affecting plant growth. In addition, high salt concentrations also affect homeostasis, osmoregulation and net photosynthesis. Photosynthesis is the most critical metabolic process for almonds. In response to salinity, osmotic stress-mediated stomatal closure prevents water loss through transpiration in plants that also restricts the amount of CO2 taken in for photosynthesis. Consequently, stomatal conductance, net photosynthetic rate and amount of chlorophyll are used as physiological parameters to study salinity tolerance in different almond varieties. In a recent study where we compared several rootstocks for salinity tolerance, photosynthetic rate was found to be the most reliable parameter to assess salinity tolerance.
Tissue Ion Composition and Salinity Tolerance
Tissue Na concentration is commonly used as a guide for the salinity tolerance of a variety. However, for some plant species, tissue Na concentration is not a true indicator of salt tolerance of a variety. Never-the-less, for almonds the negative correlation between tissue Na concentration and salt tolerance holds well. Similar to Na accumulation, salt tolerant genotypes stored least amount of Cl in leaf tissue.
Figure 1. Evaluation of salinity tolerance of 16 almond rootstocks irrigated with waters of different ion compositions. Photo courtesy of Devinder Sandhu.
Genetic Control of Salinity Tolerance
In model plants, hundreds of genes have been discovered that play critical roles in salinity tolerance. Salt-stress induced signaling pathways have also been well-dissected. Molecular mechanisms of salt tolerance or salt sensitivity is largely unknown in almonds. Expression analysis of genes involved in ion transport in almond tissue showed induction of multiple genes involved in Na+ and Cl– transport under salinity treatment, suggesting importance of both Na and Cl during salinity stress. The genes involved in Na+ transport were differentially expressed during salinity stress, compared to the control. For instance, NHX1 (a vacuolar sodium/proton antiporter) and SOS3 (SALT OVERLY SENSITIVE 3 that encodes a calcium sensor) were upregulated in leaves and on the other hand HKT1 (encodes a Na transporter) was induced in roots under salinity treatment. SOS3 is involved in Na+ exclusion from roots, NHX1 plays role in sequestering Na+ in vacuole and HKT1 is critical for retrieving Na+ from xylem back into root to protect leaves from salt toxicity. Additionally, CLC-C (chloride channel C) and SLAH3 (encodes a slow-type anion channel) that are important for Cl– transport, were highly upregulated in salinity treatments in almond roots. These observations confirmed the role of multiple component traits in salt tolerance mechanism in almonds. As seen in other plants, multiple signaling pathways and various genes are expected to be involved in establishing ionic homeostasis during salt stress. Nevertheless, there are not many studies focusing on understanding the roles of organic solutes and enzymatic or non-enzymatic antioxidants in mitigating effects of salinity in almonds. Although some genes involved in ion exclusion, and ion sequestration in vacuoles have been identified in almonds, future studies are warranted to identify additional genes. In addition, different scions should also be compared for the genetic variation involved in ion homeostasis and scavenging reactive oxygen species (ROS) produced during salinity stress, which may provide some insights into sensitivity of almonds to salinity.
Contrary to studying the importance of a few genes in salinity stress at a time, an RNA-seq based approach compares global changes in gene expression between the control and the salinity treatments. In addition to targeting genes already characterized in model plants this strategy can link different pathways involved in salinity tolerance and identify specific almond genes contributing toward salt tolerance.
Can Alternate Approaches Mitigate Harmful Effects of Salinity?
Many additional strategies have been reported in plants that have been implicated to improve salt tolerance level in response to salt stress. For instance, application of certain microbes could improve salt tolerance level of plants. Arbuscular mycorrhizal fungi (AMF) are known to form symbiotic associations with many land plants that considered to be valuable to plant growth. AMF helps host plants not only by providing essential minerals but also by impeding the translocation of toxic ions like sodium. The use of AMF in multiple plant species has shown enhanced growth, development and productivity under salt stress. Although, different Prunus rootstocks have been screened for mycorrhizal colonization, direct effect of AMF in mitigating salinity still need to be established.
Application of plant growth promoting rhizobacteria (PGPR) is also known to improve salt tolerance in different plant species. However, there are no published reports describing effect of PGPR in improve salt tolerance in almonds.
Although, AMF and PGRP show a lot of promise, the potential of their application on almond rootstocks to mitigate salinity stress needs to be explored further, along with the economic feasibility of these approaches at the commercial level.
Future Perspectives
One of the main consequences of the climate change is the length and frequency of drought periods experienced in certain part of the world. California, the main almond producing region of the world, experienced a long drought period in the recent past. Drought leads to excessive groundwater pumping and use of alternative water resources with high salinity for irrigation. Based on the current trends, salinity problem is expected to intensify in next couple of decades. Currently, salinity screening is taking a backseat in almond rootstock breeding, which is expected to change in the near future. One of the approaches for the future almond breeding programs will require screening of wild genetic material for salinity tolerance. In addition to the other important rootstock traits such as high vigor, nematode resistance, disease resistance, insect resistance, drought tolerance, salinity tolerance should also take central stage during rootstock breeding.
Identification and isolation of the key almond genes involved in salinity tolerance will be critical. Functional validation of selected almond genes by complementation assay in a model plant like Arabidopsis may provide an initial proof of functional conservation of genes between these species. Characterization of genes will facilitate identification of specific mutations that are critical for salinity tolerance. The CRISPR/Cas9 system has a great potential in fixing the both type of genes that play positive or negative roles in salt tolerance in almonds. The CRISPR/Cas9 is a precise, suitable, and efficient technology that has been used for genome editing in various crops such as rice, wheat, maize and sorghum. It is important to note that CRISPR/Cas9 modified crops are not considered as genetically modified organisms (GMO).
Identification and characterization of genes regulating ion uptake, effective compartmentalization, and tissue tolerance may provide new means to develop almond varieties with enhanced salinity tolerance.
Disease Management
