Delayed and erratic bud break from DSG vines (courtesy G. Zhuang.)
After 2021 grapevine budbreak, we received many calls about dead spurs, delayed bud break, stunted shoot growth and poor fruit set. In Fresno County, some severely impacted vineyards suffered a substantial yield loss. In many cases, the problem was delayed spring growth (DSG), and the classic vine symptoms include:
Delayed and erratic bud break
Stunted shoot growth
Excessive berry shatter and poor fruit set
The situation was apparently spread across different grape growing regions in California, and UC Davis Department of Viticulture and Enology held a virtual grower meeting to discuss it (the recorded presentation can be found on the UC Davis AggieVideo website.)
Delayed Spring Growth
Grapevine DSG is associated with insufficient rehydration of the vines and may be due to vascular tissue injury, insufficient carbohydrate reserves, excessively dry soil over winter or some combination of these factors. Symptoms can result in significant yield loss and permanent vine damage, resulting in economic hardship for growers. Some vine DSG symptoms are similar to other pest/disease symptoms (e.g., vine trunk disease or soil pests like nematodes/phylloxera.) However, most vineyards we visited had little or no sign of trunk disease or soil pests. Several factors this past fall and winter contributed to widely observed and severe vineyard DSG symptoms:
Ongoing drought and increasingly dry soils, especially over winter in vineyards which were not sufficiently irrigated postharvest, or during winter
Warmer than normal fall temperatures, including a particularly warm October
A sudden freeze in early November
According to CIMIS station data at Five Points, October 2020 was warmer than the last five years’ average and followed a sudden freeze event in November (Figure 1), and warmer-than-normal autumn is a risk factor for DSG. In addition to the November freeze event, October and November 2020 were mostly dry, and a drier autumn could make the freeze worse. Even though the minimal temperature of the 2020 winter might be lower than the last five years’ average according to the CIMIS station data, the DSG’s occurrence and severity varied significantly across vineyards in Fresno County and other parts of California.
Figure 1. Daily minimal temperature from September 2020 to April 2021 at CIMIS Station near Five Points, Calif.
Also, vineyard management, particularly postharvest and winter irrigation, could make a big difference on the results of DSG even if the ambient weather condition was similar. The geographic location as well as vineyard microclimate can sometimes mean quite different consequences in the face of freeze damage. Figure 2 illustrates the variation of daily minimum temperatures from five locations in Fresno County. Typically, vineyards located on the west side had a lower minimal temperature and suffered more freeze damage than vineyards located on the east side. Sand Ranch in particular had the lowest daily minimum temperature among five locations from October to March.
Figure 2. Daily minimal temperature from September 2020 to April 2021 at five UC IPM weather stations in Fresno County.
To make matters worse, Fresno County saw much less precipitation during the months of November and December 2020 than the last 20 years’ average (Figure 3). These drier months might offer the perfect conditions for DSG. Although precipitation amount was normal in January 2021, February was yet another dry month in comparison to historical averages. Lack of soil moisture before bud break is another major risk factor for DSG.
Figure 3. Monthly precipitation from October 2020 to March 2021 in Fresno County.
Grapevine winter freeze damage and DSG have similar symptoms and can be difficult to differentiate. Winter cold damage or freeze injury damages vascular tissues and can thus interfere with water, carbohydrate and mineral translocation, causing symptoms similar to DSG. A lack of soil moisture can impair vine rehydration, making vines suffer water stress and causing DSG symptoms directly. Additionally, vines might be more vulnerable to cold injury even though the minimal temperature in the past winter, such as 25 degrees F at Sand Ranch, might not cause significant freeze damage on most Vinifera grapes.
Maintain Vine Health
Vineyard conditions should be considered to avoid DSG and possible cold damage:
Abiotic or biotic stressed vines (e.g., severe water stress and overcrop, nutrient deficiency, pest/disease)
Young vines
Late ripening and cold tender varieties
Certain rootstocks, including Freedom and Harmony
Insufficient soil moisture during the dormant period (e.g., October to March in the San Joaquin Valley)
Generally, maintain vine health over the growing season and assess soil moisture as the vines enter dormancy, watering if needed. Too many clusters with not enough leaf area can weaken the vines and deplete the trunk and root’s carbon reserves, which are needed to maintain respiration over winter, help prevent freezing and nourish the vines as they regrow in the spring. To maintain a functional vine canopy, irrigate as necessary to support photosynthesis without stimulating excessive growth. If pests or diseases are present in the vineyard, such as powdery mildew, nematodes, grapevine trunk disease, virus, mites and leaf hopper, a good assessment of canopy health is important. Grapevines with severe defoliation or small canopies will be of great concern, and management should focus on better addressing pest and disease problems to avoid early defoliation.
A young vine has its inherent nature of vulnerability due to a lack of sufficient carbon reserves. Therefore, severe water stress and overcropping should be avoided, and irrigating the soil before a freeze event (e.g., late October and early November) can be greatly beneficial to provide heat protection for young vines.
This past spring, we noticed some susceptible varieties might suffer greater damage from DSG, and that has been consistent with the reports from other growers. Chardonnay and Pinot gris have been reported frequently on DSG, although both varieties are also susceptible to winter freeze.
Rootstock can also play an important role in DSG. Certain rootstocks (e.g., 5BB and Freedom) are more susceptible to DSG than others (e.g., 1103 P), according to results of UCCE rootstock field trials in different growing regions of California. Thus, growers who have the susceptible rootstock might want to take extra care of the vines, such as irrigating the soil during the dry winter, so that the risk of potential DSG might be minimized.
Manage Soil Moisture
Last but not least, lack of soil moisture might be the most important yet manageable factor contributing to most DSG farm calls. As discussed previously, drier October, November, December and January months posed a great risk of DSG as well as inhibited rehydration of the vines, which can also lead to a greater risk of freeze damage. However, water availability during the drought years might be significantly reduced or expensive.
Therefore, irrigation during the drought years can become the dilemma. Growers need to balance the cost and reward of irrigation vs. no irrigation during drought years. Greater than 20% yield loss has been reported for some vineyards in Fresno County in 2021, and DSG might play a large role in it, although the record summer heat and seasonal variation could also result in loss. Finally, the consequence of DSG on Fresno vineyards varied greatly. Some vineyards appeared to be significantly stunted after budbreak and later fully recovered due to irrigation. Some vineyards might suffer multi-year yield loss due to the weakened canopy and few desired canes to prune.
In the face of upcoming potential drought, growers can use multiple tools to reduce or eliminate the effect of soil moisture deficit. Many tools (e.g., shovels, soil augers, moisture sensors) can provide great benefits for assessing soil moisture and help growers determine whether or not to irrigate. Weather stations can also provide great amounts of information regarding the minimal ambient temperature as well as the amount of local precipitation, since temperature and precipitation can vary greatly from one vineyard to another. UC IPM has seven weather stations in Fresno County and one station in Madera County in cooperators’ vineyards, and those stations can offer both temperature and precipitation amount, serving the growers whose properties are nearby the station.
Though vineyard managers continually face many challenges to optimal productivity, Pierce’s Disease (PD) represents a particularly formidable threat due to limited options for effective prevention and control. PD is a degenerative, deadly and costly disease of grapevines caused by Xylella fastidiosa subsp. fastidiosa (Xff) bacteria, Gram-negative rod-shaped microbes with characteristic large pili. Though hosted by many plant species, these bacteria are easily spread to grapevines by insect vectors such as blue-green and glassy-winged sharpshooters (Figure 1, see page 41). Xff colonize the gut of sharpshooters and are transmitted to grapevines as the insects feed on vines.
Pierce’s Disease: A Major Threat
Once inside a grapevine, Xff bacteria impede the normal function of xylem tissue (transport of water and nutrients ‘up’ from roots to stems and leaves.) This damage induces the characteristic chlorosis and scorching of leaves, causing early symptoms of PD which mimic water stress. However, the insidious and cumulative damage caused by PD eventually kills entire vines in one to five years.
Figure 3. Viral bacteriophage particles of XylPhi-PD precisely targeting their bacterial host (photo courtesy Inphatec.)
PD represents a major threat to U.S. wine regions, accounting for widespread economic damage (e.g., roguing and replanting of vines, low fruit production, etc.) and costly deployment of resources aimed at disease moderation. PD has been reported in 28 California counties, covering most of all wine-producing regions. State Extension teams in Texas, Arizona and North Carolina have reported significant outbreaks in 2021. Lost production and vine replacement has been estimated to cost grape producers about $56.1 million annually as of 2014. Further, a 2016 survey of nearly 200 growers and managers in Napa and Sonoma counties revealed that 73% of respondents identified PD was one of their top three management problems.
Figure 1. Blue-green sharpshooter (top) and glassy-winged sharpshooter (bottom), two of the main insect vectors for spread of Xff (photos courtesy Inphatec.)
Few methods for controlling and treating PD have been made available, with efforts historically focused on controlling the sharpshooter vector (e.g., insecticides, trapping, monitoring, inspections) or roguing seriously ill vines (rogue, replant), all of which has achieved only limited success. However, a new option that reduces PD in grapevines is now available.
New Bacteriophage Injection for PD
XylPhi-PD is a novel, OMRI-listed, biological treatment for PD, a cost-effective break-through technology developed exclusively for viticulture by A&P Inphatec (Figure 2, see page 32). XylPhi-PD contains a cocktail of viral bacteriophages (Figure 3, see page 32) that are injected into a grapevine (‘bacteriophage’ are bacteria-killing viruses that selectively infect bacteria but do not infect the eukaryotic cells of plants or animals.) These virus particles enter and destroy Xff bacteria, thus limiting bacterial growth and the xylem-clogging damage to the plant. Hundreds of phage particles can be manufactured inside an Xff bacterial cell after infection by a single phage particle. The Xff bacterial cell eventually dies and releases all newly created phage particles to seek and destroy more Xff bacterial cells (Figure 4).
XylPhi-PD applications are made by injection of the product into the vascular system of grapevines. Applications are made directly into the active xylem tissue of the plant using a pressurized injection device, the Xyleject Injection System (Figure 5).
XylPhi-PD can be flexibly applied as a treatment when disease symptoms appear, as a preventative to protect growing vines, or whenever conditions may lead to disease. As with most any disease situation, disease prevention or treatment early in the disease process provides much better outcomes than treatment of later-stage severe infections. XylPhi-PD is available in 100-mL vials (treats up to 300 mature vines or 600 young vines.) The product has no restricted-entry interval (REI), requires minimal personal protective equipment (PPE) when used in accordance with label directions and is approved for use in organic production (OMRI-listed, Organic Materials Review Institute).
Efficacy Overview
Multiple studies have been conducted to support the development and commercialization of XylPhi-PD, and results have provided much insight regarding the product’s efficacy profile and use strategies. Brief overviews of some of these studies follow:
Greenhouse pilot study (Texas A&M, 2014):
Design: 30 greenhouse grapevines inoculated with Xff. 15 vines treated with XylPhi-PD once at three weeks post-inoculation, 15 vines received only buffer. Visual symptoms of PD assessed 12 weeks post-inoculation.
Results: XylPhi-PD reduced PD incidence by 87%.
Natural infection field trial (Texas A&M, 2015):
Design: 30 Chardonnay and 30 Cabernet Sauvignon vines in an area with high natural PD pressure randomly assigned to each of the four groups. Vines treated zero, one, two or three times with XylPhi-PD during the summer.
Results: Disease incidence in the XylPhi-PD-3X group was significantly reduced by 44% (P = 0.047) compared to controls (10% vs 18%). Activity of vectors positive for Xff was high during the trial period.
Figure 4. Death and rupture of a bacterial cell, releasing newly created phage particles to seek and destroy more bacterial cells (photo courtesy Inphatec.)
Challenge studies, prevention and therapeutic treatment (California University, 2017):
Design: Prevention study – 15 Chardonnay and 15 Cabernet Sauvignon vines treated with XylPhi-PD and then challenged twice with Xff.
Therapeutic study – 30 Chardonnay and 30 Cabernet Sauvignon vines/group challenged twice with Xff and then treated zero, one, two or three times with XylPhi-PD.
Results: Prevention – prechallenge XylPhi-PD significantly reduced incidence of PD symptoms by 75% in Cabernet Sauvignon vines (P < 0.10) and 100% in Chardonnay vines (P < 0.05) vs controls.
Figure 5. Injection of XylPhi-PD into trunk of mature grapevine. Xyleject Injection System (Pulse Biotech, LLC; Lenexa, KS)
Therapeutic – three post-challenge XylPhi-PD treatments significantly reduced incidence of PD symptoms by 90% in Cabernet Sauvignon vines (P < 0.05) and 77% in Chardonnay vines (P < 0.10) vs controls.
Multi-year natural infection field trial (2017-20; Sonoma, Calif.):
Design: Two groups of healthy Zinfandel vines were tracked in a high PD-pressure, organic vineyard for three seasons (Ridge, Lytton Springs). One group (n=71) received XylPhi-PD three times/summer, while controls (n=94) only received buffer.
Results: After three years of treatment, vines treated with XylPhi-PD show much less PD incidence than control vines as assessed by both qPCR (-60%) and visual PD symptoms (-72%). Vines treated with XylPhi-PD also generated higher fruit yields, averaging +1.34 lb/vine (+21%) more than control vines.
4-site, 3-year, Natural Infection Field Trial (2019-21; Sonoma, Calif.):
Design: A three-year, multi-location commercial (Wilbur-Ellis) field study evaluated the efficacy of XylPhi-PD against endemic PD across four sites and three production seasons. The extensive research effort began in 2019 when a study was conducted that involved 400 vines (300 Chardonnay, 100 Pinot Noir) at three Sonoma County commercial wineries with a history of PD (one winery had two test fields) (Figure 6). All four commercial vineyards were historically high PD sites, and despite continual roguing and insecticide use in the past, a persistent reservoir of Xff remained in the vineyards from previous infection cycles. Thus, each site included vines with both early-stage and chronic/severe PD.
Figure 6. Locations of four sites used for three-year field study.
Vines were randomly selected in treatment blocks at each site and assigned to either of two treatment groups as follows:
-Control (untreated): n=200 (50/site);
-XylPhi-PD: three treatments (Jun/Jul/Aug); 80 µL of XylPhi-PD injected twice in the trunk and once in each cordon (4 to 6 injections = one treatment); n=200 (50/site).
Six petioles from each vine were collected in September for analysis by quantitative polymerase chain reaction (qPCR) and confirmation of Xff infection. All study vines were also visually assessed by trained observers for PD development. Insect traps were placed at each study site in an attempt to monitor vector pressure.
A continuation of the same study protocol was followed in 2020 and 2021, allowing two additional seasons of treatments and observations for the same vines/blocks at the same sites/wineries. In addition, treatments and observations of additional vines were initiated in 2020 at each site (n=50/site, 200 total), repeated again in 2021 (Figure 7). As a result, three groups emerged from the study for tracking and evaluation:
Figure 7. Study design and timeline for three-year multi-site field study.
Vines with three years of treatment (three-year treated, n=200)
Vines with two years of treatment (two-year treated, n=200)
Non-treated controls (n=200)
Results
Comparative outcomes for vines qPCR-positive for Xff are summarized in Figure 8, see page 36. Under the conditions of only mild to moderate PD pressure and low vector populations, sequential year-to-year use of XylPhi-PD generated impressive results. Incidence of Xff positivity fell 24% and 45%, respectively, for vines receiving two or three years of treatment compared to untreated controls. Improvement in the three-year group was significant (P < 0.05) vs controls. The few vines remaining Xff-positive in the two- and three-year groups were chronic infections that would be rogued.
Figure 8. Vines qPCR-positive for Xff, or vines showing visual signs of PD. Summary of four sites in Sonoma County.
The visual assessment of vines for signs of PD was another important study parameter, and outcomes (Figure 8) were similar to those using qPCR confirmation of infection. Vines treated with XylPhi-PD for two years generated a 27% reduction in visual PD incidence compared to controls, while vines treated for three years showed double the benefit, a significant 54% reduction (P < 0.05) of PD incidence. The similarity of these data with qPCR results suggests that visual assessments can help vineyard managers tangibly gauge the efficacy of XylPhi-PD.
Figure 9. Average fruit yield for Chardonnay vines at one site (D).
Notably, XylPhi-PD continued to protect against PD infections at all four trial sites for the three years of observations, with no new infections detected in the three-year treated group. In contrast, the control group had ~2% to 4% new infections.
Fruit yield was measured at one study site (D) at the study’s conclusion in 2021 (Figure 9, Chardonnay, eight- to ten-year-old vines). Compared to untreated controls, vines in the group treated with XylPhi-PD for three years averaged 5.1 lb (+17%) more fruit per vine than controls. Vines treated for two years were intermediate (3.1 lb, +10% more fruit vs controls).
Usage Recommendations
XylPhi-PD can be applied as a preventive treatment to protect growing vines, as a therapeutic treatment when disease symptoms become visible, or anytime production conditions may lead to disease pressure.
Table 1. XylPhi-PD dosage options and number of applications per vial (100 mL).
Locations selected for application of XylPhi-PD can be based on the age of the plant, the pruning style and/or the training system utilized for the plant. The product is to be injected into the active xylem vascular tissue above ground level. Two examples of injection strategies appear in Figure 10, see page 37. On established vines, for example, one or two injections can be applied in the trunk with an additional injection in each cordon or spur. For young, recently planted or radically pruned vines, apply two or three injections into shoot, two to six inches above the ground. For all scenarios, the total number of injections administered to a vine define one ‘application’ of XylPhi-PD.
For most production situations (medium to high PD pressure), two or three applications of XylPhi-PD are recommended during each growing season at near-monthly intervals (Figure 11, see page 37). This frequency of application has been demonstrated to provide optimal PD control under various levels of PD pressure. The volume of XylPhi-PD administered can also vary depending on the age of plants being treated and PD pressure.
Figure 11. Examples of XylPhi-PD application programs involving medium to high PD pressure.
The quantity of XylPhi-PD used in XylPhi-PD treatment programs can vary based on the number of injections/vine, the concentration of product/injection and the number of applications/year. Growers and PCAs have options and flexibility to match doses and the number of applications to their specific conditions and risks. Table 1 summarizes some of these options and their impact on the amount of XylPhi-PD used and number of vines treated per 100-mL vial.
Treatment Strategies for PD Management
Several recommendations for managing PD across an entire vineyard have emerged from field use experience with XylPhi-PD. These recommendations largely depend on the scope and distribution of PD in a particular vineyard or block.
Figure 12. Strategies and options for treating areas within vineyards or blocks.
As usual, vines demonstrating severe and/or chronic PD infection should be rogued per IPM protocols.
Vines appropriate for XylPhi-PD treatment should be identified.
Figure 13. Examples of visual damages caused by PD (photos courtesy Inphatec.)
These appropriate vines should be treated with recommended doses of XylPhi-PD at two or three seasonal applications as needed for mature vines or replants (per label directions).
It is the second step, identifying which vines to treat, that can sometimes pose a quandary for production managers. To help in this process, three strategic options can offer direction for developing a customized plan for vineyard-wide PD management. Based on evaluations regarding the spread of infection and site specifics for a particular vineyard or block, a treatment strategy can be selected from the following (Figure 12):
Targeted buffer zone: Treat all vines in any defined area with high PD activity (>2% symptomatic vines) and/or vector activity, such as near a riparian area.
Precision spot: In large areas with few symptomatic vines, mark/identify affected vines and treat only them and their immediate surrounding neighbors.
Entire block: In large areas with multiple scattered symptomatic vines, in the presence of vectors, full-block treatment is needed to reduce overall PD pressure.
In addition to these three options, the duration of treatment (multiple years) is also a critical consideration. As discussed earlier, field studies have demonstrated the cumulative value of consistent treatment with XylPhi-PD over multiple years for reducing the level of PD in a vineyard or block, even when under low PD pressure. As always, prevention of severe, chronic disease is the best approach, and consistent long-duration use of XylPhi-PD appears to substantially diminish PD progression and pressure in vineyards.
Identifying PD
Some of the visual signs of PD damage are presented in Figure 13, including characteristic chlorosis (leaf scorching), irregular lignification and berry shriveling, and ‘matchstick’ petioles. In general, PD is likely present in a vineyard if the following four symptoms are observed late in the season:
Leaves scald in concentric rings or in sections
Leaf blades abscise, leaving petioles attached to the cane
Bark matures irregularly
Fruit clusters shrivel or raisin
However, damage caused by various other diseases, water stress, pests or nutrient deficiencies/imbalances may produce symptoms similar to those caused by PD. Therefore, PD must be laboratory confirmed by detection of Xff by qPCR testing on late-season/fall petioles (e.g., UC Davis Foundation Plant Services, Texas Plant Disease Diagnostic Lab, Arizona Plant Diagnostic Network).
Conclusions
PD is an extremely challenging problem for wine producers and their consultants. A biological control approach with XylPhi-PD offers a fresh opportunity to help manage PD and limit losses associated with the disease. In multiple studies, XylPhi-PD treatment of diverse wine varietals prompted reductions in PD incidence and/or severity under conditions of both natural and challenge infection with Xff. These favorable outcomes distinguish XylPhi-PD as a targeted and cost-effective strategy for effectively protecting valuable vineyards against PD.
Always read and follow product label directions. Not registered in all states. EPA Reg. No. 93909-1. Operators of injector must undergo training and be certified by Pulse and must follow instructions in device manual. XylPhi-PD is a trademark of A&P Inphatec. Xyleject is a trademark of Pulse Biotech.
Figure 1. Aerial vineyard view taken in 1985 showing sand streaks that produce weaker vines that often produce lower yields than the other parts of the vineyard. Aerial imagery taken today can be taken with specialized sensors that provide additional data for assessing a vineyards condition (photo by L. Peter Christensen.)
The 2021 season was a challenging year for western grape growers. After an unusually dry, cool winter, many growers observed poor shoot growth and various fruit maladies from bud-break through harvest. Delayed spring growth, vine mealybug and leaf hopper infestations, heat waves and water shortages were just a few of the issues that caused problems for grape growers.
Farmers and viticulturists were busy trying to diagnose various vineyard problems from shortly after bud-break through leaf fall. Diagnosing vineyard problems as soon as possible is important for minimizing yield and quality losses, especially when caused by pests that can be managed before moving into neighboring vineyards. However, when a quick diagnosis cannot be made, it is important to survey the vineyard and collect as much information as possible so damage can be mitigated.
Prompt Data Collection
When trying to diagnose abnormal plant growth, it is important to collect as much data as possible near the time that it was first observed. Data in the form of dates, symptoms (e.g., abnormal growth), signs (e.g., insects), records, etc. are important for you and those you may consult with in determining the cause of poor growth. Table 1 highlights important information to collect for proper diagnosis. The most basic information focused on vineyard characteristics, such as variety/rootstock combination, weather, soil, irrigation type, grape tissue analysis, etc., are typical data needed to solve vineyard problems. Sometimes, it can take days or weeks of consideration to determine what has affected the vines. However, there are times when a problem remains unsolved, even after data have been collected and reviewed by multiple experts.
Table 1. Data collected to help identify vineyard problems
After basic vineyard characteristics, reports (see Table 1) of varying types are important pieces of information for deciphering vineyard issues. Phenological dates, yield and quality, fertilizer and pesticides used, etc. give clues about what has happened this season and in past years that may correlate with a particular issue. The more reports that are available for review, the easier it will make solving the problem.
Photos are a great tool because they can easily and quickly be shared via email or text. Today’s phones are equipped with a camera that can capture both photos and video that make documenting vineyard problems easier. However, there is a difference between a photo and a great photo that can help solve the cause of symptoms. Great photos show detail that can aid in determining the cause of symptoms. For example, a picture of a single leaf on the bed of a truck probably won’t help identify a problem. A more useful photo would highlight multiple symptomatic leaves showing their location on the grapevine and in the vineyard, which may highlight the root cause of a problem.
Table 2. Some likely causes and symptoms* of certain patterns found in vineyards.
Water, soil and tissue laboratory analysis when taken annually help identify trends over seasons. The best approach is to pull samples at specific times (i.e., growth stages) each year so the information is available when decisions need to be made. For example, taking water samples at the beginning of the growing season will help determine how much nitrogen is coming from irrigation water sources. Once known, planned fertilization rates can be adjusted depending on the amounts in the water. Excess nitrogen applied during the season without knowing what is in well water can contribute to poor fruit quality and may further contaminate the aquifer.
Biotic vs Abiotic
Vineyard issues affecting vine health and fruit production fall into two categories: biotic or abiotic. Issues caused by insects, fungi, bacteria, viruses, rodents, birds and other living organisms are biotic. Abiotic includes non-living factors like climate, soil compaction, water, pH, nutrient presence or lack thereof, etc. Biotic factors tend to be the most common reason for poor grapevine health but aren’t always the primary cause.
Trying to separate symptoms between biotic and abiotic vine health issues can take time to sort through since symptoms can have multiple causes. For example, a vineyard may be showing signs of decline due to root rot infections. However, the real cause may be the presence of hardpan not allowing water to drain. Water accumulating around vine trunks often leads to fungal infection, decline and vine death. Symptoms from both fungal infections and poor water penetration can have similar symptoms (weak vine growth with chlorotic foliage). Identifying the primary cause is important so a solution can be developed and implemented.
Understanding Patterns
Patterns of symptomatic vines are an important piece of information needed to solve the cause of poor vineyard growth. An easy way to identify patterns is the use of aerial imagery (Figure 1) to survey your vineyard. Aerial imagery has improved tremendously and can help detect differences in vine growth, soil and irrigation issues, pest or disease problems and more. Accessing aerial imagery is as easy as using your own drone to capture footage or hiring a licensed pilot or company to take photos and video for you.
Data from aerial imagery can also be transformed into helpful indices such as NDVI, and specialized sensors such as thermal or hyperspectral images provide additional data for assessing vineyard condition. Aerial imagery can help you track irrigation problems or general vine health throughout the season by outlining patterns based on vine growth and leaf condition.
Driving and walking vine rows with a soil map in-hand can be essential for assessing problem areas. Surveying the vineyard on foot or with an ATV allows for a closer look at low-vigor areas, which may give you management clues that can complement aerial imagery. Patterns of poor growth found on vineyard edges, down rows or in small patches are sometimes easily solved when those areas are visited on foot. For example, Figure 2 shows multiple vines within a row that are weak or dead. At first glance, a disease might be suggested as the cause of poor growth and death. But after further investigation from top to bottom, it was found that the base of the vine and root system had been chewed and girdled by meadow voles. Once the problem was identified, a management plan was implemented, which ended additional vine deaths.
Symptom Diagnosis
Diagnosing abnormal growth symptoms is somewhat of an art and a science. When called to identify the cause of poor or unusual growth, a vineyard diagnostician must consider all the scenarios that might be causing symptoms.
A methodical approach that results in an accurate diagnosis is important since time is of the essence when considering management strategies to minimize yield and quality losses. Variety, rootstock, vineyard age, soil type(s) and depths, irrigation methods and timings, nutrition, common pests, diseases, climate and many other factors must be considered. A systematic approach must be taken to identify the primary cause that includes symptoms found in the vineyard, including related reports, photos and laboratory analyses.
Diagnosing grape pests or diseases can be easier than abiotic causes since three factors need to be present: the pest or disease, grape (i.e., host) and favorable environmental conditions.
Figure 2. Multiple vines in a row displaying discolored foliage that looks to represent a virus infection. However, a closer look (photo inset) reveals that the base of the trunk and larger structural roots have been chewed on by meadow voles. As a result of the chewing, the vines were girdled and displayed reddening symptoms, which is similar to a virus infection (photo courtesy S. Vasquez.)
Often, the lack of optimal climatic conditions do not allow for pest or disease outbreaks. In contrast, the cause of abiotic symptoms is more difficult to identify and costs time and resources. Once pests or diseases have been eliminated as causes, a diagnostician must spend time reviewing reports, lab analysis and walking the vineyard looking for patterns and additional clues. A shovel, shears, soil probe, and a lot of time will be needed to narrow down the cause.
Finding a Trusted Advisor
Solving vineyard problems that are negatively impacting yield and quality can be daunting, especially when you are working alone. At some point, you may need to consult with an expert. Finding a trusted advisor can also be daunting because there’s often a cost associated with hiring someone. Here are some considerations for finding and hiring a CCA, PCA, private consultant or agricultural forensic consultant.
Before hiring someone, clear goals need to be identified so they can be shared with a prospective consultant. First, what are the yield and quality goals for the vineyard and are they being impacted by the problem? Are you trying to determine what has reduced vineyard performance or what has killed vines? Second, do you want solutions to end the loss of vines and stop the yield decline? Finally, do they have the experience needed to help you solve your vineyard problem? Are they focused on your success or are they just interested in selling you something that will “correct” the problem? If it is a challenging problem, do you feel confident that they will tell you the truth if they don’t know what the cause of poor growth is in your vineyard? If you don’t already have a trusted CCA or PCA, it may take some time to interview a consultant that you feel confident will help you and have your success in mind.
Soil salinization is caused by excessive accumulation of salts in the soil and is one of the most severe land degradation problems. Globally, salt-affected soils are estimated to be about 2 billion acres and are expected to increase in the future. In California, about 4.5 million acres of irrigated cropland (more than half) are affected by salinity, causing significant problems to the state’s agriculture. When salinity levels exceed critical thresholds in the soil, the plants cannot reach their full genetic growth potential, development and reproduction.
Causes and Consequences of Soil Salinity
Soil salinity can be related to the origin of the soil (e.g., in soils from land that was once submerged under the level of the sea or a lake.) It can also be caused by humans. Irrigation in dry areas exacerbates the problem as water contains salts that are left in place after evapotranspiration, and there is not enough rainfall to help with the leaching (i.e., the process of washing off excess salts from the surface toward the deeper layer of the soils and out of the reach of plants.)
Figure 1. Effect of severe salt stress on young and mature grapevines (all photos courtesy L. Brillante.)
Salinity can be caused by excess in different kinds of salts, including table salt (NaCl), potassium chloride (KCl), etc. Table salt is the most common and most problematic. It is composed of sodium ions (Na+), which negatively impact soil physical-chemical properties and create an osmotic stress in plants, and chloride anions (Cl-), which are toxic to plants.
Accumulation of sodium in soils causes swelling of clays, destroys soil structure and reduces infiltration and water holding capacity (Figure 2). This reduces the oxygen & water availability to roots. Excess of salts in the soil generates osmotic stress, which affects grapevine physiology and performance. At low rates, it is a chronic problem that disturbs grapevine water relations, causes stomatal closure in grapevine and reduces leaf and crop size. At high rates, it can create toxicity problems that can lead to premature leaf senescence and plant death (Figure 1, see page 22).
Salt buildup can result in three types of soils: saline, saline-sodic and sodic. Salinity & sodicity terms are used interchangeably very often. However, salinity refers to the concentration of salts (of all kinds) in the soil. Sodicity is associated with the proportion of sodium in pore water or adsorbed to the mineral surface. In saline soils, there are enough soluble salts of all kinds to injure plants. Sodic soils are low in soluble salts but relatively high in exchangeable sodium. Saline-sodic soils have a high content of salts and high content of sodium relative to calcium and magnesium salts.
Figure 2. Effect of salt stress on the soil, showing reduced infiltration, surface crust formation, and clays’ dispersion.
The electrical conductivity of soil extracts can measure soil salinity, as with more salts in the water, it is easier for the current to flow. Sodicity of soil is indicated by the exchangeable sodium percentage (ESP), which is the soil cation exchange capacity occupied by sodium, or by the measure of sodium adsorption ratio (SAR), which represents the amount of sodium with respect to calcium and magnesium. Saline, sodic and saline-sodic soils can be differentiated according to their physical-chemical properties. Soils with EC > 4 dS/m, SAR <13, pH <8.5 are classified as saline soils; soils with EC <4 dS/m, SAR >13, pH >8.5 are classified as sodic soils; and soils with EC >4 dS/m, SAR >13, pH <8.5 are classified as saline-sodic soils.
Alleviating Soil Salinity
Alleviation of salt-related problems is of crucial importance to reduce the impact on crop performance and ensure the profitability of agriculture. This can be done by decreasing the amount of Na+ ions, destroying soil structure and replacing them with Ca+ ions. Adding calcium helps maintain a favorable electrolyte concentration in the soil solution, thereby preserving the physical and chemical properties such as structural stability and clay flocculation, which encourages better root penetration and air and water movement through the soil. Ca2+ ions have a much higher flocculating power than sodium and potassium ions due to their charge and size. Thus, Ca2+ ions have a higher affinity for clays and can easily replace Na+ ions during soil reclamation practices to improve soil properties.
For this purpose of alleviating salt buildups, soils are treated with calcium-based amendments, the most common being the application of gypsum (CaSO4·2H2O). Other products, such as calcium chloride (CaCl2), calcium carbonate (CaCO3) and sulphuric acid (H2SO4), can also be used for reclamation of saline soils but are not as effective as gypsum, and they are destined to specific conditions.
Calcium chloride contains chloride anions that are known to be toxic to plants and, in particular, grapevines. Sulphuric acid does not directly contain calcium required to replace the Na+ ions in the soil. Hence, it is only effective in calcareous soils (rare in the U.S.), where it can react with the calcium carbonate to free the calcium already in place. Calcium carbonate (CaCO3), or lime, is alkaline and significantly less soluble than gypsum. It is generally used to reclaim acid soils. It cannot provide sufficient Ca2+ ions for effective Ca2+- Na+ exchange as it can free Ca2+ ions only in acid soils, while salt-affected soils are generally alkaline.
Gypsum as a Solution
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate with the chemical formula CaSO4∙2H2O containing two molecules of water inside every single crystal.
Calcium sulfate can also be found in two more phases: calcium sulfate anhydrite (CaSO4), which does not contain water of crystallization, and calcium sulfate hemihydrate, also called bassanite (CaSO4∙1/2H2O), which only contains a half-molecule of water per crystal. Severe pressure, temperature and other natural conditions make gypsum outcrops lose water molecules and form anhydrite and bassanite.
Besides a high concentration of calcium and sulfur, pure-quality gypsum contains 21% water, giving it peculiar properties (e.g., different solubility) with respect to the other forms of calcium sulfate. Of particular importance under dry conditions is the ability of plants to access and use the water in the crystalline structure of gypsum.
The use of gypsum crystallization water by organisms is a critical water source for life under dry conditions, as recently demonstrated by Palacio et al. 2014. These authors reported that in natural conditions and during summer, the sap of shallow-rooted plants is 70% to 90% derived from gypsum crystallization water. The extraction and use of water molecules from gypsum by plants is accelerated in hot temperatures. Microporosity of gypsum also offers protection to the soil microbes and promotes relatively abundant and diverse microbial life in dry conditions. Soil microbes may in turn increase the availability of inorganic compounds to plants.
Figure 4. Amount of calcium in the soil solution in the year after the treatment application. The soil samples were extracted with the ammonium acetate method and measured with MP-AES. See the text for treatments corresponding to code.
Gypsum is best applied routinely, as frequent irrigations leach out calcium from the root zone. This is even more important when water for irrigation is alkaline. If gypsum is not applied regularly and calcium content decreases in the soil, the soil tends again to get compacted and the infiltration of water slows down, creating stress to plants.
Figure 5. Infiltration rate measured by the single ring infiltrometer method in the year after the application. See the text for treatments corresponding to code.
It is recommended to spread gypsum on soils as an application through low-volume irrigation sources (i.e., drip and sprinklers) and is shown to work best with irrigation water of low salinity, i.e., about 0.1 ds/cm. Liquid gypsum application can increase the water infiltration to greater depths under the emitters over time due to soil particle binding and aggregation properties of calcium. When gypsum is applied through drip lines with irrigation waters of high bicarbonate content, it requires cautionary measures and lowering of pH (<6.5) because of the formation of calcium bicarbonate precipitates.
Gypsum Research
With the intent of clarifying the effectiveness of gypsum to reclaim salt-affected soils and to provide application guidelines in vineyards, we performed a three-year-long project where we monitored the response of soil physics, grapevine physiology and fruit composition to different dosages and forms of CaSO4 (anhydrite, CaSO4 & gypsum CaSO4∙2H2O) in synergy with organic matter.
Figure 6. Yield per vine (1kg = 2.2 pounds) in the year of treatment application (2020) and in the year after application (2021).
The experiment was performed from 2019 to 2021 in a Merlot vineyard located in a fine loamy sodic soil on the southwest side of Bakersfield, Calif. After the first season of measurements (2019), to ensure no differences across treatments before application, we broadcasted chemical amendments in winter 2020 in bands under the vines (Figure 3, see page 26). The experiment was a completely randomized block design with six treatments replicated four times, and each replicate was 0.2 acres large.
The treatments were applications of gypsum at different rates: 2.5 t/ac (50% reclamation rate, code 50G), 5.1 t/ac (100% reclamation rate, code 100G), 10.2 t/ac (200% reclamation rate code 200G), application of anhydrite at 5.1 t/ac (100% reclamation rate, code 100A) and addition of compost to gypsum (5.1 t/ac gypsum + 1t/ac compost (code 100GC)). We included one untreated control (code CTRL). All treatments were applied in one single application at the beginning of the experiment.
Calcium amounts in the soil extracts (obtained after treatment of soil samples with ammonium acetate) increased in all treatments with respect to the control one year after the application, both in the top eight inches and at a depth between 8 and 16 inches. We observed the largest increase in the 100G, which had 28% more exchangeable calcium ions on the soil particles than the control in the top eight inches and 15% more in the 8- to 16-inch depth (Figure 4, see page 26). Sixteen months after the application, we started observing effects on water infiltration rate in the soil, with the untreated control showing the lowest water infiltration rate. The 200G and 100GC showed the highest infiltration rate with 65% and 63% improvement over the control, respectively (Figure 5, see page 26).
We did not observe notable differences in plant water status measured by a pressure chamber (water potentials), neither in photosynthesis nor transpiration across the treatments. In the two years after the treatment, we observed higher yields in the 200G and 100GC with respect to all other treatments and the control (Figure 6). The response was more robust in the year of the application, in particular for the 100GC, with plants producing 15% higher yield than the control, but only 2% more than the control in the second year. At the same time, the effect of the 200G was more consistent and was higher (8%) with respect to the control the same year as well as the following year (5%). The other treatments did not have positive effects on yield. We did not observe significant differences in sugar content (Brix), pH or titratable acidity of musts across treatments.
This trial showed that gypsum effectively reduces the adverse effects of salt stress on soils and vines by increasing calcium ions at the surface of clays. The increase of calcium ions improves soil structure with positive returns on infiltration rates and the increase in soil health promotes grapevine performance and increases yield. The addition of compost enhances the positive effects of gypsum.
By quantifying the metabolically active and available nutrients in the sap and assessing their balance, growers are able to determine not just if nutrient deficiencies exist but also the future potential for nutrient deficiencies (photo by Cecilia Parsons.)
Leaf sampling in agricultural crops is a long-practiced sampling method where the analysis of the collected tissues is used to assess crop nutrient status. Sampling whole leaves, drying, grinding, digesting and then analyzing the sample for nutrient levels has aided farmers in managing their crop nutrition programs and optimizing crop yield.
This method, however, has some inherent limitations. Primarily, the results of the analysis are providing nutrient levels for ALL nutrients in the sample, including those that are structurally bound within cell walls, the leaf surfaces (cuticles) and organelles. While the analysis is accurate in quantifying the nutrient levels in the sample, these bound nutrients are largely immobile and unavailable to developing leaves and fruit.
Additionally, any nutrients found on the outside of the leaf or embedded in the leaf cuticle are included in the results. As an example, if a calcium carbonate material were applied foliarly to a crop and then tissue samples were pulled, the analysis would demonstrate that our tissue’s calcium levels increased. On the other hand, calcium carbonate materials have very poor foliar uptake and are commonly used as solar protectants or sunscreens. As a sunscreen, it is necessary that the material remains on the leaf surface to do its job, but a standard tissue sample analysis cannot differentiate between “in” or “on” the leaf. Further, tissues being prepped for analysis may be rinsed or washed in an attempt to alleviate leaf surface contaminants. What is commonly overlooked with this practice is the effect that rinsing can have on nutrients within the leaf. For instance, potassium, calcium, magnesium, manganese, nitrogen, phosphorus and zinc can all be leached to varying degrees from the leaf tissue with water.
Now, if the grower is analyzing the leaf tissue because it is the crop, then knowing the nutrient levels of the entire leaf structure and surface is appropriate. However, many growers are not selling leaves but are using the leaves as the machinery to develop the structure of the plant and produce the end crop. Whether that end crop is a tuber, fruit, nut, seed or simply a flower, knowing the number and quantity of nutrients available for assimilation within the plant as well as the balance among these nutrients may spell the difference between a mediocre crop and a stellar crop. Knowing and adjusting the nutrient balance is crucial to nutrient performance and preventing fruit nutrient disorders which can impact crop storability and shelf life. Other characteristics such as fruit sugar levels, size and color also can be positively impacted by proper nutrition.
Forward Looking Analysis
An alternative method to leaf tissue analysis is sap analysis. With sap analysis, leaves are sampled in sets with new leaves and old leaves collected separately without petioles. At the lab, a proprietary process under the NovaCrop brand then extracts the sap from the leaf. This process is done without rinsing, drying, grinding, cutting or crushing the leaves and the extracted sap is largely free from leaf structural components and surface contaminants. This results in the extracted sap being more similar to a blood sample than the biopsy approach akin to classical tissue sampling and analysis. In addition to analyzing the samples for 19 nutrients and five other nutrient and metabolic indicators, having the sap of both new and old leaves analyzed separately allows for the comparison of nutrient uptake, mobility and remobilization within the plant. This comparison is also valuable for assessing the movement of sugar in the plant, which is the plant’s initial building block and energy source.
Minerals, sugars and nitrogen-containing compounds, such as amino acids and proteins, found in the sap represent the majority of plant nutrients that are immediately available for use. By intentionally sampling and analyzing leaf sap, the results provide a forward-looking picture of the nutritional environment in which the plant is currently growing. With this information, deficiencies, toxicities and nutritional imbalances can be identified and corrected in a proactive manner, often before their physiological effects are visible. With traditional tissue analysis, the results are providing information largely about what has already happened nutritionally, leading to decisions being reactive in nature. As the demand for agricultural products continues to increase, farmers are often turning to more aggressive fertility programs, which frequently leads to over-applying nutrients or missing the best opportunity for the application.
Both over-applications and mistimed applications of nutrients can negatively affect crop yields and quality. The balance of nutrients within the plant can be upset by an over-applied nutrient. This can happen with foliar-applied nutrients as well as soil-applied nutrients. In some instances, as with nitrogen, it can alter the expression/regulation of genes and lead to a shift in growth toward vegetative and away from fruit development. In other cases, the over-applied nutrient can create nutrient imbalances that present as deficiency symptoms of other nutrients despite adequate concentration levels in the sap. This can be described as like-kind interactions where minerals of similar charge are competing with each other for space within the sap (cations affect cations and anions affect anions.) Physiological responses within the plant to an applied nutrient can modify the uptake or physiological activity associated with other nutrients, either positively or negatively.
Nutrient Availability
By removing unavailable nutrients from the analytical picture, the concentrations of available nutrients and their interactions are more easily seen in the analysis and accommodated for in the grower’s nutrition program. In the soil, colloidal and mineral properties influence how various nutrients, specifically cations, populate the cation exchange locations. A key takeaway is that these soil interactions occur primarily with available nutrients. The same is true within the plant with nutrient interactions primarily between available nutrients not unavailable and structurally bound nutrients. This is where sap analysis shines.
By quantifying the metabolically active and available nutrients in the sap and assessing their balance, growers are able to determine not just if nutrient deficiencies exist but also the future potential for nutrient deficiencies. With a NovaCrop sap analysis in hand, growers are able to evaluate the balance of nutrients in their crop and better understand how excessive levels of nutrients may impact the uptake and/or activity of others. Through this analytical report, a grower might determine that the most efficient way to increase the level of a certain nutrient is NOT by applying more of that nutrient, but rather is best achieved by decreasing the rate of other applied nutrients and restoring balance. Over-application of nutrients affect the safety of ground and surface water for human consumption and the wildlife dependent on those water sources. In an ever-increasingly regulated world, leaching and runoff of nutrients caused by over-application are not merely wasted money and crop potential, but could result in the grower being fined thousands of dollars, reclamation fees and civil judgements. Fertility management plan modifications, when based on NovaCrop sap analysis, and coupled with soil analysis, improves fertilizer use efficiencies and decreases over application of nutrients.
Rhizoctonia solani most often attacks the belowground parts of plants, such as these cauliflower crowns, resulting in loss of root attachment and decline of the plants (all photos by S. Koike.)
The soilborne fungus Rhizoctonia is an extremely important pathogen of plants worldwide. Hundreds of vegetable, field, fruit and nut, and ornamental crops are susceptible to this fungus. Rhizoctonia is common and severe on cereals and herbaceous row crops but can also cause disease on woody species. Found in soils throughout the world, this pathogen has evolved survival strategies that enable it to become established wherever it is introduced. Despite the implementation of integrated pest management tools, Rhizoctonia remains a challenging pathogen that is difficult to control.
Understanding Rhizoctonia solani
Rhizoctonia is the genus name that refers to a group or “complex” of fungi. Initially, fungal species were placed in this Rhizoctonia group because they shared certain features, such as the absence of asexual (anamorph) spores (remember that fungi can have two different phases or forms: asexual and sexual); a sexual (teleomorph) stage belonging to the Basidiomycetes; distinctive cell wall structures (septa) that divide the relatively thick hypha into sections, an existence that is primarily in the soil; and being mostly pathogenic to plants. For most row crops, the species Rhizoctonia solani is the most important pathogen.
Rhizoctonia is considered a “species complex” because of the many closely related species and subspecies that make up this group. Figure 1 outlines and breaks down this complicated, diverse group of organisms. The Rhizoctonia genus, first of all, can be divided into two major categories: (A) Those species that have two nuclei per mycelial cell (called binucleates) and (B) those species that have more than two nuclei per cell (multinucleates). R. solani and other species (e.g., R. oryzae and R. zeae) are in the multinucleate group. Note that each fungus listed in Figure 1 has two taxonomic names. The Rhizoctonia name refers to the anamorph or asexual stage and is the phase of the fungus that infects plants and causes disease. The second name is the teleomorph or sexual stage. This form is rarely found in the field and probably does not cause disease in plants; however, researchers employ the teleomorph names when studying the interrelationships between the various species. Our challenging fungal foe is therefore known mostly as Rhizoctonia solani (anamorph) but is also referred to as Thanatephorus cucumeris (teleomorph).
Rhizoctonia solani forms tiny, matted mycelial clumps (sclerotia) that enable the pathogen to survive in soil for prolonged periods.
The R. solani species is itself very diverse and can be separated into distinct groups. Different isolates of R. solani fall into one of many anastomosis groups, or AGs (Figure 1, see page 4). AGs are determined in the laboratory where two isolates are grown side-by-side in culture and the resulting reaction, whether the two hyphae fuse (compatible) or do not fuse (incompatible), is viewed under the microscope. Isolates that are deemed compatible are placed together into a numbered AG. Incompatible isolates cannot be in the same AG. There are currently 14 AGs: AG1 through AG13, and AGB-1. Seven of these AGs are further divided into subgroups. Molecular techniques are also available for AG classification; such techniques can be more accurate than searching for hyphal fusion under the microscope. Molecular methods, however, can be time-consuming to complete.
Categorizing R. solani isolates into these AGs is not an academic exercise but provides insights into how this pathogen functions in agriculture. Different AGs possess different traits; while some AG isolates have a broad host range, others are more restricted regarding the crops they can infect. For example, R. solani AG2-1 tends to be the main pathogen that causes wirestem on crucifers, while AG2-2/IIIB and AG2-2/IV cause brown patch in turfgrass and root disease on sugarbeet. R. solani AG3 is common on potato and causes stem and stolon lesions as well as black scurf on tubers. R. solani AG8 is primarily a pathogen of cereals but also infects potato roots. In contrast, AG4 has a broad host range and can infect many crops. So, identifying the AG status of an R. solani isolate can provide important information on the diseases caused by that isolate and the susceptibility of subsequent crops that might be placed in that field.
Figure 1. Complexity of the plant pathogenic Rhizoctonia group, and placement of R. solani.
Finally, isolates belonging to the same AG are not all identical to each other. While sharing the same AG designation, the isolates can differ physiologically in how carbon sources and other chemicals are metabolized, how fast they grow in culture, and other features. Isolates in the same AG can also differ genetically and have varying DNA sequences. This great diversity found within R. solani isolates accounts for the difficulty that researchers have in fully understanding this important plant pathogen complex.
Diverse Diseases Caused by Rhizoctonia solani
Rhizoctonia solani causes different types of crop diseases, all of which are related to the soilborne nature of this pathogen (Table 1). R. solani can be a seed pathogen. Once seed are placed in the ground, R. solani that is residing in the soil can invade the seed and kill it before it germinates. Even if the seed germinates, R. solani can cause a decay of the roots and shoots before the seedling emerges above the soil surface; this early seedling disease stage is called preemergent damping-off. Post-emergent damping-off occurs if the diseased seedling is strong enough to grow above the soil surface, only to succumb and collapse shortly afterwards (Table 1). Collectively, seed decay, preemergent damping-off and post-emergent damping-off can result in loss of plants very early in the production cycle, causing stand loss in the field. Healthy seedlings that escape death at the seed and newly germinated stages remain vulnerable to this pathogen; established seedlings can still be infected by R. solani and develop root rots and/or lesions on stems in contact with soil (Table 1).
Table 1. Categories of Rhizoctonia solani diseases of row crops
Plant leaves are not immune to R. solani. Field cultivation practices and splashing water can move bits of R. solani-infested soil up into the foliage of plants. Under favorable conditions, the introduced R. solani mycelium can colonize the leaf tissue and cause foliar blights in crops such as endive (Table 1). For head-forming vegetables such as lettuce and cabbage, the bottom leaves are unavoidably in direct contact with the soil. If R. solani is present in the underlying soil and if conditions (excess soil moisture) favor the pathogen, extensive rotting of the lower leaves and plant base can take place, resulting in bottom rot. If fruits (e.g., cucumber) and pods (e.g., beans) happen to be in contact with infested soil, these harvestable commodities can become diseased (Table 1). Finally, sweet potato roots, potato tubers and other similar plant structures under the ground can suffer from lesions, rots, and defects from R. solani in the soil surrounding these fleshy plant parts. Table 2 lists some row crops that are susceptible to R. solani.
Disease Development
Rhizoctonia solani has evolved to be a challenging, persistent soilborne pathogen. Tiny, tightly clustered clumps of mycelium grow together to form resilient structures (sclerotia) that can withstand unfavorable conditions and allow it to survive for years without a plant host. These sclerotia are the main mechanism for survival, since R. solani is not a particularly aggressive or successful saprophyte in the soil environment. When a host plant grows next to sclerotia and favorable soil conditions are present, the dormant mycelium germinates and can infect the plant.
For lettuce and other head forming row crops, Rhizoctonia solani can cause a rot where the basal leaves are in contact with soil.
Diagnostic Considerations
Diagnosing Rhizoctonia diseases based only on symptoms is risky because Rhizoctonia is not the only soilborne pathogen that causes seedling damping-off, root rots and stem lesions of row crops. On spinach, damping-off and root rot can be caused by R. solani, Fusarium and Pythium; visually, one cannot reliably distinguish between the symptoms caused by these three pathogens. Cilantro crops can develop similar-looking root diseases caused by R. solani and Fusarium. Cauliflower transplants are susceptible to both Rhizoctonia and Pythium, both of which cause the lower stem tissue to become discolored. Cauliflower disease diagnostics is further complicated because the root maggot insect feeds on lower stem tissue and causes symptoms identical to those created by R. solani. Precise and accurate diagnosis of Rhizoctonia diseases will therefore require lab-based examination and assays. Diagnosticians usually deploy culture tests in which surface sanitized bits of symptomatic tissues are placed in microbiological agar media. These scientists then use microscopes to examine the mycelium that grows out of the plated tissue. For most fungi, spores are important structures that diagnosticians rely on for fungal identification; since R. solani produces no spores, scientists must examine the hyphal structures or employ molecular assays to confirm this pathogen.
Rhizoctonia is one of the damping-off pathogens that can affect young seedlings, such as the Swiss chard plants pictured here: infected plants (left), healthy plants (right).
Managing R. solani
Rhizoctonia is a difficult pathogen to control. Attempts to manage this fungus will require the implementation of IPM practices.
Site Selection Plant in fields that do not have a history of Rhizoctonia problems and that have well-draining soils.
Crop Rotation Avoid planting a susceptible, sensitive crop in a field known to have significant problems with this pathogen. Rotate to crops that are either not susceptible or are less sensitive to damage caused by R. solani. Remember that some R. solani AG isolates have a very broad host range and can infect many row crops; however, other AG isolates show some level of host specificity and are pathogenic on only a few crops. It is therefore useful to know which AGs are present in the field.
Rhizoctonia solani-contaminated soil particles can be moved up onto the leaves, causing a foliar blight on crops such as endive.
Time of Planting In some cases, moving the planting date to a different time of year may help reduce losses from R. solani. Planting the crop in the warmer, drier part of the year allows the seedling to grow more rapidly and perhaps escape or minimize infection from R. solani.
Fungicides Plant seed treated with a fungicide.Note that the seed treatments used to protect against Pythium have little effect on Rhizoctonia. Fungicides applied to emergent plants have little benefit.
When testing plants, diagnostic labs use microscopes to look for the brown, relatively broad, distinctive branching mycelium that characterizes Rhizoctonia solani.
Resistant or Tolerant Cultivars There do not appear to be row crop cultivars that have genetic resistance to Rhizoctonia. However, if young seedlings escape infection early in development, the maturing stem tissue will later become resistant to infection by R. solani.
Sanitation Remember that as a soilborne pathogen, R. solani will be moved and spread via mud adhering to tractors and equipment. Prevent the introduction of R. solani into plant nursery and transplant facilities by using new or thoroughly sanitized containers and trays, disposing of used rooting media and other sanitation measures.
Western Region CCA Past Chairman Jerome Pier, left, presented Backman with the CCA of the Year award at the Crop Consultant Conference in September.
This year’s Western Region CCA Crop Consultant of the Year, Keith Backman, has been advising farmers in the San Joaquin Valley since the mid-1970s on fertilizer and irrigation management and the wisdom of matching those nutrient and water applications to the needs of the crops. Today, in an environment of increased regulation and costs related to fertilizer inputs, that prudence has grown more and more important.
Backman went to work with Nat Dellavalle out of college in the 1970s and has been with Dellavalle Laboratory in one way or another ever since. Today, he splits his time in semi-retirement between traveling with his wife Gail and imparting his wisdom on plant nutrition and water analysis to a new generation of agronomists as part owner of Dellavalle Labs.
Jerome Pier, outgoing Chairman of the Western Region CCA announced Backman as the winner of the new CCA of the Year Award at this year’s Crop Consultant Conference in Visalia this past September. Pier said Backman sets the standard for the region’s 1,300 CCAs for his contributions to plant nutrition and soil science over his 40-year career and his contributions to the industry.
“He’s made a big impact on a lot of people in the Central Valley and on Central Valley ag for sure,” Pier said. “His understanding of nutrition in permanent crops is world class.”
Backman is known for having an advanced understanding about taking and interpreting soil tests, and making preplant and in-season recommendations for fertility and irrigation management that are well suited to crop needs and soil status while also being environmentally sound.
Career of Service
As a member of the Western Plant Health Association’s Soil Improvement Committee for more than 20 years, Backman helped write the chapters on irrigation management and nitrogen management for the revised editions of the Western Fertilizer Handbook. The latest edition was just released and has Backman’s expertise throughout.
He was also in on the ground floor of developing nitrogen management plans and was instrumental in drafting nitrogen budgeting and checkbook methods that have been adopted for nitrogen management plans by water quality coalitions.
“He basically authored these nitrogen management plans, and without those plans agriculture was going to get shut down by environmental activists,” Pier said. “This was the only way to show a good faith effort that we are doing something about it.”
Those principals are based on the relationship between nutrient and water management.
“Over the years I’ve come to realize that so many of our deficiencies are irrigation related,” Backman said. “Especially with nitrogen, you can’t have a good accurate nitrogen program unless you have an accurate irrigation program.”
Farmers and PCAs today have a much clearer understanding about the timing of fertilizer applications and rates at those particular times, he said.
Where growers might have done a single or split application of 150 pounds of N fertilizer a year, for instance, they now make multiple applications based on yield estimate, soil conditions, crop timing and soil and leaf analysis.
“This is where the CCA earns their money is helping a grower understand that,” Backman said. “Growers can’t guess anymore. They need proven solutions and so more and more they are relying on the diagnosis of a CCA.”
Humble Ag Roots
Raised on a small farm south of Yuba City, Backman was seventh out of eight children in an agricultural family. While getting a Master’s degree in pomology at UC Davis, Backman focused his studies on boron toxicity in orchard crops. This is a problem that became more of an issue as deep-rooted permanent crops went in up and down the Valley and water shortages made leaching out of those root zones more difficult.
He worked early on in his college career under Kay Uriu, a professor in the UC Davis pomology department in the 1970s, who did ground-breaking research on tree nutrition status and was known for being able to spot nutrition imbalances by looking into an orchard.
As a result, Backman said even today he “can’t help driving by an orchard and looking at it and thinking ‘what can I do to get it growing more efficiently?’”
In addition to his service for the industry, Backman has been a scout master in his community for more than 20 years and until recently was an active member of his church choir.
It is Backman’s dedication to his industry, community and exceptional dedication to training new agronomists that led to his nomination as CCA of the Year, Pier said.
“I was honored by the selection,” Backman said. “I appreciate the recognition; it’s nice to know what you have done for the past 45 years is valued and to be able to see the changes in the industry from some of the things I’ve introduced.”
Among the changes he has seen in recent decades: more accurate nitrogen and water applications; more accurate monitoring to take appropriate actions at the appropriate times; and the application of science in making those applications using plant science and physiology to guide those decisions.
This is the second year Western Region CCA presented an annual Crop Consultant of the Year award to recognize outstanding individuals who have advanced crop consulting throughout their careers. For information on nominating a CCA visit the Western Region CCA website at https://wrcca.org/cca-of-the-year.
Management practices that improve soil health and soil quality have gained considerable attention over the past few years, and especially during the past year, as drought conditions have impacted large areas of North America. In this article, I focus on how the living, biological components of the soil (e.g., bacteria and fungi) can be key microbial partners in your future drought management strategy.
I detail how soil microbes impact soil physical properties, including the structure (e.g., aggregation and pore space) and the ability of the soil to move and store water. Additionally, I explain how soil microbes can help crops get through drought conditions using the substances they secrete. Finally, I close with a call to action to measure your soil biology so you can make management decisions now before the next season gets underway. If needed, a CCA can help you interpret data and make an actionable plan that can help tackle the continued drought conditions that are expected for the near future.
Figure 1. Here are two concepts to help organize the contribution of microbes to soil health and structure (Concept 1) and the substances that are released by the microbes themselves (Concept 2) that help crops get through a drought period (courtesy K. Wyant.)
Let us begin with a quick reminder of what soil health means to a grower and how it is connected to the living component underground and drought management. Soil health is directly related to the interaction, or lack thereof, between organisms and their environment in a soil ecosystem and the properties provided by such interactions. When you think of soil health, think of the biological integrity of your field (e.g., microbial population and diversity) and how the soil biology supports plant growth. There is a direct link between soil health and how a soil can be managed to meet the challenges of drought conditions.
Soil Microbes and Drought Management
Soil microbes impact your ability to manage drought via two major pathways, i.e., the “Two Concepts of Drought Management” (Figure 1).
Concept 1: Soil Microbes Help Increase Water Penetration and Infiltration
Soil microbes help restore soil structure which helps water move from the soil surface downwards. This is known as water penetration. Once the water has penetrated the soil, it moves down into the soil for storage. This is known as water infiltration. If you are not capturing and moving water into the soil, you will have a tough time storing water in your field. Simply put, healthy soils have good structure, which excel at receiving and storing moisture. But how exactly do microbes improve water penetration and infiltration?
Abundant and diverse soil microbial communities produce “free” services for your farm soil, including the ability to receive and store moisture. The key to this ability lies in the ability of microbes to contribute directly to improving soil structure by binding soil particles together, which, in turn, helps water move from the soil surface and into the root zone.
Soil bacteria produce a sticky, glue-like gel called extracellular polymeric substances (EPS) that form a protective slime layer around bacteria as they grow. The EPS acts as an adhesive to bind soil particles, thereby improving overall soil structure. Fungi, another important group of soil microbes, produce miles of microscopic threads in the soil called hyphae. The threads capture and “tie” soil particles together, like a net, which improves overall soil structure. Fungi also produce a sticky protein-like substance called glomalin which, like EPS, helps bind your soil structure together via adhesion of particles.
Key Message: Soils with healthy microbial populations can restructure and re-aggregate the soil, which leads to better soil structure overall. Good soil structure allows for water (e.g., snowmelt, rainfall and irrigation water) to move from the soil surface (penetration) to below the soil surface for storage around and on the soil particles themselves (infiltration). This results in a number of benefits, including reduced runoff losses.
Research and grower experience corroborate this connection as reports show that soils with good structure often can store more water relative to degraded control fields nearby. Good soil structure also reduces runoff losses as water quickly moves downwards instead of horizontally across the soil surface, which carries materials off the field. Thus, soil microbes can be crucial partners for capturing and storing soil moisture, which will certainly come in handy during forecasted drought periods.
Concept 2: Robust Microbial Communities Release Substances to the Soil Which Can Help Crops Get Through Periods of Drought
This next concept is not so easy to visualize like changes in soil structure and the ability to store water. Imagine the microbial community on the right side of Figure 2 for the next few sentences and contrast the microbial abundance and diversity when compared to the “business-as-usual” farm soil on the left side. Now that you have refreshed your snapshot of the microbial community, we can turn our attention to the benefits that improved microbial abundance and diversity bring to a drought affected soil. Recent work has shown that soil microbes help crops get through periods of drought stress via the substances they release into the soil around the roots. These molecules include osmoprotectants and antioxidants, to name just a few (see first reference for a deeper dive).
Figure 2. The soil on the left has poor soil health and soil structure while the soil on the right has excellent soil health and, as a result, the two fields have substantial differences in soil quality and their ability to mitigate drought stress (courtesy K. Wyant.)
Key Message: A hidden benefit of maintaining a thriving community of microbes during a drought are the substances they secrete. For example, osmoprotectants play a key role in managing challenges with plant water balance under drought conditions. In another example, antioxidants help mitigate oxidative stress and internal plant cell damage observed under drought stress. Studies show that when a robust microbial community releases certain substances belowground, a crop is better able to weather the stress of drought (e.g., low rainfall, higher temperatures) more successfully aboveground.
Managing Soil Biology
Now that we have examined how soil microbes can be crucial partners under drought conditions, we can now turn our attention to a crucial next step: managing the soil biology. I will walk through the basics on how to measure the biological activity of the soil and make the case to ask your trusted CCA for assistance if this management strategy is new to your operation or if you need help with interpretation of the results.
Tests are commonly used to measure chemical constituents of the soil (e.g., pH, nitrate, phosphate, etc.) or physical aspects of the soil (e.g., soil texture, cation exchange capacity). However, these tests do not determine how “alive” the soil is. You can accomplish this goal with a broad set of soil tests that target the living components of soil. Soil biology tests are diverse and include measurements of carbon dioxide respiration, extraction of DNA for microbial community analysis, and other key metrics (Table 1), depending on what parameter you are interested in measuring and your patience level to see a measurable change.
Table 1. There are several soil tests available to help you quantify the living components of your soil and their response to field management decisions. Please see the “Comprehensive Assessment of Soil Health” from Cornell University for an exhaustive list on what is available or call your local lab. Listed are a few common testing options in ag laboratories.
Testing the biological components of the soil is new to many growers and some have reported frustration about which test to choose, how to design a sampling program, and how to interpret and write an actionable plan based on the test results. This is where a Certified Crop Advisor can step in and help reduce the learning curve.
Final Thoughts
Soil microbes can help you mange drought in ways that are readily observable (e.g., changes in soil structure and water holding capacity) and in ways that are not (e.g., release of substances that help with drought stress). In any event, microbes are essential partners for dealing with drought conditions and their usefulness should be leveraged in any crop production program.
Dr. Karl Wyant currently serves as the Vice President of Ag Science at Heliae® Agriculture. To learn more about the future of soil health, you can follow his webinar and blog series at www.phycoterra.com.
Significant findings in a California Walnut Board-funded trial show that post-plant applications of nematicides in walnut orchards frequently require multiple years before plant yields improve (photo by C. Parsons.)
When growers think about nematodes, they often only think about plant parasites. These are only about 10% of nematode species known. They pose a threat to walnut production, but among those that do cause loss of growth, vigor and production in trees, the walnut root lesion nematode is the most damaging. Other nematode species, so-called free-living species, are very important in nutrient cycling. They feed on soil bacteria and fungi. There are other species as well.
Research into management of plant-parasitic nematodes with rootstock selection and chemical control at the UC Kearney site was presented by UC Riverside Nematologist Andreas Westphal during a recent field demonstration.
Westphal said that about 85% of California walnut orchards have some parasitic nematode infestations. When planning and preparing a new planting, prior soil samples can show nematode species and infestation levels to help decide if soil fumigation is necessary. Westphal noted that post-plant application tools are needed when orchards become re-infested. Westphal and other UC and USDA-ARS researchers actively pursue rootstock selection for tolerance and resistance to nematodes, crown gall and Phytophthora rots.
Root lesion nematode is tightly associated with plant growth disruption, Westphal said. Rootstock trials and chemical trials are continuing at Kearney to develop management strategies.
Significant findings in a California Walnut Board-funded trial show that post-plant applications of nematicides frequently require multiple years before plant yields improve. Some reductions of nematode numbers by post-plant applications can already reap some benefits, but in pre-plant soil treatments, efficacy needs to be even higher to protect new plantings. New plantings have to cope with transplant shock, and any additional stress has a likelihood to damage them severely. Pre-plant treatments can be more challenging due to the scarcity of highly efficacious material and the need for optimal soil conditions. If nematode infestations are confirmed in a field, such treatments are highly desirable for successful establishment of walnut orchards.
When it comes to rootstock tolerance and resistance to nematode infections, Westphal said that tolerance alone will not ensure productive trees long-term. The level of nematode reproduction in orchard soils can have an effect on the productive life of a walnut tree.
The determination of a rootstock’s tolerance to nematode infestation is ongoing at Kearney. Westphal noted that environment and drought stress are also components in the health and function of a rootstock. The trials are using different treatment to see how the rootstocks and the scions respond to these stresses.
“We are still looking for answers to how different genotypes respond to stress and nematode pressure,” Westphal said.
Soil samples, root sampling and even use of drones have been used to detect patterns of tolerance or resistance to nematode pressure over time. Finding a balance between rootstock and the scion is important.
Trials used both pre-plant and post-plant fumigants as well as anaerobic soil disinfestation to manage nematode levels. However, Westphal noted that there is much adjustment work to do with chemical control. Root lesion nematode levels can build again over time, and researchers need to find the combination that will protect trees.
If nematodes are in a field, they are there to stay, growers need to manage them over time. Methods are being developed at Kearney to enrich growers’ tool cabinets to deal with these challenges.
In winegrape vineyards, mechanical box pruning entails pruning the grapevine’s bearing spurs from the top, bottom and sides of the canopy (photo by G. Zhuang.)
Dormant pruning in winegrape vineyards is one of the most labor-intensive practices with an estimated 80% of labor costs accrued in pruning and harvest operations. Labor cost and availability have accelerated the planting of new vineyards to accommodate mechanical pruning, said Fresno County UCCE Viticulture Advisor George Zhuang.
“Almost all new vineyards in Fresno County are now designed for mechanical pruning,” Zhuang said.
He also noted that mechanical pruning operations in vineyards have had no negative effect on grape yield or quality. In some cases, he added, yields and berry quality have increased.
Mechanical pruning in table grape vineyards presents more of a challenge due to their trellising system. But Zhuang said progress is being made toward that goal. Raisin grape growers have also been moving toward mechanical pruning due to labor shortages. Delays in harvest have led to crop damage from early rain events.
In winegrape vineyards, mechanical box pruning entails pruning the grapevine’s bearing spurs from the top, bottom and sides of the canopy. Box pruning is not as selective as hand pruning, leaving all the nodes within the perimeter of the cuts, but box height and width can be manipulated by the machine operator. A pre-pruning pass may leave a 0.3-meter-wide by 0.4-meter-tall box and is recommended in frost-prone areas. A more precise pruning pass may leave a 0.10 to 0.15 box.
Types of mechanical pruning include minimal pruning to develop high numbers of clusters, pre-pruning and mechanical shoot thinning follow-up.
With minimal pruning, the high numbers of clusters would be balanced by early growth of numerous vegetative shoots. The result is high yields with smaller clusters and berries.
Pre-pruning leaves a 0.40-meater-tall by 0.10-meter-wide box retaining 120% to 200% of the desired number of buds. After bud break, the desired shoot density is achieved by manual pruning or shoot thinning.
With mechanical shoot thinning follow-up, dormant canes are mechanically pre-pruned to 0.1-meter-wide by 0.3- or 0.4-meter-tall, retaining 120% to 200% of the predicted bud load. After bud break, the desired shoot density is achieved by shoot thinning.
Hedger bar pruners are mostly used in minimal pruning applications in the dormant season, for summer pruning or as part of combination pruners. They have a single plane of cut and low penetration into the dormant canopy.
