Mulch is applied around base of Satsuma mandarin. Trials conducted in this foothill mandarin production region showed soil moisture must be monitored when applying mulch. Growers will have to adjust irrigation scheduling to avoid over watering (photo by C. Fake.)
Keeping orchard floors bare is standard practice in California citrus production, but a Placer and Nevada County study on mulching mandarin trees is breaking ground for alternative management.
As the study enters its fifth year, UCCE farm advisor Cindy Fake noted some of the key findings. Mulched orchards have lower soil temperatures during the hot summer months and soil moisture is rarely depleted. Excessive fruit drop is mitigated and herbicide use has declined.
Parts of this foothill mandarin growing region east of Sacramento have much poorer soils than other growing regions. There are also hillside plantings where erosion is an issue, Fake said. Summer heat can be intense, and the trend toward longer dry conditions adds stress to the trees. Frost damage at the elevation of this growing region is rare and use of microsprinklers provides frost protection.
The mulching trials were initiated to help mandarin growers maintain and improve tree health and resilience in the face of climate change.
A downside of the mulch applications is the time and labor it takes to mix and apply.
Growers participating in the trials applied mulch composed of 50% horse manure and 50% wood chips, as both materials were readily available in the area. Mulch was applied under the canopies of the trees at depths ranging from four to six inches. The depth was reduced to four inches after two very wet years. Mulch was applied in the spring, Fake said, and it is critical to apply to moist soil. If it is applied to dry soil, it will take three to four major irrigations to move water through the mulch into the soil.
The trials also showed that when applying mulch to an existing orchard, soil moisture must be monitored. Growers used to applying the normal amount of water to their trees will have to adjust their scheduling, Fake said, as about 30% less water is needed.
The trial showed that moisture levels under the mulched tree were consistent and the profile is able to retain moisture with minimal depletion at a 6-inch to 12-inch depth. Most moisture depletion is occurring in the top six inches of soil and is lower than the control. Moisture levels in the control depleted at a more rapid rate throughout the soil to a depth of 12 inches.
By applying mulch in the spring, Fake said it would decompose over the next six months and not be a food safety issue at harvest. She also recommends mulching new trees to help maintain moisture for root growth.
More information on the mulch trial can be found at
Placer/Nevada Foothill Farming website. There is also an information sheet on how to use mulch on the UC ANR website.
Adult walnut husk fly. These insect pests emerge from the soil during the summer months and lay eggs inside the walnut husk. UC IPM specialist Jhalendra Rijal said early season WHF infestation can also cause shriveled kernels (photo by Jack Kelly Clark, courtesy UC Statewide IPM Program.)
All walnut varieties are susceptible to damage from walnut husk fly (WHF). WHF are distinguished from other fruit fly varieties by their yellow coloring at the base of their wings and by the dark triangular band on their wing tips. Size is similar.
Damage to developing walnuts occurs after the female husk fly lays eggs inside the walnut husk. Feeding by larvae results in a soft and dark husk inside.
UCCE IPM advisor Jhalendra Rijal presented information on the biology, monitoring and control of this walnut pest for the West Coast Nut walnut conference.
Rijal said that WHF damage appears different than sunburn damage on walnut husks. Sunburn will cause a dry appearance, typically in the sun-exposed side of the nut, while nuts infested by WHF will appear soft and moist at the feeding site. Larval feeding inside the hull causes staining on shells and makes husks difficult to remove after harvest. Early season infestation can cause shriveled kernels.
WHF produce one generation per year. Adults emerge from soil in the summer months. Female WHF will feed on some sugar and nitrogen sources out in the orchard including aphids or bird feces for a week or so prior to laying eggs. Larvae hatch from eggs, feed on the husk and drop to the ground to pupate under the soil over winter.
Rijal said monitoring of adult WHF emergence is important to time insecticide applications for control. Yellow sticky traps with the attractant ammonium carbonate should be hung on the north side of the tree as high as possible and shaded by foliage. Check these traps two to three times per week to help to determine if WHF is emerging and laying eggs.
Rijal advised using a lens to identify female WHF in the traps. They are larger than the males and have light-colored segments on legs. Crushing their abdomen will reveal if they are carrying ‘rice-grain’ looking eggs.
Insecticide applications may be needed as soon as the first female WHF with eggs is found, depending on the orchard damage history, Rijal said.
The control materials target adult WHF and spray applications should be made before large numbers of egg-carrying females are found. The most effective control insecticides can be found at on the UC IPM web site.
Multiple applications may be necessary to cover the entire susceptible period as adult flies emerge over two to three months in the summer.
Rijal said research is needed to determine if cultural control (disking orchard soils to break life cycle) is possible or use of potential biocontrol agents such as insect pathogenic fungi or nematodes is needed to kill the larvae or pupae when they are in the soil. There is not natural biological control known for WHF.
A field demonstration of mechanized vineyard pruning at the Kearney Agriculture Research and Education Center. Transitioning existing vineyards to a single high wire system has been most successful for mechanized pruning, reports Fresno County UCCE farm advisor George Zhuang (photo courtesy G. Zhuang.)
Mechanical pruning operations continue to increase in San Joaquin Valley wine grape vineyards as a way to save on labor costs. Dormant pruning, suckering and leaf removal in vineyards can all be done mechanically.
George Zhuang, UCCE viticulture farm advisor in Fresno County, in a presentation at the San Joaquin Valley virtual Grape Symposium, said existing trellis systems in vineyards can be converted to mechanical pruning systems and retain production and fruit quality.
The most common trellis systems in San Joaquin Valley wine grape vineyards have been two-wire bilateral cordon, “California sprawl,” and quadrilateral, and both can be adapted to mechanized pruning, Zhuang said. The single high wire system is the standard system for mechanization.
Recent trials have looked at differences in cordon height and how that affects vineyard production. Transitioning existing vineyards to a single high wire system has been most successful for mechanized pruning operations. This trellis system has a single high wire at 62 to 66 inches in height, is single canopy, non-shoot positioned and has around 35% exposed leaf area. Production is at 18 to 24 months and yields are 11 to 24 tons per acre with 7×10-foot spacing.
Zhuang said the other trellis system in use in vineyards where mechanized pruning is done is the quadrilateral. This system has a divided canopy and a higher percentage of exposed leaf area. Production depends on variety and spacing of vines.
Setting up the box size is important in mechanized pruning. The spur height sets the height of the bearing surface. Precision pruning is four inches, while 6- and 8-inch set ups may require some hand pruning to keep from overloading the vines as the bearing surface increases.
Two Fresno county trials were done to compare single wire height in trellis systems for winegrapes. Vines were planted in 2017 and hand pruned the first year of production in 2019. In 2020, vines were mechanically pruned. Heights tested were 68-inch cordon and 52-inch, the classic “California Sprawl” height. First yields were comparable in both systems. Sugar (measured by Brix) was increased in the high wire system due to more leaf area, Zhuang said. Water use was lower in the higher wire system.
He said the trial would continue to determine if there are significant differences as the vines mature.
Availability of mechanical pruning custom operators may be a limiting factor going forward. The machinery is a significant investment more suitable for larger acreages. Smaller growers would need to hire a custom operator. Zhuang said he has seen efforts by winegrape growers to design and build their own pruning machinery.
At least four species of garlic bulb rots have been recovered from samples submitted to UCCE researchers by garlic growers. Fusarium proliferatum has been the most common, being recovered from all storage facilities and farms (photo courtesy T. Turini.)
Lesions on garlic cloves, which affect crop quality, have been associated with several species of Fusarium, said Tom Turini, UCCE vegetable crops farm advisor. Similar garlic diseases caused by this soilborne fungus were reported in other production areas, but recent studies investigating this issue in Central California until now have not been conducted.
The Fresno County 2019 crop report noted that 24,180 acres of garlic were harvested that year, making garlic one of the higher acreage vegetable crops in the county. In addition, the 2018-19 CDFA Agricultural Statistics Review documents that more than 80% of the garlic produced in the state was in Fresno County based on gross crop value.
Turini is working with UC Davis plant pathology specialist Cassandra Swett, who is receiving samples collected from fields and those submitted by growers and processors. In 2020, Swett ran tests to identify the Fusarium species associated with the clove lesions. The work planned for this growing season should provide a robust set of detailed information regarding the identity of the fungus responsible for the crop damage.
Swett reported that at least four species of garlic bulb rots have been recovered from samples submitted. Fusarium proliferatum has been the most common, being recovered from all storage facilities and farms. F. oxysporum and F. falciforme are also common, present in about 50% of farms or facilities. These three species have been confirmed to be capable of causing a clove rot in preliminary trials.
proliferatum is the only species found occurring alone, suggesting it might be the primary driver of bulb rot. A fourth species, F. brachygibbosum, was also found and is being tested for pathogenicity. Neither this species nor F. falciforme have previously been described as garlic pathogens. Swett said that with multiple species causing garlic bulb rot, it will be critical to identify strategies that can co-manage all species. Understanding when and how infections are occurring can help with identifying critical target periods for control.
Garlic bulb rots are not a new issue in garlic production, Turini said. Fusarium was previously associated with garlic rot in the Fresno production area and the fungus was considered ubiquitous. Current observations are that the issue is more common, and the character of the symptoms is not identical to those described decades ago. The work in progress will provide insight into what fungi are involved in the disease and other details critical in taking reasonable steps to mitigate the damage.
Research is ongoing to find the best strategies for material applications that help walnut trees overcome warmer winters (photo by C. Parsons.)
Walnut varieties with higher chill requirements during winter dormancy may be facing production challenges in the future. Climate prediction models are showing the amount of winter chill needed by trees may not be achievable every year.
UCCE Orchard Systems Advisor Katherine Jarvis-Shean said walnut varieties, including Chandler, might need some help to achieve a robust leaf-out in the spring. Research is ongoing to find the best strategies for material applications that help walnut trees overcome warmer winters.
Growers need to become familiar with the UC Davis Fruits and Nuts research on chill portions in preparation for warmer winters in the future, Jarvis-Shean said. Chill hour calculations differ from the newer chill portions models. Information on chill hours and portions can be found here.
Climate prediction models show that the amount of chill needed by walnut trees may not be achievable every year. A regional look at climate prediction models show more warming than cooling. There will still be variability, Jarvis-Shean said, but there will be more low-chill winters and more winters with lower chill than what is now experienced. By mid-century, the models are predicting less chill in the southern San Joaquin Valley than is necessary for the Chandler variety.
Research done over the last 30 years shows that dormancy is influenced by a number of factors, including hormones, transport capacity, oxidative stress and metabolism. Budbreak in the spring is normally preceded by a big upswing in starch production. Warm winters cause trees to lower their starch production and keep sugar levels stable. After trees make this adjustment, Jarvis-Shean said, they need more heat than normal in the spring to achieve high starch pre-budbreak.
Effects of a low-chill winter on walnut trees become apparent at leaf-out. Some buds don’t break, resulting in fewer flowers. A wider maturity window means there will be size variability in nuts. Early setting nuts are larger, taking up the available carbohydrates, and leaving later-maturing nuts much smaller.
Dormancy-breaking chemicals that can stimulate the hormone, transport capacity, oxidative stress and metabolism systems are under study to determine if their use can compensate for lower winter chill. Joint UC ANR/UC Davis studies are looking at applications of hydrogen cyanamide, also known as Dormex, a nitrogen cocktail and hormone analogue, over a couple of winters to understand how trees respond to treatments and to achieve consistent results.
Roof rat damage in citrus can vary depending on the variety. Extensive girdling has been observed in Lisbon lemons (photo courtesy UCCE.)
Invasive roof rats are causing damage in citrus orchards throughout California.
Kern and Tulare counties are sites of roof rat infestations in citrus as well as Southern California citrus growing regions. Once a population of roof rats becomes established, they can be found throughout an orchard, girdling tree branches and damaging fruit.
Roger Baldwin, UCCE wildlife specialist, said a multi-year study is underway to develop management strategies for roof rats, which are causing significant damage in a number of tree nut and fruit crops.
Roof rats are active year-round, building nests in citrus trees or burrows near the base of the trees. They forage away from their burrows, and signs of activity are sometimes difficult to see. Their burrows are about 1.5 to 2 inches in diameter, distinguishing them from ground squirrels.
Baldwin said roof rat activity may vary depending on the variety of citrus. In Meyer lemon, for example, he said rats have been found to eat the rind and leave the fruit. Extensive girdling of tree branches has also been observed in Lisbon lemon, but little branch girdling and more extensive fruit damage in navel orange. In navels, rats chew holes in the rind, eat the fruit and leave a shell behind. Another sign of a rat infestation, in orchards with snail populations, is piles of empty snail shells.
Baldwin said the UC study would be focused on cost-effective methods of control, including rodenticides and traps. He said the new A24 trap on the market, which use lures and CO2 cartridges triggered by rat activity, may save labor costs. However, these traps have not been tested in ag fields, so their efficacy is currently unknown. Rodenticides are another alternative, but restrictions may limit use. Diphacinone grain can be used, but bait stations are more effective when placed in trees rather than on the ground. Bait is also less likely to be eaten by non-target animals when placed up in the tree.
UC researchers plan to study rat movement patterns in orchards, which will help them test efficacy of management tools. Growers interested in participating in the study should contact UC research associate Ryan Meinerz at rmeinerz@ucdavis.edu for more information. More information on controlling roof rats can be found at UC ANR publication 8513, Managing Roof Rats and Deer Mice in Nut and Fruit Orchards.
Branched broomrape attaches to the root of a host plant, extracting water and nutrients. Methyl bromide was used to eradicate this weed in the past. Work is being done to find new control methods (photo courtesy G. Miyao, UCCE retired.)
The ‘delicate’ issue of reporting branched broomrape infested tomato fields continues in California.
Infestations of this parasitic weed in tomato fields can severely decrease yields because the weed attaches to tomato plant roots and extracts water and nutrients.
Gene Miyao, retired UCCE farm advisor from Yolo, Solano and Sacramento counties, said control options for branched broomrape are limited and expensive. The potential exists for spread of branched broomrape from field to field via equipment and field workers. Severe infestations of this weed occurred in Sacramento County in 1959. The California processing tomato industry responded with funding to begin an eradication effort which lasted for two decades. The program ended with successful control of the broomrape weed. Eradication was possible with the use of the fumigant methyl bromide.
Branched broomrape is classified in California as an “A” pest, an organism of known economic importance,and is subject to enforced action, including eradication, quarantine, regulation, containment, rejection or other holding action.
Extent of the branched broomrape infestation in tomato fields is not fully known, Miyao said. It has been reported in several tomato fields north of Stockton in recent years. Consequences of reporting an infestation to a county agricultural commissioner is complete loss of the crop without harvest within the infested area of the field. Miyao said not reporting an infestation also has consequences. At risk is the likelihood of increased spread of this weed and further crop loss in the future.
Research by UCCE weed specialist Brad Hanson found that emergence of broomrape occurs during early tomato fruit set and continues over a 10-week period. Attempts to hand weed broomrape can reduce the problem as it prevents seed production, but the weeding crew must carefully bag all shoots and stay out of uninfested parts of the field to contain the spread via seed. The hand removal must continue almost weekly for an extended period. Branched broomrape produces large amounts of tiny seeds, and seed set occurs within a week of emergence. Shoots of this weed are often hidden by the tomato plants and can go undetected.
Miyao said a grower who chooses to knowingly harvest through a broomrape-infested area will spread the seed down the row and will risk spreading seed across the field including into other fields. A grower who unknowingly harvests through an infestation is equally at risk.
The California Tomato Research Institute sponsored an herbicide trial to control branched broomrape in tomatoes. The trial in an infested field used two herbicides not currently registered for use in California. Compared to a successful Israeli program, it has not been as effective in the California trial. The herbicide rates and timing will be modified in the 2021 season. Miyao said this control program would be helpful, but if even a small percentage of the weed escapes, the seed bank in the soil will increase.
Miyao said an effort to eradicate this weed while the number of impacted acres is limited would be more cost effective than waiting until the infestation is widespread. The issue for the California processing tomato industry is that CDFA rates branched broomrape as a class “A” pest, which creates the ‘quarantine-like’ conditions of a no-harvest. Allowing the weed to not be under CDFA regulation could potentially cause substantial crop loss from this parasitic weed without an adequate control program.
Vine mealybug are the main mealybug species causing economic damage in California vineyards (photo courtesy Stephanie Bolton, Lodi Winegrape Commission.)
Control of vine mealybug (VMB) in grapevine continues to be stymied by this pest’s ability to take refuge underneath bark of the grapevine trunk and below the ground.
Kent Daane, UC pest management specialist, in a presentation for the San Joaquin Valley Virtual Grape Symposium, explained how VMB infestations persist in spite of insecticide applications and healthy parasitoid populations.
Daane said VMB is the worst invasive mealybug species due to its potential for causing extensive crop damage. Highly prolific and present year round in most California grape growing regions, VMB produces copious amounts of honeydew, causing vine defoliation and making fruit clusters unmarketable. VMB is also known to transmit viruses.
Vine trunks, roots and canes all provide protection for VMB from contact insecticides and natural enemies. In addition, ants drawn by honeydew will tend mealybug, kill parasites and remove honeydew from VMB hiding places. Knockdown of VMB on leaves with newer systemic materials can be achieved, Daane said, but population densities make complete control difficult.
Mating disruption may help control VMB, but the populations that persist under the bark remain an issue.
Trials using organic production approved materials showed they were only effective on contact, leaving the population hidden under the bark and below ground to grow.
In the San Joaquin Valley, systemic insecticides Movento and Applaud were effective on exposed areas of the vines, but left the hidden population. Daane explained that VMB infestations are clumped in that they start small in a few places that are difficult to find. It may be more cost-effective to spot spray where an infestation is found rather than treating an entire block.
Parasitoids have provided some control, but mainly on exposed areas. They cannot parasitize VMB under bark and can be killed by ants. Ants in vineyards aid VMB by not only removing honeydew, but making underground habitat available during periods of high temperatures.
There are strategies being developed to reach VMB populations under bark of the trunk and underground, Daane said, but they are expensive. Spot spraying of systemic insecticides is one alternative. Girdling the trunks can expose VMB and applications of tanglefoot directly on the exposed population is being studied. Barriers with insecticides placed around the trunk have also been tried to slow the movement of crawlers. Flaming the bark is another control tactic that is being studied.
Another study is the use of nematodes to impact VMB populations on the roots.
Figure 1: Holistic management tools that help improve soil structure and soil quality includes physical and chemical controls (Part I in this article series) and biological controls (Part 2).
You may have heard or read about improving your soil health or soil quality over the last year as this area of field management gains more attention. But what exactly do the terms mean and how can you incorporate the concepts in your day-to-day soil management practices? In Part 1 of this article series, the physical and chemical connection to optimizing soil quality by focusing on the structure of your soil will be explored.
Soil Quality and Health
Soil quality has broad application to your farm. It refers to how well a soil functions physically, chemically and biologically and how well it does its “job” (Fig. 1, page 6). For example, a forest soil has a different job than a farm soil, and soil properties can be measured on how well the soil is performing (e.g. soil structure).
Many factors influence the soil quality on a farm and are summed up in Figure 2, on page 7. In this article, the focus will be on the physical and chemical management practices that maximize soil quality, expressed here as soil structure (Fig. 3).
Soil health refers to the interaction between organisms and their environment in a soil ecosystem concept and the properties provided by such interactions (e.g. ecosystem stability). When you think of soil health, think of the biological integrity of your field (e.g. microbial population and diversity) and a focus on supporting plant growth. This will be the focus of Part 2 of this article series.
Poor soil structure and the resulting decrease in soil quality continues to impact yields in many farming areas. This is because many factors can negatively impact soil structure, including soil compaction from field equipment, poor salinity management, rainfall and irrigation droplets, excessive tillage, etc. Fortunately, fields that have poor structure (see Fig. 3 for a visual reference) can be fixed once you determine what is causing the issue. I strongly recommend that you put your field detective hat on and work with your favorite Certified Crop Adviser (CCA) to diagnose why your field is not performing as expected.
Figure 2: Crop productivity is influenced by several interrelated concepts, which have an impact on the soil quality of a field (courtesy K. Wyant.)
Physical Controls
Physical and chemical management strategies are important to help reverse poor soil structure and thus improve the overall soil quality of your field.
The physical controls on soil structure generally relate to reducing disturbance to the soil and protecting the soil from future disturbance. With this broad mandate, there are a variety of techniques out there to accomplish this goal and help restore soil structure, but reducing tillage and incorporating cover crops into the growing operation are the most important.
Tillage Practices and Soil Structure
Field activities like tillage are crucial for any successful growing season, whether in the short term or long term. However, excessive tillage can be hard on your soil structure as common implements can slice, compact or crush soil aggregates and quickly change a soil from having excellent tilth (see right side of Fig. 3) to one that lacks those properties (see left side of Fig. 3). The physical destruction of aggregates can have an immediate impact on your soil quality and can impact your operation’s bottom line. Fortunately, there are many modern options for avoiding excessive tillage, including conservation tillage and residue management, strip tillage and even a complete elimination of the practice (no-till). Every operation is different, and changes in tillage need to take local growing practices into account before moving forward. Reducing tillage has been shown to improve soil structure and soil quality; however, pulling it off can be tricky. It is worth noting that some tillage can be beneficial (e.g. deep ripping of hard pans) but should be employed only when necessary to help avoid impacts on soil structure.
Cover Crops
This concept is related to keeping the soil covered and physically protected from disturbance into the future after the crop is planted. The cover crop, usually grown in between the rows of permanent crops (e.g. trees and vines) or in the ‘off-season’ for annual crops, can be used to shield the soil from wind and water erosion and also help open up soils that have a history of poor soil structure. Briefly, the canopy of a cover crop can intercept and slow the velocity of raindrops and break up wind gusts, which helps keep soil on the field.
Belowground, the root systems of cover crops can poke through clods and hard pans and help open up channels in the soil profile to help move water downward into the profile. Fine root hairs can also tie soil particles together, improving soil structure and quality.
Please contact your preferred CCA for more advice on tillage practices and cover crop selection and to see if it fits into your farming operation.
Figure 3: The soil on the left has poor soil structure while the soil on the right has excellent aggregation and structure. As a result, the two fields have substantial differences in soil quality and their ability to support optimized crop growth (photo courtesy K. Wyant.)
Chemical Controls
A common issue in fields that have poor soil structure and soil quality is related to a chemical relationship in the soil. A loss of soil structure due to a mismatch between soil structure, calcium and other elements is called deflocculation. Under normal conditions, soil particles with ample calcium naturally come together to form aggregates, which improves soil structure. When the calcium is displaced by other ions (e.g. sodium), the soil structure collapses, and soil quality can quickly deteriorate. One of the main goals of a chemical remediation program is to provide more calcium to the soil, thus reversing the collapse in soil structure and driving an improvement in soil quality. The question is, “Which calcium amendment do I use?”
I cannot stress enough the need to start your program with a soil sample and to utilize the experienced advice from a CCA. Taking a soil sample will help you figure out the following important parameters critical to your chemical program: 1) Which amendment to use (e.g. gypsum, lime, sulfur product); and 2) Dosage of correct amendment to apply.
Critical parameters on the soil report worth looking at are soil pH, fizz test results, EC (dS/m), SAR, sodium and chloride results. For example, the fizz test measures the amount of free lime in a soil (CaCO3), and this is either expressed qualitatively (low, high, etc.) or quantitatively (%). There are a few major bulk calcium amendments that are available to help restore your soil structure: lime, gypsum and sulfur products.
Lime
Lime (CaCO3) is commonly used in soils that are acidic with soil structural issues and should not be confused with ‘free lime’ that shows up on a soil test report. Lime can provide a calcium source while also neutralizing acidic soil pH. Thus, soil quality can be improved on two separate fronts (soil structure and soil pH) with one program. Calculating the liming rate for a field (lbs/acre) can be tricky since you have to factor in both the amount of calcium needed to fix your soil structure problem and the amount needed to adjust the pH. A good soil sample and an experienced CCA can help determine the optimum rate for your field.
Gypsum
Gypsum (CaSO4) is a calcium amendment when one has low or very low fizz test readings. This is one reason why I recommend that you start your program with a soil test. When gypsum dissolves, it can supply calcium directly to the soil, which improves soil structure and allows excess salts (e.g. sodium) to drain. However, unlike lime and sulfur products, gypsum will have little impact on soil pH and should only be used to provide calcium to help restore soil structure and soil quality. An experienced CCA can help interpret your soil test and provide a reasonable gypsum recommendation (lbs/acre) to fix the issue.
Sulfur Products
Sulfur products, such as sulfuric acid and elemental sulfur, require the presence of free lime in the soil (CaCO3). As such, soils that have medium to very high fizz test ratings are a great candidate for the use of sulfur products in your chemical reclamation program. Briefly, sulfur products react with the free lime in the soil to create gypsum as a by-product. The calcium in the gypsum is then able to go to work to improve soil structure. One advantage of using sulfur products is that you generally do not need as much material to get the job done as you do with gypsum (1 ton gypsum = 0.57 tons sulfuric acid = 0.19 tons elemental sulfur), which can impact field logistics and application costs.
Also, repeated applications of sulfur products can reduce problematic soil pH areas by moving the soil pH from alkaline to neutral. One disadvantage to using sulfur products is that they have several reaction steps that can slow the reclamation speed of the field relative to lime and gypsum. Also, sulfuric acid products present some safety concerns that must be considered from a worker safety and transport level. Like lime and gypsum, an experienced CCA can help interpret your soil test and provide a reasonable recommendation (lbs/acre) to fix the issue.
Conclusion
Physical and chemical factors can have a profound impact on your overall soil structure and, thus, the soil quality of your field. Generally, poorly structured soils have a difficult time supporting optimized crop growth due to the severe reduction in water storage capacity, low oxygen, surface crusting and seed bed issues, accumulation of salinity, etc. If your soil looks like the example on the left side of Figure 3, it may be well worth your time and money to start implementing soil improvement practices as outlined in this article. You have a variety of management options, including the implementation of practices that improve the physical components of soil quality (e.g. tillage reduction and use of cover crops) or the chemical components such as adding bulk calcium amendments to your program. A bit of detective work beforehand determining why your field is having a soil structure problem can pay off in turning your field around and using your input dollars most effectively.
In Part 2 of this article series, we explore the biological components that influence soil structure and soil health. We will define soil health and go through plenty of examples on how the living component of a soil can impact your soil structure and overall soil quality. Furthermore, we will discuss how to test for soil health in the field.
Botrytis gray mold in trials at the UC Shafter Research Station (all photos by S. Dara.)
Strawberry is a high-value specialty crop in California and is susceptible to multiple pathogens that infect roots, crowns, foliage, flowers and fruits. Verticillium wilt caused by Verticillium dahliae, Fusarium wilt caused by Fusarium oxysporum f. sp. fragariae and Macrophomina crown rot or charcoal rot caused by Macrophomina phaseolina are major soilborne diseases that cause significant losses if they were not controlled effectively. Chemical fumigation, crop rotation with broccoli, nutrient and irrigation management to minimize plant stress and non-chemical soil disinfestation are usual control strategies for these diseases. Botrytis fruit rot or gray mold caused by Botrytis cineaea is a common flower and fruit disease requiring frequent fungicidal applications. Propagules of gray mold fungus survive in the soil and infect flowers and fruits. A study was conducted to evaluate the impact of drip application of various fungicides on improving strawberry health and enhancing fruit yields.
Propagules of gray mold fungus survive in the soil and infect flowers and fruits.
Methodology
This study was conducted in an experimental strawberry field at the Shafter Research Station in fall-planted strawberry during 2019-2020. Cultivar San Andreas was planted on October 28, 2019. No pre-plant fertilizer application was made in this non-fumigated field which had Fusarium wilt, Macrophomina crown rot and Botrytis fruit rot in the previous year’s strawberry planting.
Both soilborne diseases were present throughout the field during late spring 2019 with symptoms of wilt or crown rot appearing in many plants. In the current study, each treatment was applied to a 300-foot-long bed with single drip tape in the center and two rows of strawberry plants. Sprinkler irrigation was provided immediately after planting along with drip irrigation, which was provided one or more times weekly as needed for the rest of the experimental period.
Each bed was divided into six 30-foot-long plots, representing replications, with an 18-foot buffer in between. Between November 6, 2019 and May 9, 2020, 1.88 qt of 20-10-0 (a combination of 32-0-0 urea ammonium nitrate and 10-34-0 ammonium phosphate) and 1.32 qt of potassium thiosulfate w ere applied 20 times at weekly intervals through fertigation. Treatments were applied either as a transplant dip or through the drip system using a Dosatron fertilizer injector (model number D14MZ2). The following treatments were evaluated in this study:
Untreated control: Neither transplants nor the planted crop was treated with any fungicides.
Abound transplant dip: Transplants were dipped in 7 fl oz of Abound (azoxystrobin) fungicide in 100 gal of water for four minutes immediately prior to planting. Transplant dip in a fungicide is practiced by several growers to protect the crop from fungal diseases.
Rhyme: Applied Rhyme (flutriafol) at 7 fl oz/ac immediately after and 30, 60 and 90 days after planting through the drip system.
Velum Prime with Switch: Applied Velum Prime (fluopyram) at 6.5 fl oz/ac 14 and 28 days after planting followed by Switch 62.5 WG (cyprodinil + fludioxinil) at 14 oz/ac 42 days after planting through the drip system.
Rhyme with Switch: Four applications of Rhyme at 7 fl oz/ac were made 14, 28, 56, and 70 days after planting with a single application of Switch 62.5 WG 42 days after planting through the drip system.
Parameters observed during the study included leaf chlorophyll and leaf nitrogen (with chlorophyll meter) in February and May; fruit sugar (with refractometer) in May; fruit firmness (with penetrometer) in April and May; severity of gray mold twice in March and once in May; other fruit diseases (mucor fruit rot caused by Mucor spp. and Rhizopus fruit rot caused by Rhizopus spp.) once in May, three and five days after harvest (on a scale of 0 to 4 where 0=no infection; 1=1-25%, 2=26-50%, 3=51-75% and 4=76-100% fungal growth); and fruit yield per plant from 11 weekly harvests between March 11, 2020 and May 14, 2020. Leaf chlorophyll and nitrogen data for the Abound dip treatment were not collected in February. Data were analyzed using analysis of variance in Statistix software and significant means were separated using the Least Significant Difference test.
Fruit yield per plant from 11 weekly harvests between March 11, 2020 and May 14, 2020.
Results and Discussion
Leaf chlorophyll content was significantly higher in plants that received drip application of fungicides compared to untreated plants in February while leaf nitrogen content was significantly higher in the same treatments during the May observation. There were no differences in fruit sugar or average fruit firmness among the treatments.
The average gray mold severity from three harvest dates was low and did not statistically differ among the treatments. However, the severity of other diseases was significantly different among various treatments with the lowest rating in Abound transplant dip on both three and five days after harvest and only three days after harvest in plants that received four applications of Rhyme. Unlike the previous year, visible symptoms of the soilborne diseases were not seen during the study period to evaluate the impact of the treatments. However, there were significant differences among treatments for the marketable fruit yield.
Leaf chlorophyll and nitrogen content between February 4, 2020 and May, 15 2020.
The highest marketable yield was observed in the treatment that received Rhyme and Switch followed by Velum Prime and Switch and Rhyme alone. The lowest fruit yield was observed in Abound dip treatment. Unmarketable fruit (deformed or diseased) yield was similar among the treatments. Compared to the untreated control, Abound dip resulted in 16% less marketable yield and such a negative impact from transplant dip in fungicides has been seen in other studies (Dara and Peck, 2017 and 2018; Dara, 2020). Marketable fruit yield was 4% to 28% higher where fungicides were applied to the soil. Although visible symptoms of soilborne diseases were absent during the study, periodic drip application of the fungicides probably suppressed the fungal inocula and associated stress and might have contributed to increased yields. The direct impact of fungicide treatments on soilborne pathogens was, however, not clear in this study due to the lack of disease symptoms.
Considering the cost of chemical fumigation or soil disinfestation and the environmental impact of chemical fumigation, treating the soil with fungicides can be an economical option if they are effective. While this study presents some preliminary data, additional studies in non-fumigated fields in the presence of pathogens are necessary to consider soil fungicide treatment as a control option.
Marketable fruit yield on various harvest dates. Marketable yield was 4% to 28% higher where fungicides were applied to the soil with the exception of Abound dip, which resulted in 16% less marketable yield.
Acknowledgments
Thanks to FMC for funding this study and Marjan Heidarian Dehkordi and Tamas Zold for their technical assistance.
Dara, S. K. and D. Peck. 2017. Evaluating beneficial microbe-based products for their impact on strawberry plant growth, health, and fruit yield. UCANR eJournal of Entomology and Biologicals. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=25122.
Dara, S. K. and D. Peck. 2018. Evaluation of additive, soil amendment, and biostimulant products in Santa Maria strawberry. CAPCA Adviser, 21 (5): 44-50.
