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Walnut Husk Fly Management

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Walnut husk fly poses particular challenges for developing a truly integrated pest management (IPM) program due to the nature of its life cycle (one generation per year with a long emergence period) and lack of natural enemies. As a result, best practices for management rely heavily on monitoring and insecticide treatments. Precise timing based on monitoring method and rotation of chemistries to minimize resistance risk are keys to successful long-term control of this pest.

Figure 1. Life cycle of walnut husk fly.
Photos courtesy of University of California Statewide Integrated Pest Management Program

Susceptible Varieties
Walnut husk fly (WHF) damage earlier in the season causes shriveled and darkened kernels, increased mold growth, and lower yields. Later season infestations result in little kernel damage, but may stain the shells and make husk removal difficult. All commercial English walnut cultivars are susceptible to WHF infestation, although they differ in their relative degrees of susceptibility and thus damage potential. In general, Hartley, Tulare, Franquette, Payne, and Serr are considered more susceptible, with Howard, Ashley, Chico, and Chandler exhibiting less susceptibility (in order as listed). However, even less susceptible varieties can be damaged by high populations of WHF. Black walnut is also a preferred host, therefore proximity to black walnut can significantly increase WHF pressure in commercial English walnut orchards.

The varietal differences in susceptibility have been correlated to fruit characteristics including husk color, husk hardness, fruit size, trichome density, and plant volatile profiles, in addition to temporal factors (i.e., more severe earlier season damage may be more evident in earlier-leafing cultivars). Current research led by Dr. Steven Seybold (United States Department of Agriculture (USDA) Chemical Ecology Entomologist) is characterizing the plant volatile profiles associated with differences in varietal susceptibility, which may lead to improvements in monitoring and control products, as well as inform plant breeding approaches for genetic resistance or tolerance to WHF infestation.

WHF Life Cycle
The life cycle and basic biology of walnut husk fly is fairly well understood (Figure 1). There is a single generation per year, with adult emergence historically beginning in early to mid-June and lasting through September in the Central Valley. In coastal areas, and recently some inland valley locations, emergence can be detected earlier, in mid- to late-May. Peak emergence is generally observed July through mid-August in most locations. Females must mate and develop eggs prior to the initiation of oviposition into the walnut husk, a period which averages approximately two weeks after emergence. Once eggs are laid, maggots emerge within approximately four to seven days, and feed on the husk for a typical period of three to five weeks. After this period, mature maggots drop to the ground and pupate in the soil. Most adults emerge the following year, but a portion of the population may remain in the soil as pupae for two or more years before emerging as adults.

Extended Emergence Period
The extended emergence period of the single generation of WHF, and significant differences in the timing of initial emergence, peak emergence, and end of the flight based on location, year, and other factors, have been the subject of much research. As opposed to some other key pests (e.g., codling moth), there is not yet a validated phenology or degree-day model available for growers and pest control advisors (PCA) to readily adopt to predict key WHF development and adult activity timings. Two recent publications out of University of California (UC) Berkeley (Emery and Mills 2019a, 2019b) investigated the effects of temperature and other environmental parameters on walnut husk fly development and timing. One study evaluated 18 years of historical trap catch data from 49 walnut orchards spanning the Central Valley to determine which factors most influence emergence timing and thermal requirements for development (degree days to emergence), with the goal of developing a phenology model that can be used to predict initial and peak emergence. Some of the factors evaluated included latitude, walnut cultivar, orchard age, winter precipitation, winter chill, and degree-day accumulation. While this model requires refinement for adoption by orchard practitioners (growers and PCAs), it represents a great step forward in improving our understanding of WHF developmental requirements to aid in our IPM program development.

Photo 1. Male (left) and female (right) walnut husk fly adults.

Biological Control Agents
Biological control agents for walnut husk fly in California walnuts are virtually non-existent. The pest in general appears to have few natural enemies. Some reports from the state of Washington indicate that a predatory mite and anthocorid bug species have been observed feeding on WHF eggs, and some spiders and ants may feed on larvae and adults. In addition, chickens and other birds are said to be among the natural enemies of WHF. However, any naturally-occurring WHF biological control agents that may be found in walnut orchards are not known to provide any significant level of population reduction. Other mortality factors, particularly those that may impact the overwintering pupal stage in the soil (e.g., intentionally augmenting soil moisture, various cultivation practices, effects or augmentation of insect-parasitic nematodes or other microorganism populations, soil insecticide applications) have been explored to some degree with no specific recommendations or guidelines emerging as a result.

Photo 2. Female walnut husk fly with eggs.

WHF Management Guidelines
In spite of some of these challenges for WHF management, guidelines regarding treatment timing and options are available, and when employed properly tend to provide adequate control of WHF in many situations (albeit with more insecticide intervention than may be desirable or sustainable in the long-term). Because WHF activity and population abundance can vary significantly from orchard to orchard (even those in very close proximity), site-specific monitoring is necessary to get the most effective results from insecticide applications.

Photo 3. Walnut husk fly sting.

Monitoring
Monitoring should begin earlier than the June 15 historical guideline (no later than June 1 in the Central Valley is the more recent recommendation). Some reports of late May catches in 2016 further support the “earlier-is-better” practice—there is little harm in counting zeroes for a few weeks. Yellow sticky card traps baited with ammonium carbonate lures should be hung high in the canopy (minimum 2 per 10 acres) in dense foliage on the north side of trees and checked two to three times per week. Each orchard should be monitored individually for WHF activity to best determine if and when to treat. A summary article regarding the efficacy of available traps/lures for WHF monitoring was published in 2014 (http://www.sacvalleyorchards.com/walnuts/insects-miteswalnuts/walnut-husk-fly-trap-and-low-volume-spray-study/).

Treatment Timing
Treatment timing can be based on one of three monitoring methods (the first two have typically been most effective).

  1. Detection of eggs in trapped females. This is a simple process that requires slightly more time than counting overall trap catches and can increase the efficacy of treatments by timing applications to specifically target female oviposition activity. Females can be distinguished from males by the shape of the abdomen (pointier in females) and color of the front leg (female leg is entirely yellow, male leg is black close to the body, Photo 1). After females are identified, gently squishing the female abdomen will squeeze out eggs if they are present (Photo 2). Eggs resemble small grains of rice. Previous guidelines indicated that the treatment window is one week after egg detection. However, recent modifications suggest that treatments should be considered as soon as the first female with eggs is found because in practice there is often a lag time in getting the treatments out, and trap checks (even two to three checks per week) may not be frequent enough to represent initial egg development in the female population. Therefore, planning to treat as soon as possible after eggs are detected may be the best option to minimize infestation and damage. [Note that this is the preferred method for timing treatments unless using GF-120® alone; see below].
  2. Overall trap catches. For low to moderate populations, consider treatment when a sharp increase occurs in trap counts. In high pressure orchards or if using GF-120® alone, treatment should be considered when any flies are detected rather than waiting for a sharp increase in catches.
  3. Stings on nuts. This is the least preferred method, as damage has already occurred. However, examining nuts for stings (Photo 3) can provide indication of efficacy of your management program when using one of the first two methods. If using this method to time treatments, consider treating when the first sting is observed using full cover neonicotinoid materials that have some ovicidal activity mixed with an adulticide.

Continued monitoring throughout the season is crucial. Short-residual insecticides plus bait will generally kill WHF for seven to ten days. Target subsequent applications at two- to four-week intervals based on the efficacy of the previous spray and trap catches. Clean traps the day after application and check three to four days later. If the number of flies drops to near zero, the spray was highly effective and a longer treatment interval may be used. If post-treatment catches increase or eggs are detected in trapped females, and the residual period of the previous treatment has elapsed, additional treatments may be required if harvest is more than three weeks away.

There are several materials effective against WHF, both for conventional and organic orchards. All materials aside from GF-120® (which contains its own bait) should be applied with a bait (e.g., Nu-Lure®, molasses, etc.). However, very high population orchards with extensive previous damage may require full coverage sprays (no bait needed) to achieve adequate suppression. Keep in mind that rotation of chemistries (based on the Insecticide Resistance Action Committee (IRAC) mode of action classification) is critical to minimize resistance development for pests that are treated multiple times each season. Proper aphid management can also help limit movement of WHF within and between orchards by reducing honeydew accumulation (a food source for adult WHF).

The UC IPM Pest Management Guidelines (ipm.ucanr.edu/PMG/r881301211.html) lists insecticides, baits, and rates for WHF. A summary of efficacy data for selected materials (updated September 2016) are summarized at (www.sacvalleyorchards.com/walnuts/insects-mites-walnuts/walnut-husk-fly-biology-monitoring-and-spray-timing/).

Referenced articles:
Emery, S. A. and N. J. Mills. 2019a. Effects of temperature and other environmental factors on the post-diapause development of walnut husk fly, Rhagoletis completa (Diptera: Tephritidae). Physiological Entomology 44: 33-42.

Emery, S. A. and N. J. Mills. 2019b. Sources of variation in the adult flight of walnut husk fly (Diptera: Tephritidae): a phenology model for California walnut orchards. Environmental Entomology 48: 234-244.

Evaluation of Grafted Tomato Plants for California Fresh Market Production Systems

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All photos courtesy of Brenna Aegerter

Why graft?
Grafting involves joining a fruit-producing shoot (called the ‘scion’) of a desirable cultivar onto the disease-resistant rootstock of another cultivar.  For example, let’s say you normally grow the cultivar ‘QualiT-47’ for fruit production, but that cultivar is susceptible to a soilborne disease problem in your fields, then you could graft the top part of a ‘QualiT-47’ seedling onto the root-portion of a more disease-resistant cultivar. In the case of tomato rootstocks, the majority of the cultivars are interspecific hybrids between cultivated tomato (Solanum lycopersicum) and wild tomato species (most commonly Solanum habrochaites, or less often S. peruvianum or S. cheesmaniae). Solanum habrochaites is known from other published research to be tolerant of salinity, drought, cold temperatures, and resistant to many soilborne diseases and many of these benefits have been demonstrated to be conferred to the grafted plant when an interspecific hybrid rootstock is used.

Most of us are familiar with grafting as a standard practice for California fruit and nut trees and grapevines, but it has experienced only limited commercial adoption among annual crops in California thus far. Grafted tomato transplants are commonly utilized in the commercial greenhouse industry, where tomatoes are produced under protected culture and are generally grown over a much longer production cycle, often a 10-month period. There are greenhouse producers in Southern California, but it is more common in British Columbia, Ontario, Mexico and other US states (Arizona and others).  In many countries in Latin America, Europe and Asia, grafted plants represent a large percentage of the tomato industry. For example, in Spain, 50 to 70 million grafted plants are grown annually for greenhouse production systems. There has also been some adoption of grafting by high-tunnel tomato growers in the eastern United States. In Southern California, the nursery Plug Connection is marketing grafted tomatoes to home gardeners, dubbing them as a “Mighty ‘Mato”. This allows a home gardener to grow an heirloom tomato variety, which often has little or no disease resistance, without worrying about rotation or other soilborne disease control measures.

Our goal was to evaluate the potential for grafting standard tomato cultivars onto rootstock cultivars that possess resistance to soilborne diseases and nematodes. Our primary objective was to evaluate the yield performance of grafted plants in replicated trials in commercial fresh market (“mature green”) production fields in the northern San Joaquin Valley. Our team consisted of myself, Scott Stoddard with University of California Cooperative Extension (UCCE) in Merced County, and Michael Grieneisen and Minghua Zhang in the Department of Land, Air and Water Resources at the University of California, Davis. This project produced the first publicly-available research results on grafted tomatoes for California production systems.

How is it done?
For each tray of grafted tomatoes to be produced, two trays of seed are sown; one tray of the rootstock seed and another tray of the scion seed. At approximately one month after sowing, the young seedlings are grafted. Both seedlings are cut at the hypocotyl, and the scion shoot is spliced onto the rootstock stump. The method we used is a commonly used splice-graft with small, soft, silicone clips to hold the scion and rootstock together during healing. Grafted transplants cost more than non-grafted transplants due to increased seed costs and the labor required to do the grafting. Thus far, grafted tomato plants are only available from a few sources in California, and we won’t know what the cost for grafted tomato transplants will be until they are being produced in larger volumes here. The use of fully- or semi-automated grafting robots is emerging as a way to reduce labor costs and improve the survival rate of grafted plants. This of course requires significant capital investment. About 20 seed companies offer tomato rootstock seeds (see list at http://www.vegetablegrafting.org/tomato-rootstock-table/). However, for a nursery facility or grower considering doing their own grafting, building a healing chamber may be a hurdle. There is research underway by others to look at conducting the one-week healing period inside the greenhouse. For more information on the logistics of grafting on a commercial scale, please see the Vegetable Grafting Manual, the link for which is provided at the end of this article under “More information”.

Figure 2. Production of splice-grafted tomato plants for our field trials.

Field Trials in the Northern San Joaquin Valley
The trials were conducted in commercial production fields at six locations over three years from 2016 to 2018; three locations in San Joaquin County and another three locations in Merced County. The treatments included all combinations of the scions and rootstocks listed in Table 2.

Table 2. Scion and rootstock cultivars used in our field trials. *Note: Galilea is a roma/saladette type, while the other seven cultivars are all round types; all but Dixie Red were developed for the Western U.S. mature green production system.

The plots were laid out in a randomized complete-block design with four replicate blocks, each block measuring approximately 80 by 40 feet. The cooperating growers managed the experimental plots similarly to the rest of their field with respect to pest control, fertilization, irrigation, and other management practices. Plants were mechanically transplanted into prepared beds at a 4- to 5-inch depth per normal practice; the graft union ended up well below the soil surface. In staked or trellised production systems in other regions, the graft union is typically kept above ground to realize the full benefit of the rootstock pathogen resistance. With graft union buried below the soil surface, soilborne pathogens may attack the scion crown tissues or adventitious roots arising from the scion.  Due to the lack of significant pathogen pressure in our fields, we believe this was not an issue for these trials.

In our trials, grafted plants were more vigorous and had better foliage cover of fruit at harvest than non-grafted plants of the same cultivar. We also measured NDVI (Normalized Difference Vegetation Index, a measure of the “greenness” or how much of the bed is covered with actively photosynthesizing foliage) and it was also slightly higher in grafted plots. Averaged across all six trials, marketable yield increased only 12 percent when grafting with ‘Maxifort’ or ‘DRO138TX’ as the rootstock, although the results were better in some individual trials. At the San Joaquin County sites, yields of non-grafted vines were similar to the statewide average yield and grafting increased yield significantly (25 to 40 percent depending on the year). Some scion-rootstock combinations were as much as 68 percent higher than the non-grafted plants of the same scion (e.g. ‘QualiT-27’ on ‘Maxifort’ at the San Joaquin site in 2018). At the Merced sites, yields of non-grafted vines were well above-average and grafting was much less beneficial. Many published field trials indicate that the yield advantages of grafted plants are greatest under sub-optimal growing conditions. Field sites with heavy soilborne disease pressure, or abiotic stresses may be the best candidates to see improvements with grafting.

Fruit Size and Quality
Many published studies have found that grafted plants produce a higher percentage of fruit in larger size classes than those produced by the non-grafted scion varieties. Averaged over all our trials, the differences in fruit size distribution between grafted and non-grafted were fairly small. In some trials, however, plants on vigorous rootstocks did have larger fruit. Some published studies provide measures of fruit quality, such as dissolved sugars, pH, total dissolved solids, vitamin C, lycopene, or even “taste-test” data. Those studies indicate that the quality of fruit from grafted plants seems to be slightly inferior to fruit from the non-grafted plants, though still commercially acceptable. Our field trials focused on yields, and we did not measure any fruit quality data. However, we did not notice any fruit defect problems in grafted vines. Also, in 2018 we did cut open both red and mature green fruit at harvest to make sure that there were no problems inside the fruit.

Variability From Trial to Trial or Field to Field
A study in Florida with determinant type cultivars has shown yield increases of 25 to 42 percent using certain rootstocks, but year-to-year variability also increased as compared to non-grafted plants. This variation underscores the importance of considering variable outcomes to determine the feasibility of grafted tomatoes here. Some fields will likely benefit more from grafting than others, and this may not always be predictable in advance.

Economics
Costs of field establishment are increased significantly with grafting. Materials costs for transplanting (seed plus nursery costs) alone might be $2,000 per acre or higher or more than with conventional transplants. However, we don’t yet really know what the costs might be if this were adopted commercially in California, so our plant costs are based on small volume sales prices. If we assume a cost of $0.40 per grafted plant, then a yield increase of 19 percent at a market price of $6.55 per 25-pound box would pay for the increased plant cost.

On-going and Future Work
Other research projects looking at grafting tomatoes are being conducted in California. A United States Department of Agriculture (USDA)-funded project with processing tomatoes is underway with collaboration of Gene Miyao, UCCE Yolo, Solano and Sacramento counties, Zheng Wang, UCCE Stanislaus County and myself, in addition to proprietary research being conducted by the industry. Rootstocks for heirloom tomato production are being evaluated by Margaret Lloyd, small farms advisor with UCCE in Yolo, Solano and Sacramento counties.

Acknowledgements
The California Department of Pesticide Regulation provided partial funding for this project but does not necessarily agree with any opinions expressed, nor endorse any commercial product or trade name mentioned. In addition, this project was supported by the Specialty Crop Block Grant Program at the U.S. Department of Agriculture through Grant 14-SCBGP-CA-0006. The contents of this report are solely the responsibility of the authors and do not necessarily represent the official views of the USDA. We also thank our grower-cooperators (Live Oak Farms and Pacific Triple E), Growers Transplanting Inc. for producing grafted plants, and the following companies that supplied the seeds: Monsanto/De Ruiter Seeds, Gowan Seed Company, Harris Moran Seed Company, and Syngenta Vegetable Seeds.

For more information:
Additional information on our field trials:
https://ucanr.edu/sites/veg_crop_sjc/Grafted_tomatoes/

Detailed information on how to undertake vegetable grafting is available at: http://www.vegetablegrafting.org/resources/grafting-manual/

List of tomato rootstocks including disease resistances and where to order seed: http://www.vegetablegrafting.org/resources/rootstock-tables/solanaceous-rootstock-table/

Weed Identification: A Crucial Component of Weed Management

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Weed identification is the foundation for weed control. For both cultural controls (tillage, weed-whacking, etc.), and herbicides, misidentification can lead to wasted time, money, and resources. But even for experienced weed scientists and botanists, weed identification can be difficult. Traditional keys, for example, primarily rely on our ability to distinguish between plants at flowering, and often require a fair amount of knowledge of botanic terms, and possibly even a microscope. Aside from the difficulty of using the keys, identification at flowering is usually too late for weed control, particularly for the use of many herbicides.
There are many tools available to use for weed identification, ranging from books to cards, to online databases, and even computer programs and smartphone apps. The resources found below are just a few of the plethora of weed identification resources, highlighting some of the most relevant for California, and many that are free or low-cost.

Print
Printed materials may be a bit difficult to carry into the field, obviously, but they can be a good resource, especially for learning more about the biology and ecology of the weed species once it is identified.

1. Volume 1-2 of Weeds of California and Other Western States (DiTomaso and Healy, 2007).

This 2-volume set is available through the University of California Division of Agriculture and Natural Resources (UCANR) website (as well as many other websites), and contains many, many weeds found in California, as well as those that may be likely to move into California from surrounding states. It includes over 700 weed species, in over 60 families. It also has tables to help distinguish between commonly-confused weeds.

2) Weed Identification Cards (DiTomaso, 2013)

This set of cards is adapted from the Weeds of California and Other Western States book listed above, and contains the 48 most widely distributed weed species in California. It is available from the UCANR website. The weeds are divided into the following eight plant groups, for easy searching:

  • Broadleaf annuals, erect
  • Broadleaf annuals, low growing
  • Broadleaf annual, scrambling
  • Broadleaf perennials, not viney
  • Broadleaf perennials, viney
  • Grass annuals
  • Grass perennials
  • Sedges

The cards are small and can be held in the hand while in the field. They are laminated, so even if they get moist, they will not be ruined.

Computer Based
There are a couple of resources available (one USB-loaded program and one web-based application) if you need some help identifying weeds, especially during early growth stages. While the USB can only be used on a computer, the web-based application can also be used on a smart-phone (in the field), although it does require connectivity to the web to be able to do so.

1) Weed ID USB (DiTomaso, 2014)
The Weed ID USB replaces the CD’s that came with the original 722 broadleaf species and 200 weedy grasses. Using the program, it is possible to identify weeds even at the seedling stage, using key characteristics visible to the naked eye, instead of requiring a microscope or hand lens. For example, key traits such as stem cross-section, leaf shape, hairs on the leaves, etc. It also allows the user to select the family, or genus, if known. After making selections, the program will, through process of elimination, show a list (with photos and descriptions) of all the weed species fitting those characteristics. Once down to a few species, the user can visually compare the photos of their specimen to the photos in the database.

The USB is available through the California Invasive Plant Council Website (www.cal-ipc.org), or by contacting Dr. DiTomaso directly (jmditomaso@ucdavis.edu).

1) Weed ID USB (DiTomaso, 2014)

2) Online Weed Identification Tool (University of Wisconsin-Madison):
The University of Wisconsin-Madison hosts an online weed identification tool that is very similar to the USB drive (above). It is freely available online, at https://weedid.wisc.edu/weedid.php. Several states are available, so be sure to select California as the search location. There are far fewer species in this database in comparison to the USB drive, but the identification method is similar. The user has to select plant characteristics, and through a process of elimination, the possible weed species with those characteristics will remain. The user can then use the photos to identify their specimen.

2) Online Weed Identification Tool (University of Wisconsin-Madison).

Mobile Apps
Although there are several smartphone applications available for both Androids and IPhones, testing of several yielded only one with enough species to make it worthwhile to use in the field.

1) Pl@ntNet (www.plantnet.org)
The Pl@ntNet app (available for both Android and iPhone) was created by a consortium of universities and public research institutions. It contains plant species from all over the world, so the user has to be careful to select the correct continent (North America). It does not cover only weeds, however, which is important to note. It also contains native and naturalized species.

The app works by matching key photo characteristics with identified photos already in the database. The user takes a photo, then specifies which characteristic to focus on (leaf, fruit, bark, flower, habit, or other), then the app matches the photo with potential specimens in the database. The user then selects the species that most closely matches, and submits it to the app, where it is reviewed (seemingly by experts). Upon testing it, it was able to accurately identify several grasses, broadleaves, and shrubs.

In-Person Assistance
When in doubt, ask another person to assist in identification. Fellow pest control advisers, growers, and other colleagues are great resources. However, if it appears to be a new or unknown weed species in a particular cropping or natural system, there are other resources available to help as well.

1) University of California Cooperative Extension advisors and specialists
County-based advisors and campus-based specialists can be helpful in providing weed identification. Local offices are located in almost every county in California, and advisors there can tap into larger networks of weed scientists for help with identification, if they are unable to identify the weed themselves.

2) Weed identification at the UC Davis Herbarium
For really tricky cases, the UC Davis Herbarium has botanists on staff that can assist in identification. Visit the herbarium website at https://herbarium.ucdavis.edu/services.html for collection and sample delivery protocols. The herbarium can identify up to five samples per person per year at no charge, and after that, an hourly rate for identification applies.

There are many more tools available for weed identification beyond the ones listed above, including many helpful keys and online resources. In order to identify a weed, it may be necessary to utilize many tools and second opinions, particularly if it is a less well-known species, or if it is new to a cropping or natural system. While identification can be time-consuming, especially when we are anxious to get rid of a weed, ensuring proper identification before deciding on a plan for control can save a lot of time, energy, and money over the long run.

Evaluation of Nitrogen Stabilizers to Improve Corn Yield and Plant Nitrogen Status

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Nitrogen (N) is part of a balanced, natural cycle in the environment among the atmosphere, soil, plants, animals, and water. Nitrogen is the most important element needed by crops, and we often add nitrogen fertilizer to optimize crop productivity. Nitrogen use in agricultural systems must be reported for regulatory compliance under the Irrigated Lands Regulatory Program and the Dairy Order to help ensure that a greater fraction of the applied N is recovered in the harvested crop and not lost to the environment. Nitrogen management gives consideration to the four R’s:

  • Right source: selecting a fertilizer source that matches with crop need and minimizes losses.
  • Right rate: applying the right amount based on crop need and nutrient availability through other sources.
  • Right time: applying the nutrient when the crop can use it.
  • Right place: fertilizer placement that optimizes the crop’s ability to use it.

The four R’s address management considerations (e.g. fertilizer program, irrigation), but site characteristics (e.g. soil, cropping system, weather conditions) also influence N recovery in the crop. Also important to improving crop N recovery is understanding barriers to adopting best management practices, such as costs or risks to crop quality or yield.

While the four R’s articulate four principles for nitrogen management, the N cycle in cropping systems is complicated. Nitrogen can be introduced and lost by various paths. We generally add N with fertilizer or organic matter amendments—such as crop residues, compost, or manure. Fertilizers provide N in plant-available forms—ammonium (NH4+) and nitrate (NO3–). Organic matter amendments must be mineralized before the N is available for plant uptake. Mineralization is a process that involves soil biology converting organic N to NH4+. The timeline of this conversion will depend on the properties of the amendment, environmental conditions—such as soil temperature and moisture, and the activity and abundance of soil microbes.

In the soil, NH4+ has different fates. It can be immobilized by microorganisms, taken up by plants, fixed to soil particles due to its positive charge, volatilized to ammonia gas (i.e. lost from the system), or converted to NO3–—a process known as nitrification. Nitrification is a two-step process. The first step is the conversion of NH4+ to nitrite (NO2–) by Nitrosomonas bacteria. The second step is the conversion of NO2– to nitrate (NO3–) by Nitrobacter bacteria. These two steps generally occur in close succession to prevent the accumulation of NO2– in the soil. Conditions that affect nitrification include soil aeration, moisture, temperature, pH, clay and cation content, NH4+ concentration, among others. Just as NH4+ has different fates in the soil, so too does NO3–. Plants preferentially take up NO3–, but if NO3–  is present when plants are not in need of it, then NO3– may be immobilized by microorganisms, volatilized to nitrogen gas (i.e. lost from the system), or leached out of the root zone (i.e. lost from the system).

Fig.-1. Nitrogen stabilizers applied at sidedress fertilizer application.

Technologies have been developed to mitigate N losses from the system. These technologies are collectively known as enhanced efficiency fertilizers (EFF) and include additives, physical barriers, and chemical formulations that stop, slow down, or decrease fertilizer losses. Nitrogen stabilizers, slow-release fertilizers, and polymerized fertilizers are examples of EEF. Nitrogen stabilizers are fertilizer additives intended to improve crop N use efficiency and reduce N losses to the environment by interrupting the microbial processes that change N to its plant-available forms. We developed a trial to evaluate two N stabilizer products with the objective of determining whether the treatments improved corn silage yield or plant N status compared to fertilizer alone. We did not attempt to measure N losses from the system (e.g. leaching, denitrification), as these are very challenging to quantify.

The products in our trial were Vindicate (Corteva Agriscience) and Agrotain Plus (Koch Agronomic Services). Vindicate delays the nitrification process by inhibiting the Nitrosomonas bacteria that converts NH4+ to NO2–. Vindicate has bactericidal activity, and the active ingredient is nitrapyrin. Agrotain Plus has two modes of action—reducing ammonia volatilization and delaying nitrification. Ammonia volatilization is the conversion of NH4+ in the soil to ammonia gas (NH3) in the atmosphere, and it is reduced by inhibiting the urease enzyme. Ammonia volatilization is most problematic when the N source is urea-based and not incorporated or watered into the soil. The active ingredients of Agrotain Plus are Dicyandiamide (DCD), which delays nitrification, and N-(n-butyl)-thiophosphoric triamide (NBPT), which reduces volatilization. DCD has bacteriostatic activity, which means it slows the metabolism of Nitrosomonas. We hypothesized that N stabilizers would improve yield and N uptake over the fertilizer-only treatment, providing growers with a tool for nutrient stewardship.

Methods
The trial took place in San Joaquin County on a DeVries sandy loam soil. The field had a winter wheat crop that was cut for forage in the late spring. Dry manure was applied to the field between wheat harvest and corn planting, which occurred on May 24, 2018. The variety was Golden Acres 7718. At-planting fertilizer provided approximately 12 lb N per acre (4-10-10). Sidedress fertilizer application occurred on June 21st and provided approximately 105 lbs N per acre (UAN 32). Four treatments were applied at sidedress, when plants were at V3-4 stage of development (Fig. 1). The N stabilizers were applied at the label rates, and the treatments were: 1) Vindicate at 35 fluid ounces per acre, 2) Agrotain Plus at 3 pounds per acre, 3) combination of Vindicate and Agrotain Plus at aforementioned rates, and 4) fertilizer-only, no stabilizer product (“untreated”). Plots were 35 feet across (i.e. fourteen 30-inch rows), in order to adapt to equipment of different widths, by 900 feet long. Treatments were randomly applied in three replicate blocks. Aside from the treatments, the trial was managed by the grower in the same manner as the field.

We evaluated soil N status, plant N status, and silage yield. Prior to planting, 20 soil cores were randomly collected from across the trial and aggregated by foot-increments, down to two feet. Mid-season leaf and soil samples were collected when the corn was in the R1 stage (i.e. silking). Soil was collected from 10 in-row locations in each treatment, and aggregated by foot-increments, down to two feet. Leaves were sampled from ten plants in each treatment, sampling the leaf one-below and opposite the earleaf. Harvest occurred on September 20th. All fourteen rows were harvested for weight, and samples were collected at the silage pit for aboveground biomass N analysis. The samples were dried at 60⁰C for 48 hours for calculating dry matter (DM). Post-harvest, 10 in-row soil cores were collected to one-foot depth and aggregated for each treatment. Laboratory analyses were conducted by Ward Laboratories (Kearney, NE; https://www.wardlab.com/). We used Analysis of Variance to detect differences in treatments and Tukey’s range test for means separation (JMP statistical software). Treatments were considered statistically different if the P value was less than 0.05.

Results and Discussion
There were no statistically significant differences among treatments for plant tissue N, yield, dry matter, or total N removed at harvest (Table 1). Mid-season leaf N averaged 2.88 percent across treatments, and aboveground biomass N at harvest averaged 1.12 percent. At mid-season, leaf N from 2.7 to 3.5 percent indicates that the plants had sufficient N to carry the crop to harvest, and at harvest, whole plant N from 1.0 to 1.2 percent indicates that the N fertilization program was adequate for maximizing yield [1]. Calculated to 30 percent dry matter, average yield across treatments was 38.8 tons/acre, and dry matter was 35 percent. There was a trend for the two treatments with Vindicate to have a higher N removed than the two treatments without it, but the difference was not statistically significant. The low coefficient of variation (CV), which is a measure of variability in relation to the mean, indicates low variability among replicates for all of these parameters.

The pre-plant (post-dry manure application) soil nitrogen status was 17 parts per million (ppm) NO3-N and 4 ppm NH4-N for the 0-12 inch depth, and 7 ppm NO3-N and 2 ppm NH4-N for the 12-24 inch depth. When soil NO3-N is below 25 ppm in the top foot of soil, it is recommended to apply N fertilizer in order to prevent yield reductions [2]. There were no differences among treatments in soil N status at the mid-season sampling, but there were differences at the post-harvest sampling (Table 2). Mid-season soil NO3-N averaged 32 ppm across treatments in the top foot of soil, and 10 ppm in the second foot of soil, which is an adequate concentration to carry the crop through to harvest. Soil NH4-N averaged 4 ppm and 2 ppm across treatments for the top foot and second foot, respectively. The CV was high for mid-season soil data, which indicates high variability among replicates. Post-harvest soil NH4-N averaged 2 ppm across treatments in the top foot of soil, but soil NO3-N was higher than at any other time during the season, averaging 46 ppm across treatments. These results may indicate that the dry manure mineralized later in the season, after the peak demand of the corn. Post-harvest soil NO3-N above 20 ppm is considered high and indicates that this crop was not deficient in N [2]. The low CV for NO3-N indicates low variability among replicates. The significant differences among treatments are not well-understood, particularly as the control (fertilizer-only) treatment had soil NO3-N that was not different from any of the treatments. Interestingly, Vindicate had the lowest post-harvest soil NO3-N and a trend toward higher N removed (though not statistically higher), which may indicate that product use made N available at a time that optimized N uptake.

Summary
N is part of a balanced, natural cycle in the environment and is the most important nutrient in cropping systems. Giving consideration to N management will help ensure that a greater fraction of the applied N is recovered in the harvested crop and not lost to the environment, and keeps growers in regulatory compliance. Enhanced Efficiency Fertilizers, such as N stabilizers, have been shown to improve crop yield in regions like the Midwest and the Northeast, and may help to mitigate N losses from the environment. In our trial, we evaluated the efficacy of N stabilizer products for improvements in corn silage yield or plant N status compared to fertilizer alone. Under the management and environmental conditions of this trial, we found no differences in yield or plant N status; however, plant and soil tests indicated that N was never limiting in the trial. If N was lost from the system, the loss was not large enough to result in N limitation in the control. Future study should test these products using different N sources and N rates (e.g. grower rate and grower rate minus 50 lbs N/acre). It may be possible to reduce the fertilizer N rate without sacrificing yield.

References
1. Nutrient Management for Field Corn Silage and Grain in the Inland Pacific Northwest. https://www.cals.uidaho.edu/edcomm/pdf/PNW/PNW0615.pdf.
2. Nutrient Management Guide – Silage Corn (Western Oregon). https://catalog.extension.oregonstate.edu/em8978.

Managing Citrus Thrips is Especially Difficult During Drought Years

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(originally March/April 2017)

Citrus thrips is a common pest of California citrus, attacking leaves and the calyx end of newly forming fruit, when the epidermal cells are quite sensitive.  In leaves, this causes distortion of the leaves and light lines of scarring

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Thermal Infrared Sensors for Postharvest Deficit Irrigation of Peach

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(originally published January/February 2017)

California has been in a historic drought and the lack of water has been a major problem for agriculture especially for crops that depend on irrigation. Deficit irrigation may be used in some cropping systems as a potential water saving strategy (Goldhamer et al., 1999). The term “Deficit Irrigation” simply means irrigating at less than the full amount required by crop evapotranspiration needs. For fruiting trees such as peaches, because fruit yield and quality at harvest may not be sensitive to water stress at some developmental stages such as during the non-fruit bearing postharvest season, there is an interest in applying deficit irrigation strategies. Deficit irrigation has not been widely used due partially to the lack of effective and fast methods of monitoring plant water stress in near real-time and determining associated risks of applying deficit irrigation. When crops are managed under deficit irrigation, the margin of error in timing and amount of water application becomes smaller before causing yield losses. Monitoring the soil and plant water status is more critical for reducing risks of a crop failure or permanent damage to the trees.  However, current established techniques of monitoring the soil and plant water status such as neutron probe readings of soil water profile and pressure chamber measurements of stem water potential are labor intensive, and lack the timeliness needed for irrigation scheduling purposes.

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New Efforts on Fusarium Wilt of Lettuce Brings Disease Front and Center

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(originally published September/October 2016)

Shortly after hiring on as the inaugural executive director of the University of Arizona’s new Yuma Center of Excellence for Desert Agriculture, a public-private partnership devoted to applied agricultural research needed by the desert agriculture industry, Paul Brierley asked his stakeholders what they would like the Center to address first.  The answer was resounding:  Help us mitigate plant diseases!  And not just any plant disease – help us with the seemingly impossible-to-eradicate Fusarium wilt of lettuce.  And thus began the Center’s odyssey against the insidious disease that is wiping out entire fields during warm-season production of iceberg lettuce – costing the industry millions of dollars.

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Tomato Spotted Wilt Virus Management in Tomatoes

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By: Thomas A. Turini, University of California Agriculture and Natural Resources, Vegetable  

Tomato spotted wilt virus is a thrips-transmitted virus that can infect many crops and weeds. In California’s Central Valley, in an important processing tomato production area, this virus disease may cause substantial economic damage. The most recognizable symptoms include fruit with oval protruding oval deformities or irregular concentric ring color patterns and this virus can kill shoots and plants, so both quality and yield are affected. The host range of this virus includes many common crops and weeds and likely survives the winter on a few weed or crop plants, but quickly amplifies on tomatoes in spring. Therefore, risk increases during the season. The virus is transmitted by thrips; primarily Western Flower Thrips, Frankliniella occidentalis in the Central San Joaquin Valley. The vector must feed on an infected plant as a nymph to be capable of transmitting the virus as an adult. Risk of loss due to TSWV can be reduced but management in high risk situations is going to depend upon several tactics.

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Grapevine Red Blotch or Leafroll Disease

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By: Larry Bettiga, Viticulture Farm Advisor University of California Cooperative Extension Monterey, San Benito and Santa Cruz Counties

(originally published May/June 2018)

Grapevine leafroll and red blotch disease are two virus-associated diseases that should be on the radar of all grape growers. The following article will hopefully provide you an update on these virus diseases based on our current knowledge. Summer surveying of vineyards for visual leaf symptoms is a great time to assess vineyard blocks for the presence of disease.

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