Melon nematicide trial 30 days after transplanting. (all photos courtesy J. Sidhu.)
California is one the largest producers of melons in the U.S., and melons most commonly grown in California are cantaloupes and honeydew types. Although some acreage is reported throughout the state, most are grown in the Southern desert valleys and San Joaquin Valley. Root-knot nematodes (RKN), Meloidogyne spp., are the most significant plant parasitic nematodes affecting melon production in California, especially in light-texture soils. The nematodes are widespread in Central and Southern California.
Damage results from feeding of second stage juveniles (J2) inside melon roots, and the roots respond to nematode invasion by formation of root galls. Nematode-infested plants become stunted and less vigorous with severe galling of roots. Deformed roots due to galls are unable to sustain the water and nutrient needs of the plant in hot weather, leading to wilting and poor growth of plants, reduced yield and poor fruit quality. Nematode-infested plants may also become more vulnerable to other soilborne pathogens.
Currently, there are no resistant cultivars in melons, and RKN management has mainly relied on the use of preplant soil fumigants and soil-applied nematicides. Management with these products is expensive and involves safety and environmental risks. New fumigant regulations by the Department of Pesticide Regulations (DPR) have been put in place to restrict the emissions of volatile organic compounds from the use of soil fumigants. These regulations include limits on the amount of soil fumigants a grower is allowed to use in a year, caps on the amounts allowed within a township and new expanded buffer zones, meaning large parts of a field may not be treated all. These new regulations by DPR may mean that there will be some fields not treated for nematodes because of caps placed on the amount a grower is allowed to use or caps on the amount of fumigants allowed in a township.
Alternative control options that have high efficacy, are economically viable and environmentally safe need to be evaluated under field situations. The goal of this project was to evaluate the efficacy of Salibro, an organic product and a developmental product (DP1) in comparison to Nimitz (fluensulfone) in melons applied through deep-buried drip tube. Nimitz is registered on melons in California.
Root galling at harvest. 1) Untreated control 2) Nimitz 3) Gropro 4) Salibro 5) DP1
2020 Field Trial
This study was conducted as a small plot field trial on our RKN-infested site at the Shafter research farm. A western shipper-type melon variety, ‘Durango’, was hand transplanted onto 60-inch beds on June 30, 2020. There were four replications and five treatments in this trial arranged in a randomized block design. Rates, timings and method of application for each treatment is listed in Table 1, on page 18. Each plot was 20 feet in length with a five-foot buffer between plots along the bed. Treatments were applied either as a pre-plant or post-plant application through buried drip. Before chemigation, water was run for ten minutes to ensure all treatment tubing was filled, and after, chemigation water was run for about 20 minutes to flush the lines. The plots were irrigated using a surface drip and maintained using standard agronomic practices.
Table 1: Treatments, rate/A, application schedule and timings.
Before applying the treatments, soil samples were collected from all plots using a sampling tube at a depth of eight to 10 inches and submitted for analysis to determine the RKN count. Soil samples were collected and analyzed for nematodes again at harvest. Melon roots were evaluated for galling at mid-season and at harvest. Data on nematode counts and root galling was analyzed using SAS (statistical analysis software).
Data and Results
Plant vigor for each plot was rated visually on August 11, 2020 on a scale of 1-5, with 1=worse and 5=best plants. Vigor included the size of the vines and general appearance or health of plants. On the same day, five plants from each plot were randomly selected and visually rated for severity of root galling on a scale of 0-10; (0=no galls, 10=completely galled roots). The average galling on these five plants was used to give a galling index for each plot. The fresh shoot weights of these plants (without fruits) were determined.
At harvest on September 22, 2020, soil samples were collected from each plot for RKN count. All plants in each plot were dug and the severity of root galls on these plants was visually rated on a scale of 0-10 (0=no galls, 10=completely galled roots). The average of the galling on these plants in each plot was used to give a galling index for each plot.
Plant Vigor
No obvious differences were observed in plant vigor among treatments (Table 2). Some plots were a little more vigorous than others, but these differences were not attributed to treatment effect.
Table 2: Mid-season average plant vigor of melon plots in five treatments during the 2020 growing season. Vigor on a scale of 1-5, with 1=worse and 5=best plants.
There were no significant differences observed in fresh shoot weight of melon plants during mid-season evaluation on August 11 (Table 3). However, Nimitz resulted in higher shoot weights than the other treatments.
Table 3: Average plant weight (g±SE) of melon vines in five treatments during the 2020 growing season.
Root Galling
The severity of root galling was assessed at mid-season and at harvest. At mid-season evaluations, root galling was moderate and ranged between 2.4 in the Nimitz treatment and 4.6 in the untreated control (Table 4). Root galling in Nimitz and Gropro treatments was significantly lower than the other treatments. At harvest, there was a little increment in root galling across all treatments, however there were no significant differences among treatments. Surprisingly, Salibro and the developmental product was not beneficial in the trial and had higher root gall ratings than the non-treated control plots at harvest.
Table 4: Average root galling of melon plots in five treatments during the 2020 growing season. Galling on a scale of 0-10 (0=no galls, 10=completely galled roots).
Conclusion
In our 2020 trial, there were some treatment effects on mid-season root galling with Nimitz and the organic product Gropro having statistically lower root galling index compared to other products. However, none of the treatments were significantly different at harvest, and the results indicate that none of the treatments had a long-lasting effect on RKN levels in the soil. Therefore, further evaluations are needed to better determine the efficacy of these products as sole treatments and in combination with other products and their potential and continued use by the melon industry.
This project was funded by the California Melon Research Board.
Pierce’s disease leaf symptoms. Leaf margins turn yellow (or red in red varieties) then burn back from the margins to center in patches (photo by K.T. Lund, UCCE.)
Pierce’s disease is caused by the bacterium Xylella fastidiosa. These bacteria live within xylem, the vascular tissue through which water travels in a plant. As the bacteria population grows, it stimulates the plant to produce tyloses. The combination of bacteria and tyloses cause vessel plugging, which restricts water movement in the plant, thus causing many of the disease symptoms. These blockages will eventually lead to the vine’s death. It is estimated that Pierce’s disease costs the California grape industry $56.1 million a year in lost productivity (Tumber et al., 2014). To minimize losses, it is important to understand the biology of the disease, including the bacteria’s host range, how the bacteria moves from plant to plant, and how to identify infected plants will help growers prevent losses and control the disease.
About the Bacterium
The bacterium X. fastidiosa has a large known host range. The European Food Safety Authority maintains a database of known hosts for X. fastidiosa (their updated list approved in April 2020 can be found at doi.org/10.2903/j.efsa.2020.6114.) Research done throughout California has identified many hosts, including weeds such as shepherd’s purse (Capsella bursa-pastoris), filaree (Erodium spp.), cheeseweed (Malva parvifolia), burclover (Medicago polymorpha) and annual bluegrass (Poa annua) among many others (Shepland et al., 2006 and Costa et al., 2004). Overall, at least 350 host plants have been identified from over 75 plant families as hosts for X. fastidiosa. From a control standpoint, once X. fastidiosa has been introduced to a geographic area, it will be virtually impossible to eliminate it from that location with such a wide variety of possible hosts.
X. fastidiosa does have another level of complexity. To date, four distinct subspecies of X. fastidiosa have been identified. X. fastidiosa ssp. fastidiosa is the subspecies that causes Pierce’s disease in grapevine, while X. fastidiosa ssp. multiplex is the subspecies that causes almond leaf scorch (Rapicavoli et al., 2018). This does mean that almond trees with almond leaf scorch would be unable to be the source of Pierce’s disease in a vineyard. To narrow down the definitive host range, grape-specific PD (X. fastidiosa ssp. fastidiosa) was inoculated into a range of possible host plants using glassy-winged sharpshooters as a vector. After an incubation period, multiple positive ELISA results were obtained for several plants including black mustard (Brassica nigra), black sage (Salvia mellifera), mirror plant (Coprosma repens), Spanish broom (Spartium junceum), Mexican elderberry (Sambucus mexicana), almond (Butte) (Prunus dulcis), white sage (Salvia apiana), sycamore (Platunus racemose) and coast live oak (Quercus agrifolia), confirming that they could host the bacteria (Costa et al., 2004). This does ultimately lower the number of possible plant hosts for X. fastidiosa ssp. fastidiosa; however, it still includes a long list of common plants found in and around vineyards, enough that even including this reduced host list, managing only these species, and other hosts yet to be identified, is still outside the ability in most growing regions.
Insect Vectors
Bacteria within the xylem tissue of one plant may be spread to another plant through the feeding activities of certain xylem-feeding insects. In vineyards, two groups of insects have been identified as possible vectors: sharpshooters and spittlebugs. Spittlebugs have been shown to vector X. fastidiosa in controlled settings, but their importance as a Pierce’s disease vector in vineyards is unclear. Sharpshooters, on the other hand, are known to be effective vectors of Pierce’s disease in vineyards.
There are several different sharpshooters in California that vector X. fastidiosa. The most important of these in the coastal portions of California is the blue-green sharpshooter. This sharpshooter is not adapted to the hotter climate of the San Joaquin Valley (SJV). In the SJV, there are three other sharpshooters: the green sharpshooter (Draeculacephala minerva), the red-headed sharpshooter (Xyphon fulgida) and the glassy-winged sharpshooter (Homalodisca vitripennis).
From left to right: green, red-headed and glassy-winged sharpshooters. Size comparison measurements are in the bottom right of each individual image (photos by Jack K. Clark, Regents of the University of California.)
Green and Red-Headed Sharpshooter
The green sharpshooter and the red-headed sharpshooter are both small and prefer to feed on grasses. The red-headed sharpshooter is specifically drawn to and reproduces on Bermudagrass. Both the green and red-headed sharpshooter can be found in irrigated pastures and along waterways such as streams, creeks, canals and ditches. Neither of these sharpshooters prefers to feed on grapevines, however they may do so under certain conditions and thus transmit Pierce’s disease. Since they don’t prefer grapevines, they tend not to spread deeply into vineyards; thus, when these vectors transmit Xylella, it is usually only to grapevines along the edges of a vineyard, whereas vines in the middle or the sides away from the green or red-headed sharpshooters’ preferred habitat are not affected.
Glassy-Winged Sharpshooter
The glassy-winged sharpshooter is twice the size of either of the other two sharpshooters. Their large size makes them more effective as a vector for Pierce’s disease because they can travel further than smaller sharpshooters and feed more effectively on a wider variety of plants, including woody plants such as grapes. To date, over 350 plants have been identified as hosts of glassy-winged sharp shooter (cdfa.ca.gov/pdcp/Documents/HostListCommon.pdf). Many of the hosts for glassy-winged sharpshooters are also hosts for X. fastidiosa. One of the key hosts for both glassy-winged sharpshooters and X. fastidiosa in the SJV, and for local control of Pierce’s disease, is citrus. The large feeding range of the glassy-winged sharpshooter also means that it can spread the disease throughout the vineyard instead of just to the edges.
The glassy-winged sharpshooter is not a native California insect, only arriving in California in the late 1980s (first recorded in 1989). As this non-native pest is such a dangerous vector, the CDFA tracks their distribution. Most of Kern county, parts of Tulare and Fresno counties, and a very small sliver of Madera county just over the San Joaquin River are all hosts to naturalized populations of glassy-winged sharpshooters within the SJV. In SoCal, Ventura, Los Angeles, Orange, San Bernardino, Riverside and San Diego counties as well as portions of Santa Barbara county and a small section of Imperial county all play host to endemic populations.
CDFA map for April 2020 of glassy-winged sharpshooter distribution in California (photo courtesy CDFA.)
Identification of glassy-winged sharpshooters within and near these areas is important for controlling their spread as well as the spread of Pierce’s disease. At the top, the insect has a deep brown color with creamy white dots on the head and thorax. These colors and dots continue onto the abdomen; however, here they are covered with transparent wings (the source of their glassy name). Highlighting the glassy wings are red lines and patches which can be seen from both the top and side.
The other main identifying mark is the flat white marking along the side of the of the abdomen. When sitting on a stem, this white mark stands out under and through the wings of this sharpshooter. Younger nymph glassy-winger sharpshooters have yet to develop their namesake wings. Their bodies are a lighter grayish-brown with very small white dots. In this stage, the standout feature is their red eyes. The red is the same color that will soon highlight the parent’s wings.
Later-stage nymphs have started to transition to the adult body color, and the red color in the eye is mostly lost. However, the red color has transitioned onto the wing pads in a pattern that has started to develop the adult wing’s patterning.
Images of glassy-winged sharpshooter. A. Top view of adult glassy-winged sharpshooter. B. Side view of adult glassy-winged sharpshooter. C. Top view of young nymph glassy-winged sharpshooter. D. Side view of late-stage glassy-winged sharpshooter (photos by K.T. Lund, UCCE.)
Vector Monitoring and Treatment
Monitoring for glassy-winged sharpshooters can be done using yellow sticky cards. It is recommended to use cards that are at least 5.5” x 9” in size. One card should be placed for every 10 acres and checked weekly for recent activity. Monitoring should be done from budbreak through November. If a glassy-winged sharpshooter is found, and you are outside of a known population center, please contact your local agriculture commissioner’s office or cooperative extension office. Green and red-headed sharpshooters are not attracted to yellow sticky cards, so to monitor their populations you will need to use a sweep net. Sweep lush green grasses near and within your vineyard in April and May to assess population size.
For both green and red-headed sharpshooters, finding two adults in 50 sweeps warrants a response. Unfortunately, as both sharpshooters are only incidentally on grapevines, treating the grapevines will not help the situation. The preferred habitat (lush grassy areas) will need to be addressed. Due to the overlapping generations seen in these sharpshooters, insecticide treatments are often ineffective. Removal of preferred habitat is a more effective treatment option for these sharpshooters.
For glassy-winged sharpshooters, a single find warrants a response. A list of treatment options for glassy-winged sharpshooters can be found on the UC IPM webpage at ipm.ucanr.edu/PMG/r302301711.html. The most common insecticides used for glassy-winged sharpshooter control contain the active ingredient imidacloprid. As with all chemical control, it is important to rotate active ingredients regularly.
Research conducted in 2017 showed that several other insecticides had long-term control of glassy-wing sharpshooters. These included Sivanto (a.i. Flupyradifurone) and Assail (a.i. Acetamiprid), which still showed greater than 90% mortality 7 weeks after application; Actara (a.i. Thiamethoxam), which still showed greater than 90% mortality 5 weeks after application; Harvanta (a.i. Cyclaniliprole), which still showed greater than 90% mortality 4 weeks after application; and Sequoia (a.i. Sulfoxaflor), which still showed greater than 90% mortality 3 weeks after application (Haviland and Rill 2019). This research was conducted in citrus because relying on vineyard-only management is not enough for glassy-winged sharpshooters. With the larger range of the glassy-winged sharpshooter, it is important to focus on an area-wide approach. A pilot program with cooperation between grape and citrus growers has shown great promise in Kern county. Citrus groves are a primary overwintering spot for glassy-winged sharpshooters. When treatments can be applied to these locations, it can lower the number of glassy-winged sharpshooters and, thus, the presence of PD in the area.
Pierce’s disease symptom on shoots. Patchy bark maturation on current year’s shoots leaving green islands (photo by K.T. Lund, UCCE.)
Vine Infections
Early identification of infected vines is the final step in preventing a larger problem from Pierce’s disease. Infected vines can be a source of the disease for vectors to spread to neighboring vines. They are also a strong indicator that the bacteria and a vector are present in your location.
The leaves of infected vines will turn yellow (for green varieties) or red (for red varieties) along the margins. This discoloration will then work inwards from the margin, with the discoloration quickly turning to brown/dried dead tissue. This often happens unevenly or in sections. Shoot tissue also shows an uneven maturation process, leaving green islands within lignified brown tissue.
Affected leaves eventually fall off, but will sometimes leave the petiole still attached to the shoot. Not all these symptoms will be found on every infected vine. If you suspect a vine is infected with Pierce’s disease, you can contact your county’s viticulture advisor for corroboration. Ultimately, a diagnostic analysis is required to confirm the presence of X. fastidiosa in the suspected vine. Table 1 lists laboratories within California that offer Pierce’s disease testing.
Table 1: California Laboratories that offer Pierce’s disease testing.
References
Costa, H. S., Raetz, E., Pinckard, T. R., Gispert, C., Hernandez-Martinez, R., Dumenyo, C. K., and Cooksey, D. A. 2004. Plant hosts of Xylella fastidiosa in and near southern California vineyards. Plant Dis. 88:1255-1261.
Haviland, D. and Rill, S. 2019 Evaluation of glassy-winged sharpshooter mortality following exposure to aged insecticide residues, 2017. Arthropod Management Tests, 44(1), 1–1. doi: 10.1093/amt/tsz075
Rapicavoli, J., Ingel, B., Blanco-Ulate, B. Cantu, D. and Roper, C. 2018. Xylella fastidiosa: an examination of a re-emerging plant pathogen. Mol. Plant Pathol., 19(4), 786–800
Shapland, E. B., Daane, K. M., Yokota, G. Y., Wistrom, C., Connell, J. H., Duncan, R. A., and Viveros, M. A. 2006. Ground vegetation survey for Xylella fastidiosa in California almond or orchards. Plant Dis. 90:905-909.
Tumber, K. P, Alston, J. M, & Fuller, K. 2014. Pierce’s disease costs California $104 million per year. California Agriculture, 68(1-2)
Fertilizer label on a storage tank (photo courtesy Jacob Hernandez, JH Ag Consulting.)
The goal of fertilization for any crop is to ensure the optimum levels of nutrients are available to the plant at key stages in the growth cycle. Balancing these factors is an art as well as a science. The first step is identifying what nutrients to apply. The second step is deciding how much fertilizer to apply. The third step is choosing the best time to make the application.
Tissue Analysis
In an ideal world, all of the plant’s needs would be met by nutrients available from the soil. In wine grapes, however, soil tests are not a useful predictor of fertilizer needs as the vine’s uptake of nutrients is affected by soil chemistry such as pH and the dynamics of different soil types. Therefore, tissue analysis in the forms of leaf blade and petiole analysis are required. Tissue should be sampled twice per year. Bloom time petiole analysis describes the vine’s nutritional needs during the growing season. Tissue sampling at early veraison is useful for making decisions about macronutrient adjustments postharvest as nitrogen, phosphorus and potassium are all mobile in the vine between harvest and leaf fall.
Bloom time tissue analysis provides a good picture of shortages in micronutrients such as zinc, magnesium and boron. Leaf blades and petioles are separated and analyzed separately. For bloom time sampling, leaves should be selected opposite the first cluster, ideally at 50% bloom. When sampling at veraison, select the most recently matured leaf — usually the fifth or so leaf from the tip. The most important things to consider are sampling at the same time each year and accurately reflecting variation within the block.
Tissue analysis results need to be interpreted in the context of other information. Are vines exhibiting symptoms of a nutrient deficiency or excess? What is the overall vegetative growth of the vine? Is fruit set less than desired or uneven? Nutrient deficiencies are relatively easy to spot, but not always easy to diagnose. Tissue analysis will identify or confirm what those deficiencies are. Assessing vegetative growth is relatively easy. These data are only meaningful when compared year to year, so several years of data collection may need to be conducted before the relationship between fertilizers applied and their effects can be identified.
Therefore, record keeping is an important part of making fertilizer decisions. Information including tissue analysis and fertilizer applications from previous years provide insight into how in-season and post-season fertilizer applications affected nutrient status over time. Data on pruning weights and estimates of canopy growth at bloom can also show trends suggesting whether too much or too little nitrogen is being supplied.
Fertilizer Rates
As every vineyard and every block within a vineyard is different, the only way to accurately (i.e. efficiently) determine amounts of nutrients to apply is to adjust published ranges for the deficiency or excess of nutrients based on personal experience. Translating tissue analysis results into specific amounts of fertilizer to apply is not an exact science. Differences in soil can have a sizable impact on nutrient uptake and, therefore, fertilizer requirements between areas. Consider flagging rows or using GPS coordinates to sample the same areas. Year to year comparisons will tell you if your fertilizer decisions are accurate and effective.
Nitrogen and potassium are the primary nutrients that need to be supplied with fertilizers, although phosphorus and calcium are also important. Both have downsides if oversupplied, however. Excess N results in excessive growth and overcropping while excess K yields an unacceptably high pH in the juice at harvest. Replacing minerals is very important as they are transported off-site in the crop and not recycled back into the soil like leaves or canes. Every situation is different, but believable ranges are 3 to 5 pounds N, 5 to 8 pounds K, and 1 to 2 pounds Ca are removed from the vineyard each year. Most of these nutrients are taken up by the vine during the postharvest period if they are available.
Grapevines are composed of 1 to 2% N. 30% of N that the vine uses is taken up during the period between harvest and leaf fall. The same is true for K, although the rate of uptake drops off dramatically about a month after harvest. N is an important requirement for the production and function of proteins and is a major component of chlorophyll. Wine grapes will consume 40 to 50 pounds N per acre during the growing season. Much of this will be returned to the soil in the form of prunings and leaves, but the breakdown of pruning wood into bioavailable forms of N takes years, and much may be lost as nitrous oxide during the process. N is released from soil organic matter at the rate of approximately 20 lbs/acre/percent organic matter. Therefore, on soils with less than 2% organic matter, the rate of N provided through fertilization should be increase by 10 to 20%. On soils with greater than 3% organic matter, consider reducing the amount of N delivered. Nitrate levels in irrigation water can be meaningful contributors to total N supplied to the vine. In some areas of California, nitrate levels are high enough that they could be considered fertilizers.
Rates of N application in wine grapes vary from 0 to as much as 60 pounds per acre per year. Making decisions about the rate of nitrogen fertilizer to apply is complicated as excessive applications can result in nitrates leaching into groundwater and/or the generation of the greenhouse gas nitrous oxide. In leaf blades at bloom time, less than ~2% total N is considered deficient. N deficiency is expected with levels less than ~1.5% in leaf blades at veraison.
K uptake is negatively affected by high magnesium in the soil. Therefore, even if K is plentiful in the soil, additional K may need to be provided to avoid deficiency. K levels are deficient below 1% in both petiole and leaf blades during the spring and ~0.7% in the fall. A reasonable target is 2 to 3%.
Timing Application
N may be applied in a split application, with half being applied just after berry set and the other half being applied postharvest. In-season applications of N may not be necessary at all depending on the results of tissue analysis. This is an especially important consideration given that too much N in the vine during the growing season can result in excessive growth, shading inside the canopy and higher disease incidence. To avoid this, spoon feeding vines during the growing season can afford more control.
The goal of postharvest fertilization is to deliver N, K and Ca to the root zone during a time when the vines will take up the nutrient and store them in the trunk so that they are available when the vine breaks dormancy and the demand for these nutrients is the highest. For example, one study found that 50% of N in the canopy comes from N which had been stored in the trunk and roots of the vine. Timing is critical. Too soon and N may kick off a new flush of growth and delay dormancy. Too late and the vine is no longer moving water into the trunk. The canopy needs to be healthy and functional. Hitting this window with late ripening varieties may be difficult, especially if a frost knocks the leaves off. Also, care must be exercised in cold areas because excessive N postharvest can cause hardiness issues. Fertigation is the preferred method of delivering N and K fertilizers as foliar applications can result in an overly rapid uptake. Also, there needs to be sufficient soil moisture for vine roots to take up the elements.
The amount of time required for vines to restore their carbohydrate and mineral reserves varies by crop load. Vineyards with a crop load of 2 to 4 tons per acre need very little time to recover and restore after being harvested. Vineyards in the 4 to 8 tons per acre range require about a month for restoration. Vineyards cropped above eight tons per acre need between 4 and 8 weeks to build their reserves back up.
A Note on Compost
A grower may want to apply compost to their vines for multiple reasons, including delivering nutrients, inoculating the soil with microbes, increasing soil organic matter and improving soil structure. When using compost as a fertilizer, one must know the chemical analysis of the material and take into account the range of nutrients compost will contribute to the vine, including but not limited to P and K. Good quality, finished compost will have a C:N ratio of less than 20:1. Of the total N per ton in the compost, about 30% is available to the vine. Of that, approximately half is available the first year after application, with the remaining N becoming available over the next three to four years.
Although grapevines can survive in depleted soils, maintaining adequate crop loads and vine health requires replacing the elemental nutrients removed in fruit and to account for the long time required to recycle N and K from prunings and leaves back into the soil. A successful fertilization program provides enough of the required elements without producing excessive growth, high juice pH or generating pollution. Scientific data and historical records combined with experience can achieve the goals of the fertilizer program.
Thanks to Dan Rodrigues of Vina Quest LLC for his contribution to this article.
Pruning wound protectant spray after pruning (photo by Dr. Doug Gubler, UCCE.)
Pruning and disease management are important vineyard practices that need attention in the dormant season. Pruning directly affects the upcoming season’s potential yield and quality and can directly and indirectly affect the vineyard’s long-term productivity. Pruning practices, and the care of pruning wounds, can also help manage trunk diseases. Pruning practices that optimize productivity and quality, and protect against trunk disease, will help extend the productive lifespan of vineyards.
Dormant Pruning and Yield Management
Grapevines are pruned for three main reasons:
To keep the vine in a shape that conforms to the trellis system and facilitates vineyard operations.
To remove old wood and retain fruiting canes or spurs for the current season crop, plus spurs for future wood placement.
To select a quantity and quality of fruiting wood that is in balance with vine growth and capacity.
The choice of pruning method is largely influenced by fruitfulness characteristic of vine variety. For instance, most raisin varieties are cane pruned because their basal buds produce shoots with fewer and smaller clusters than apical buds (Fig. 1). In contrast, most wine varieties are spur pruned, since most wine varieties have adequate basal bud fruitfulness and spur pruning is less laborious and costly than cane pruning. However, cane pruning is sometimes preferred for certain wine varieties, like Carmenere, which have low basal bud fertility, or in cool climate regions where cool spring weather might reduce basal bud fruitfulness in other varieties. Understanding the factors determining the bud fruitfulness provides insight as to the best pruning practice for a given vineyard.
Figure 1: Bud fruitfulness from different nod positions on 15-node and 20-node canes of common raisin varieties in California. Figure is elaborated from Cathline et al. 2020. Catalyst: Discovery into Practice. 4: 53-62.
Grapevine yield is formed over a two-year cycle that begins with the initiation of cluster primordia within compound buds. Cluster primordia are initiated in basal buds first, around bloom time, with more apical buds forming cluster primordia in succession, and most buds having formed whatever cluster primordia they will have by veraison (Fig. 2). Sunlight promotes cluster initiation, so sunny, warm weather between bloom and veraison helps maximize cluster primordia formation, whereas cool and cloudy weather can lead to less fruitful buds (Fig. 3). Because basal buds tend to form cluster primordia earlier than the apical buds, spring weather can have a greater impact on the fruitfulness of some varieties than others. For example, cane-pruned varieties initiate cluster primordia over a longer time period than spur-pruned varieties.
Figure 2: Timing of cluster differentiation in the primary bud along one 15-node cane of Thompson Seedless. Figure is elaborated from Williams 2000, Raisin Production Manual (UC ANR Publication 3393).Figure 3: Bud fruitfulness generally correlates with the light exposure. Note different varieties respond to light exposure differently. Figure is elaborated from Sanchez and Dokoozlian 2005, American Journal of Enology and Viticulture. 56: 319-329.
Raisin growers have long been advised to retain “sun” canes, which generally have more fruitful buds than “shade” canes. Cane and spur morphology can also indicate potential fruitfulness. Mature, round canes and spurs having moderate thickness and internode length are often the most fruitful. Narrow canes and spurs often indicate weak growth with inadequate starch content to support cluster primordia formation. In contrast, exceptionally thick canes with long internodes and a flattened shape, commonly referred to as “bull” canes, may also be expected to have poor fruitfulness. Stressed vines may have insufficient carbohydrate content to support maximum bud fruitfulness. Insufficient water, inadequate nutrition and poorly managed pest or disease issues (e.g. nematodes and powdery mildew) can all stress the vines and reduce bud fruitfulness. As previously mentioned, node position also affects fruitfulness, cluster size and fruit quality of cane-pruned varieties. Leaving longer canes could increase yields if the vines do not become overcropped.
Cluster initiation is generally completed by veraison, so, by late summer, the number of clusters a designated bud may have in the following season has already been determined. Therefore, before pruning, growers can collect and dissect representative buds from a vineyard and observe and count the cluster primordia with the aid of a dissecting microscope. This information may be used to help predict yield potential and adjust their pruning severity to help achieve a desired number of clusters per vine. As growers gain experience with this method, it might also help them adjust their canopy or irrigation management practices to help improve fruitfulness, since shoot exposure to light improves bud fruitfulness.
After a pruning strategy has been decided on, and the vines were pruned, the maximum potential number of clusters per vine has been fixed. One of the main goals of pruning is to retain the optimal number of buds per vine to regulate the crop size. If too many buds are retained after pruning, the vines may become overcropped, leading to poor canopy growth, unripe fruit and possible carry-over effects on the following year, resulting in erratic and delayed budbreak, slow canopy growth and poor yield and fruit quality. In contrast, if too few buds are retained, the vines may be undercropped, resulting in suboptimal yield and excessive canopy growth, which can cause self-shading, reducing fruit quality. Therefore, understanding bud fertility and potential crop load can help inform pruning decisions and thereby optimize yield and quality.
Disease Management
Pruning practices also have implications for grapevine trunk diseases, which can seriously reduce vineyard productivity. Trunk diseases are caused by different fungi including Esca or black measles, Botryosphaeria (Botryosphaeria canker), Eutypa lata (Eutypa dieback) and Phomopsis viticola (Phomopsis cane and leaf spot). All of these fungi can enter the vines through pruning wounds, especially after precipitation. After a fungal infection has been initiated, it grows toward the roots, slowly killing the vascular tissue, decaying the wood and eventually killing the vines. The typical symptoms from trunk-diseased vines are cankers, dead arms and cordons, and trunks, with vines collapsing in a few years. The economic loss can be dramatic, and trunk diseases may significantly reduce the productive life of vineyards.
Top left: Dead arms/cordons on vines with trunk disease. Top right: Botryosphaeria canker on trunk cross section. Bottom left: Esca symptoms on grape leaf. Bottom right: Eutypa dieback symptoms on grape (photos courtesy G. Zhuang.)
Pruning methods can affect the potential disease risk. Cane pruning typically has less trunk disease risk than spur pruning systems since cane pruning leaves fewer pruning wounds than spur pruning. The best mitigation strategy for trunk disease is prevention. Selective pruning, sometime referred to as “vine surgery”, can remove infected wood, sometimes resolving an established infection. However, vine surgery is laborious and will not be effective unless all the diseased wood is removed. Retraining may be needed to replace arms or cordons removed in surgery, and the surgery will result in large, open pruning wounds that could easily become infected if not protected with pruning protectants. The labor cost to renew the cordon or trunk is typically economically prohibitive in the San Joaquin Valley, and it also does not offer the long-term solution, since those fungi can slowly reinfect the vine if complete elimination of diseased wood was not achieved by the vine surgery. The current preventative measures include double pruning or delay pruning, pruning wound protection and vineyard sanitation.
Double pruning or delayed pruning helps prevent the exposure of final pruning wounds until February or March when most rain events finish and weather is warming. Less rain with warm weather helps the vines seal the pruning wounds and prevent the fungi entering through pruning cuts. However, double pruning or delayed pruning does have some barriers for some growers to adopt (e.g. pre-pruner and labor availability). For growers who can adopt it, double pruning or delayed pruning offers an effective way to minimize trunk disease.
Pruning wound protectants (mostly fungicides) are another option when double pruning or delayed pruning is impractical. Dr. Akif Eskalen, UC Davis, has been evaluating different pruning wound protectants in California since 2019, and the results from those trials can be found here: ucanr.edu/sites/eskalenlab/Fruit_Crop_Fungicide_Trials/. Fungicide efficacy is variable, but the application of pruning wound protectants before the rain event can help prevent the fungal infections. However, pruning wound protectants cannot provide complete protection, and we still do not know how long the protection lasts after the spray. More than one spray might be needed if rain events occur more frequently after pruning.
Vineyard sanitation should be also integrated into the trunk disease management plan. Because numerous fruiting bodies can be found on pruning debris left in the vineyard, complete destruction is desirable to reduce the source of inoculum and avoid new infections. An extensive sanitation of the vineyard should be practiced, keeping the inoculum level as low as possible. This can be accomplished by pruning out all diseased wood, removing it from the vineyard and destroying it by burning or burying.
Recently, mechanical pruning has become more popular due to the increased cost and declining availability of farm labor. However, mechanical pruning may leave more than double the number of spurs per vine compared to traditional hand pruning. Delayed pruning or pruning wound protectants should be applied after pruning to reduce the risk of trunk disease. In all, trunk disease does not only affect this year or next year’s yield and general vine health, but also reduces the longevity of vineyard production life.
An anemometer, found at the top of the weather station, measures
wind speed and direction and can be placed in or near the vineyard.
(all photos courtesy S. Vasquez.)
Talk to a farmer; at some point, the discussion will focus on the weather. It is the number-one topic discussed amongst farmers because weather can impact farming in significant ways. A rain event in June may benefit one crop but negatively impact another. Weather forecasts are important for scheduling farm activities, and for many years, farmers relied on the Farmer’s Almanac for long-range forecast.
Fortunately, today’s California farmers have multiple options to obtain weather forecasts and real-time data to make farming decisions. One of California’s oldest publicly accessible weather networks is the California Irrigation Management Information System (CIMIS). Built to improve irrigation efficacy, UC Davis and the California Department of Water Resources (DWR) established CIMIS in 1982. CIMIS has provided California growers with weather data accessed via the internet at cimis.water.ca.gov/. Growers can select one of the 145 stations near their property, download the data to a computer and manipulate it for their use (e.g. calculating ET). However, todays farmers are busier than ever, and having to rely on downloading climate data throughout the season can be challenging, especially when data from multiple locations are needed or when there are not any CIMIS stations nearby.
Growers can log onto CIMIS once an account has been setup
The CIMIS weather station network has been, and continues to be, a valuable tool for agriculture. But growers and researchers need weather data that better represents their farm or research location. Some of California’s first “local weather stations” used in vineyards were hygrothermographs that recorded temperature and humidity. Several UC grapevine pest and disease models were developed and tested in the late 70s and early 80s using hygrothermographs placed throughout California’s vineyards. However, in the late 80s, hygrothermographs were replaced with weather stations equipped with radio telemetry and controlled by a data-collecting base station. Since then, as technology improved, weather stations have become more sophisticated and provide real-time, on-demand data, 24 hours/day.
Some of the first weather stations recorded temperature, humidity, precipitation and wind speed and direction, displaying the data as graphs that needed some interpretation. Today’s grape growers have access to the same data, but data presentation is clearer with user-friendly web portals and phone and tablet apps. Additionally, climate, environmental and irrigation sensors have been improved or newly developed to generate data that can be used to make farming decisions (e.g. solar radiation, atmospheric pressure, leaf wetness, plant and fruit growth, plant health, soil moisture, electrical conductivity, pH, nitrogen, phosphorus, potassium, microbial activity, water pressure and usage, and more). Having access to numerous types of data allows growers to manage their vineyards in a much different manner than just a decade ago. However, access to that much data can be overwhelming if it is not understood or managed properly. Growers should be trained or have dedicated personnel to help interpret the information and share it with colleagues that will be making farm management decisions.
Some Practical Uses of Vineyard Weather Station Data
Advanced sensor technology has made it easier for growers to install, build, maintain and expand a reliable in-house network of stations collecting different information. Access to local data helps decision-making that impacts crop yield and quality. For example, in addition to knowing how much precipitation a vineyard experienced during a rain event, information such as leaf wetness, relative humidity, soil moisture and wetting depth can help forecast diseases (i.e. bunch rot), future fertilizer and irrigation applications and general vineyard activity that involves tractor work. Combine that information with vineyard characteristics (i.e. variety, soil type, etc.) and suddenly growers can evaluate more acres with fewer vineyard visits that save them time and money. The improved collection and transmission of data from base (aka weather) stations equipped with unique sensors have become valuable tools for managing vineyards. Base stations now incorporate weather, phenological, pest and disease models developed by university researchers to enhance their offerings via portals and apps. The following are some additional applications that can benefit grape growers interested in designing their own network of sensors.
Pesticide Applications
Pesticide applications must follow California’s laws and regulations. Prior to any application, climatic conditions must be checked so pesticide applications are optimized. Having a base station within a vineyard can improve pesticide application efficiency and efficacy. An anemometer, which measures wind speed and direction, can help pesticide applicators decide if wind conditions will permit a pesticide application. Wind speeds need to be between 2 to 10 mph to make a legal application. Knowing the wind direction can help decide the potential movement of a pesticide to an undesirable target (i.e. organic field). Tracking vineyard temperature can help determine if temperatures are hot enough to cause spray mist evaporation or phytotoxicity. In that situation, waiting for daytime temperatures to cool or spraying at night could help solve the issue. Being able to check a vineyard’s temperature prior to sending a crew to apply pesticides will save time, money and improve pesticide planning. Temperature sensors can also be used to detect inversion layers that can contribute to pesticide drift. Placing temperature sensors at multiple heights (e.g. 5 feet and 30 feet) will determine if the lower layer is cooler than the upper layer. When this happens, pesticides can move horizontally from thousands of feet to miles from the original point of application. When a vineyard experiences inversion layers, temperature sensors and a base station can detect the scenario and send an alarm to the person planning pesticide applications.
Degree Days and Temperature Modeling
Temperature, measured in degree-days (DD), influences grapevine growth throughout the season. From grapevine bud dormancy to fruit maturity, temperature regulates vine and fruit development. DD model predictions can help growers prepare for seasonal cultural practices (i.e. bloom sprays). For grapes, temperatures greater than 50 degrees F have been determined as the developmental DD threshold. In California, Thompson Seedless development as a function of DD has been determined. Approximately 50% bud break for this cultivar is observed when 155 DD are obtained with a February 20th start date. To reach 50% bloom, an additional 741 DD are needed, with maturity reached between 2880-3240 DD. A grower can use this information to track and identify DD specific to the varieties that they grow that have similar growth characteristics to Thompson Seedless. UC has also developed DD models for western grape leafhopper, omnivorous leafroller, powdery mildew and other pests and diseases to help grape growers make management decisions.
Chilling Hours
Grapes require a specific number of chill hours to complete dormancy, break bud and begin a new season. The minimum number of chill hours required for grapes to produce a commercially viable crop averages approximately 150 hours, which is low compared to stone fruit (i.e. cherries, peaches, etc.) that need ≥800 hours. Without adequate chill hour accumulation, bud break and yield become erratic, increasing farming costs significantly. Knowing when chill hour requirements are not being met can help a grower decide when to apply chemicals that improve bud break. A local weather station can better define chill hour accumulation than regional weather data (i.e. CIMIS). Most weather stations can automatically calculate the chill hours and portions. The chill portions algorithm accounts for warmer times of the day and presents a clearer forecast for predicting cold temperature needed for uniform bud break. More information about chill hour accumulation can be found here: fruitsandnuts.ucdavis.edu/Weather_Services/chilling_accumulation_models/about_chilling_units/.
Worker Protection
Worker protection is undoubtedly one of the most important responsibilities a grower has when people are working in the vineyard. Excessive UV or heat exposure can result in chronic and acute health issues. Weather stations not only track and monitor adverse weather conditions, but can also be programmed to send warning emails prior to critical UV or heat events. Since growers must record these events, weather station data is an easy way obtain documentation.
Weather stations have many uses in the vineyard beyond temperature and precipitation. The complexity of the station will depend on a grower’s need. They can generate an enormous amount of data that will need to be interpreted correctly so good decisions can be made. Table 1 shows what types of information can be determined from the different kinds of sensors.
Types of sensors needed to make management decisions.
Selecting a Weather Station that Makes Sense
Purchasing the right weather station will depend on the type of data needed to meet your needs. With multiple weather station options, growers can design a simple or complex weather station network that best suits their farm operation. Working with a weather tech company will help a grower determine their specific needs. After an initial needs assessment, a few follow-up meetings with a vendor will help finetune a weather network design. A 40-acre vineyard might easily be covered by a single station, especially when site characteristics are similar (i.e. climate, soil). However, using a single station to cover a 400-acre vineyard could result in misinterpreted climate data that may vary over a large property. Advancements in communication and sensor technologies now make it possible to have multiple sensors communicate remotely with a base station and have the information organized into an easy-to-read format.
Hardwired vs Solar Panel
The decision to use hardwired or solar power will depend on where the station will be located. Easy access to electricity and/or communication lines is usually the determining factor. Open space near buildings or rooftops are sometimes preferred because electricity is in close proximity. If in-field hardwired installation is preferred, wires placed in conduit to the correct depth and properly wired will be needed to avoid damage or interference with data collection. Solar-powered stations have become more affordable and are also easy to install. Solar panels and batteries will need to be checked and maintained biannually to avoid data collection interruptions. Whichever type you choose, it’s important to avoid installing stations near tall objects (e.g. trees, utility poles, buildings), paved roads or bodies of water because they will interfere with data collection. Poor site selection can result in poor data collection.
Number of Soil Moisture Sensors Per Acre
Temperature, humidity and wind speed and direction are less variable than soil moisture and can be located in one or two areas that represent the property (e.g. pump station). However, the number of soil moisture sensors required per block will depend on how variable the soil texture is. If soil and topography are homogeneous (i.e. uniform), and several blocks have similar characteristics (e.g. cultivar, rootstock, age, irrigation and management), two or three sampling points should be sufficient. Soil moisture measurements should be set at a minimum of one, three and five-foot depths to determine water movement. However, soil moisture sensors can take measurements every foot, which may be needed for certain situations. Additionally, if a vineyard has different types of soils that represent different blocks, multiple soils moisture sensors may be needed to collect accurate data for irrigation scheduling.
Basic vs High-End Stations
All weather stations will provide some type of climate data, but vineyard size (acres), one’s knowledge and confidence in interpreting the data and data access frequency will help you identify they type of weather station that’s right for your operation. A basic station will offer traditional climate data like temperature, humidity and, in some cases, rainfall amounts. In addition to traditional climate information, more sophisticated weather stations will include wind speed, soil moisture, water pressure and amounts, etc. that will result in a lot of data that someone will need to track if weekly decisions are going to be made. It’s important to evaluate the presentation of the data that weather station vendors offer. Portals and apps have simplified the way that growers see and use large amounts of data, which makes it easier for vineyard decisions to be made. If you have a larger operation or want to have a more automated system, a station that is connected to the internet can be a better option. With this station, you will be able to collect the data in real time in the field or remotely. General and more detailed data can be accessed from your computer or phone app. Some manufacturers offer services to set alarms for pest and diseases based on models, degree days and chilling days. The alarms can be sent to your phone via text, email or automated call. Services varies from providers.
General and detailed weather station information from an app.
No matter what type of base station and sensor configuration you choose to purchase, it is important that you first identify what information will help you make better management decisions. Additionally, you should identify someone that will be tasked with monitoring the system and sharing information with farm personnel. This person should be involved in the discussions with the vendor(s) since they will also be in constant contact with the vendor’s customer service representative.
Disease model (left) and degree days threshold option (right).
New guidance is being developed for regulating Biostimulants as their use in California crops increases.
Biostimulant products have become widely used in agricultural production, but there are still unresolved issues around definitions, labels and efficacy claims.
Nick Young, senior environmental scientist with CDFA’s Fertilizing Materials Inspection Program, provided an update on biostimulant labeling from a regulatory perspective as part of the UC Ag Experts Talk webinar
The pending USDA definition of a biostimulant is, “a substance, microorganism or mixtures thereof that, when applied to seeds, plants, the rhizosphere, soil or other growth media, act to support a plant’s natural nutrition processes independently of the biostimulant’s nutrient content. The plant biostimulant thereby improves nutrient availability, uptake or use efficiency, tolerance to abiotic stress and consequent growth, development, quality or yield.”
No state fertilizer program currently permits the term “biostimulant” on labeling, Young said, but there are established standards for many biostimulant ingredients and a path to distribution.
Per CDFA requirements, labels for biostimulant products do require that the specific ingredient be named. If the ingredient isn’t recognized, then efficacy data may be required. Recognized ingredients should appear in a guaranteed analysis.
The issue with some biostimulant products, Young said, is that they may be required to have registration both as a fertilizing material and a pesticide. Adding the pesticide label is more costly and time consuming, and manufacturers would rather avoid the pesticide label if possible.
Young noted that just because a product is registered with CDFA as a fertilizing material, that doesn’t preclude CDPR jurisdiction.
An update draft of the Guidance for Plant Regulators and Claims, including Plant Biostimulants, was released in November by EPA, and Young said it has clarified many issues with biostimulants for industry and regulators, but questions still remain.
“It was a step in the right direction and there are a couple of flaws, but it is going to help the industry and regulators know what to do,” Young said.
Claims are a key EPA consideration in determining if a biostimulant is a pesticide. Products that are not considered pesticides are plant nutrients and trace elements, plant inoculants, soil amendments and vitamin-hormone products.
The regulatory approach from CDFA for products that have multiple plant regulator and non-plant regulator modes of action is if it can be demonstrated that a particular product has the activity claims on the product label and does not make any plant regulator claims on the label, it can be excluded from FIFRA regulation.
Eskalen noted that sudden vine collapse vines he tested were planted on Freedom rootstock and had presence of vine mealybug as well as girdling at the graft union, lack of starch in the rootstock and lack of feeder roots (photo by A. Eskalen.)
Grapevines can die for a number of reasons, but increasing incidences of Sudden Vine Collapse across all California grape growing regions have sparked investigation into possible causes for this disease.
In the 2020 San Joaquin Valley Virtual Grape Symposium, UC Davis plant pathologist Akif Eskalen shared what he and other researchers know about Sudden Vine Collapse and some management options.
Eskalen said this condition causes sudden death of grapevines in the middle of summer. Sudden Vine Collapse disease has occurred in vineyards in the Lodi and Monterey grape growing regions as well as San Luis Obispo and Santa Barbara counties and San Joaquin Valley counties including Fresno, Tulare and Kern. Similarities in Sudden Vine Collapse include vines planted on Freedom rootstock and the presence of vine mealybug. In his investigation, Eskalen also found symptoms of girdling at the graft union, lack of starch in the rootstock and lack of feeder roots on infected grapevines.
Conclusions from a sampling study done by Eskalen found that in each of the vines with moderate to severe decline symptoms, GLRaV-3 and GVA, grapevine leafroll virus and grapevine virus A were present. Grapevine trunk disease pathogens and Fusarium solani were isolated from the symptomatic grapevine samples, but no consistent fungal pathogen was found among all vine samples.
He also noted that Freedom rootstock is most susceptible to co-infection by grapevine leafroll associated viruses and vitiviruses. Virus infections can also cause graft incompatibility in certain rootstock, Eskalen noted.
Results from the sampling study also showed that efforts of the rootstock to reject the scion following infection causes girdling at the graft union, preventing flow of starch throughout phloem of the vine. The inability of the plant to transport starch leads to starch depletion in the roots and subsequent lack of feeder roots, further preventing the vine to acquire nutrients from the soil. Finally, Eskalen said, interactions between the grapevine viruses and Fusarium solani may play a role in the vine death.
Management options for grape growers, Eskalen advised, include first testing to confirm presence of viruses. Lodi Grape Growers recommends testing both healthy and collapsed vines for common leafroll viruses in California.
Testing in the fall (before the leaves senesce) is the best time of year to sample for these viruses.
Mealybug control in the vineyard is also advised as is removal of infected grapevines.
When replanting, Eskalen recommended using less sensitive rootstocks and continued control of grapevine trunk diseases in the vineyard.
An uneven appearance and cracking on the outer surface of citrus fruit called puff and crease, also called crease and split. This condition impacts fruit quality and can be caused by environmental conditions and nutrient imbalance (photo by Robert G. Platt, UC Statewide IPM Program.)
Nutritional status and environmental conditions contribute to puff and crease in citrus.
This disorder, seen as an uneven surface on the fruit, affects fruit appearance and quality.
Puff and crease indicates a separation of the albedo (the white material under the outer peel) from the outer surface of the fruit. This internal damage is visible externally as an uneven appearance of the rind surface, where some of the fruit surface appears puffy while other areas are indented. Satsuma mandarins, early navel varieties, and Valencia’s are susceptible to this rind defect.
Dustin Stewart, a crop advisor with Ultra Gro in Fresno, Tulare and Kings counties, said due to recent warm fall days, puff and crease is more prevalent on some early maturing Satsumas.
Puff and crease can be caused by late nitrogen applications in an orchard. Evidence linking nitrogen to puff, crease, smaller fruit size and staining exist. These negative effects are most significant at nitrogen levels greater than 2.6% N, Stewart said, but it is more likely that nutritional imbalance combined with warm daytime temperatures in the fall drives this condition in citrus. Once puff and crease has begun, it can’t be reversed, Stewart said, but this condition can be mitigated by making sure calcium, molybdenum, zinc and sulfur levels in the plant are adequate to assuage puff and crease.
These nutrients play a role in retarding senescence, which in the early stages is an organized phase of metabolism and not just a breakdown of tissue.
Application of plant growth regulators cytokinin and auxin or gibberellic acid can also delay development of puff and crease in citrus fruit. Cytokinin is a plant hormone that influences growth and stimulation of cell division. Cytokinin moves in the xylem and pass into leaves and fruit. Cytokinin also acts in conjunction with auxin, another plant hormone, to retard senescence.
Naturally occurring plant growth regulators must be absorbed by plant tissue to be effective. UC IPM guidelines advise spray applications to be done when climatic conditions are favorable. Good spray coverage is also important. Uptake of naturally occurring plant growth regulators (PGRs) by the trees is improved in warm and humid conditions. Use of a naturally occurring plant growth regulator to delay maturity and prevent puff and crease will be more effective in healthy, well-watered orchards with adequate nutrition. Tree size, canopy density, location of fruit and suitability of spray equipment for those conditions are other considerations for an effective PGR application.
Gibberellic acid treatments, applied at the correct time in fruit maturation, delays rind aging and softening. Timing of treatment in early mandarin varieties is two weeks prior to color break in orchards where harvest will be delayed.
Evaluation of a potential pistachio orchard site calls for understanding pistachio life cycle, tree water use, rooting characteristics, tree spacing and canopy structure as well as harvest requirements and field traffic (photo by C. Parsons.)
Production potential of a pistachio orchard is maximized with deep, well-drained soils, but this hardy tree nut species can also produce on marginal ground.
During the 2020 UCCE Pistachio Short Course, UCCE Nut Crops Farm Advisor Mae Culumber in Fresno County outlined considerations in site selection for pistachio production. Pistachio trees’ tolerance for alkaline and saline soils has led many orchards to be planted in less-than-ideal soils or locations.
In her presentation, Culumber noted that site selection for an orchard should begin with understanding pistachio life cycle, water use, rooting characteristics, tree spacing and canopy structure as well as harvest requirements and field traffic.
Evaluation of the potential orchard site should include land cost, soil texture, drainage, chemistry and amendments. Development considerations include the cost of land leveling as well as irrigation system installation, energy requirements, pressure, filtration and maintenance. Distribution patterns and the necessary irrigation set frequency should also be evaluated. A UC publication that includes sample costs to establish a pistachio orchard can be found at coststudies.ucdavis.edu/en/current/.
When considering a site, Culumber said that cost of a soil evaluation represents only a small portion of total orchard development costs, but the information will be valuable in terms of management decisions. Soils should be evaluated for structure, permeability, stratification, drainage and salinity/fertility. The UC Soil Resource lab and the SoilWeb Earth in Google Earth provide a range of soil information including historical images of past crops at the site and their effect on the site and the soil. Assessment of historical images of the site can reveal if the ‘problem’ is soil or management related. Culumber noted that online survey data might not match ground observations due to the scale of the surveys and the influence that agriculture land management and irrigation can have on salinity levels.
On-site observations are the best means of determining conditions that will reduce tree performance.
Characterization of the soil profile can be accomplished by digging backhoe pits or auguring. Samples should be taken at several depths from at least one backhoe pit to six feet for each 40 acres in one soil type identified with web soil survey tools. Depths of layers, texture, lime, hardpan, rooting and drainage should be recorded, which will help in choosing equipment for land modification and amendments for reclamation if needed.
Permeability of soils can be improved by deep ripping prior to planting, using calcium supplying amendments, adding organic matter and planting cover crops. Choosing an irrigation system that matches the water infiltration rate is also important to establish and maintain tree health.
Soil and water samples should be submitted to a certified ag lab to determine salinity levels. Long-term productivity may be impacted if the water source is 4.5 to 6 dS/m EC. Salt increases osmotic potential, costing plants energy and interfering with water uptake.
Bacterial leaf spot, shown here in iceberg lettuce, is found in most
lettuce growing regions of the country, including California, Arizona and
Florida (photos courtesy Richard N. Raid UF/IFAS.)
Bacterial Leaf Spot (BLS) of lettuce was first reported in the U.S. in 1918 in South Carolina. The disease then expanded to other production areas in California, Arizona and ultimately in Florida. The disease causes losses of entire production when outbreaks are significant. This disease is particularly devastating to the leafy vegetable industry because it is favored by warm and humid conditions. Besides the U.S., the disease has been reported in lettuce production in Europe, Asia and South America as well.
BLS is caused by the bacterium Xanthomonas hortorum patovar. vitians, formerly described as X. campestris pv. vitians (the pathovar (pv) vitians is unique to lettuce.) The bacterium can attack any type of cultivated lettuce; no relationship between lettuce type and immunity to the pathogen is known. The bacterium has three races identified thus far: races 1, 2 and 3, but race 1 has been reported in western and eastern lettuce production areas in the U.S. BLS is sporadic in lettuce with losses up to 100% in subtropical productions areas. Xanthomonas hortorum pv. vitians reproduces quickly when high humidity, leaf wetness and high temperatures are conducive for disease development. The infection starts as small brown spots that in later stages of disease development coalesce to form bigger spots.
Here in Florida at least one small outbreak is reported by growers during the growing season from October to May. Lettuce growers in Florida are the most affected by this disease because the warmer and more humid conditions in the state are more conducive for disease development; after a severe infection occurs, lettuce cannot be commercialized.
In the last five years, growers have been able to contain the disease from spreading to other lettuce farms nearby by destroying infested crop areas with lettuce BLS; therefore, small losses were manageable, and growers did not lose entire crops.
Control Methods
The disease is not prophylactically controlled as other lettuce diseases such as downy mildew, sclerotinia drop and powdery mildew because it is uncertain when the pathogen population will increase and develop to cause diseases. There are no bactericides that can eradicate BLS from lettuce production. Copper-based compounds can be effective in reducing the severity and incidence of outbreaks of BLS when the disease first appears. However, there is a potential for development of resistance to copper in the pathogen population following repeated applications.
The high variability of disease outbreaks each year makes it impossible to predict when preventative applications of copper should be used. A combination of a copper fungicide with mancozeb may be effective to reduce BLS in lettuce; both compounds have some efficacy against bacteria but are protectants and not curative. Systemic Acquired Resistance based fungicides probably have limited activity but can be used as preventative as well.
Several practices may help reduce outbreaks of lettuce BLS. These practices, however, should be part of an integrated disease management program instead of recommended alone.
Crop destruction at the point of infection and surrounding areas have proved to be effective to avoid other neighboring lettuce fields from becoming infected. This strategy has been successful in containing disease spread to other fields during recent small outbreaks in Florida. However, when the disease is highly spread, this strategy may not be economically feasible. This recommendation may only help in early detection of the disease.
The bacterium is believed to be transmitted in infested seed, which is the most common avenue of disease introduction. The use of disease-free seed is highly recommended, but to date there is no effective method to detect the pathogen on seeds and assure cleanliness from BLS. Seed production should be conducted in dry, cool environments with less likelihood of bacterium development.
The BLS pathogen spreads by rain and overheard irrigation. Drip irrigation can be used to mitigate spread of the disease by keeping foliage as dry as possible. In California and Arizona, most lettuce fields are drip irrigated. However, drip irrigation is not economically feasible in Florida’s commercial field production currently.
An adequate weed control in nearby areas of lettuce fields is highly recommended because the pathogen may be epiphytic on weeds. Many weed species such as those in the families Asteraceae, Amaranthaceae, Aizoaceae, Chenopodiaceae, Portulacaceae, Solanaceae and Malvaceae may host the pathogen, X. hortorum pv. vitians.
Ultimately, host resistance is the most efficient control method against the BLS disease. BLS resistance can be found in certain heirloom lettuce cultivars that are not acceptable for commercial production. Disease resistance towards race 1 strains of the pathogen can be easily transferred to romaine, iceberg and leaf lettuce cultivars using traditional breeding methods. There have been releases from the USDA Agricultural Research Service of lettuce lines with resistance to race 1 for the California/Arizona lettuce production system. The University of Florida is developing such resistances for the Florida production system. Resistance to races 2 and 3 against X. campestris pv, vitians remains to be reported.
Further Research
A partnership was formed between plant breeders, geneticists, plant pathologists, weed management scientists and extension agents from the University of Florida Institute for Food and Agricultural Sciences, Pennsylvania State University and USDA-ARS. This partnership will investigate the lettuce BLS interactions using several approaches that include breeding, lettuce genetics, pathogen genetics and detection. Researchers will improve lettuce cultivars against several races of X. hortorum pv. vitians and provide information on pathogen and lettuce genetics that will help the industry to efficiently manage this detrimental disease; this information will be released to growers, producers, the seed industry and other stakeholders in English and Spanish.
Baterial leaf spot symptoms in romaine lettuce(top) and iceberg lettuce(bottom).
