Author/s: Richard Smith, Joji Muramoto and Patricia Love
UCCE Farm Advisor, Extension Specialist and Staff Research Associate
In the fall, following the crop production season, residual soil nitrate-nitrogen (N) levels increase when N-rich residues from crops such as broccoli are incorporated into the soil. Soil temperatures in the fall and early winter are adequate to allow decomposition of crop residues, as well as continued mineralization of soil organic matter. The resulting pool of residual soil nitrate-N is vulnerable to leaching by winter rains. Winter-grown cover crops trap a significant portion of this nitrate in their biomass, thereby providing a useful practice to reduce nitrate leaching during the winter fallow period; however, although growers may want to include cover crops in their rotations for the benefits that they provide, conflicts with spring planting schedules and economic hurdles such as high land rents often preclude their use. As an alternative practice, we conducted trials to evaluate the potential of using high carbon:nitrogen (C:N) organic amendments during the winter fallow period to immobilize soil nitrate-N and reduce leaching. Immobilization is a process whereby the soil microbes utilize a readily available source of C and available soil nitrate to stimulate rapid growth. As a result of this microbial activity, the pool of nitrate in the soil is reduced which in turn reduces the risk of nitrate loss by leaching. On the Central Coast, it is a common practice for grower to apply compost in the fall to improve soil tilth and health. Compost made from yard waste is the most commonly used material used on the coast and it has a C:N ratio that typically ranges from 13 to 20. By contrast high carbon compost is made from materials such as tree limbs and trunks that contain little N and has a C:N ratio of >40 and is capable of immobilizing larger quantities of nitrate. If the use of high carbon compost could be substituted for normal compost applications, it could potentially provide a best management practice (BMP) to immobilize soil nitrate-N during the winter fallow and help growers comply with water quality regulations enforced by the Regional Water Quality Control Board.
Studies were conducted each winter from 2016 to 2020 to evaluate reductions in nitrate leaching with the use of high carbon compost amendments. Each trial was instructive in helping us understand this practice and evaluate if it was an effective and economically viable practice that growers could incorporate into their operations. The following are the highlights of evaluations of potential materials:
- One of the most effective materials was almond shells ground to particles 0.5 mm in size (Photo 1). The C:N ratio of this material varied from 59 to 70. It reduced the load of nitrate-N in the top three feet of soil by as much as 34 to 47%, at 5 and 10 tons/A, respectively. Almond shells are readily available in the Central Valley, but transportation and grinding costs make the expense of this material an issue.
- Glycerol is source of highly labile carbon which soil microbes are capable of utilizing immediately (Photo 2). It reduced the load of nitrate-N in the soil by as much as 48%. This material is highly effective, but high cost at the rates found to be effective (1.25 – 2.5 tons/A, equivalent to 240 and 479 gallons/A, respectively) is the main issue with this material.
- Locally sourced compost made from tree limbs and trunks was obtained from the Marina Landfill that had C:N ratios that varied from 185 to 215. The main issue with this material getting it ground fine enough so that it could serve as a rapid source of C to allow the immobilization process to proceed during the winter fallow. We obtained a material called Forest Mulch Compost which is triple screened and, in one study, had sufficient fines to effectively immobilize nitrate (Photo 3). However, the material also contains coarse material that is not active in immobilization during the winter fallow. This material costs about the same as typical yard waste compost that is commonly used and therefore provides an affordable option. Forest Mulch Compost is the most practical option for a high carbon amendment that we have found. If a market for this material is stimulated, there could be an incentive to grind it to a smaller size which would increase its effectiveness.
High carbon amendments ideally immobilize nitrate during the winter fallow period and then stop immobilizing immediately when the subsequent cash crop is planted. In actual practice however, that is not always achievable. We observed issues with the high carbon compost tying up nitrate in the subsequent crop and the following are some details from these observations:
- The high carbon compost works best when thoroughly incorporated into the winter bed in order to place it in the proximity of the nitrate in the soil (e.g. in the top foot of soil). In our trials, the material was applied to the land planed field. The soil was then chiseled, and the beds listed (Photo 4). That process seemed to incorporate the material adequately for 40-inch beds, but less well for 80-inch beds. There is a need to explore ways to get more thorough incorporation into 80-inch beds.
- In one trial, 10 tons/A of almond shells caused continued immobilization in the subsequent lettuce crop and caused yield reduction. The 5 tons/A rate did not reduce lettuce yield. There is a need to continue to explore appropriate rates and incorporation methods.
- We conducted a trial in which we intentionally put too much high carbon compost onto beds and then applied various rates of a starter fertilizer to overcome the effect of immobilization. Higher rates of starter fertilizer were able to reduce the negative impact of immobilization.
The bottom line is that high carbon amendments can provide a useful practice to reduce the load of nitrate in the soil during the winter fallow. The use of a material like Forest Mulch Compost is affordable and has been shown to be effective. Like all new practices, grower will have to start off treating small areas and build up their expertise and confidence when employing this practice. It is our hope that the Regional Water Quality Control Board will consider high carbon amendments on the “R” (removal) side of the proposed “A (applied) – R” equation which is the metric that is currently proposed to measure compliance with the limits on the quantity of N applied to crops.
Photo 1. Finely ground almond shells. Particle size <0.5 millimeter
Photo 2. Glycerol being applied to 40 inch beds.
Photo 3. Forest mulch compost (made from tree branches and trunks and triple screened).
Photo 4. Chiseling almond shells to incorporate into the soil.
Agro Gold WS was found adulterated with glyphosate and diquat and CDFGA has issue a stop order for use on organic farms in the state of California. The press release is shown below:
CDFA ISSUES STOP USE NOTICE AND STATEWIDE QUARANTINE ON ORGANIC HERBICIDE AGRO GOLD WS
SACRAMENTO, December 4, 2020 – The California Department of Food and Agriculture (CDFA) today announced that a Stop Use notice and statewide quarantine have been issued for the organic fertilizer product AGRO GOLD WS to all organic operations registered in California. CDFA lab analysis of the product detected the presence of Diquat and Glyphosate, which are substances prohibited by the U.S. Department of Agriculture (USDA) National Organic Program for use in organic production. Continued use of this product in organic production may jeopardize an operation's organic status.
Pursuant to authority under the California Food and Agricultural Code (FAC), Division 17, Chapter 10, CDFA's State Organic Program (SOP) in coordination with the Fertilizer Materials Inspection Program (FMIP) issued a Stop Use notice today for AGRO GOLD WS to all organic operations in California registered with the SOP. CDFA's FMIP also announced today that all California operations registered as organic in possession of AGRO GOLD WS must hold the product and contact CDFA for quarantine instructions on how to handle it.
AGRO GOLD WS is manufactured by Agro Research International, LLC. It has been distributed in a co-packaged box that also contains the product WEED SLAYER. CDFA continues to provide follow up to this investigation and is working with state and federal agencies. CDFA received a complaint about the AGRO GOLD WS product and program staff collected product samples from various locations to conduct lab analysis in CDFA's Center for Analytical Chemistry. FMIP is an industry-funded program that ensures consumers receive fertilizing materials that meet the quality and quantity guaranteed on the product label. Investigators located throughout the state conduct routine sampling and inspections, respond to consumer complaints, and enforce the laws and regulations that govern the manufacturing and distribution of fertilizing materials in California. CDFA's State Organic Program protects the organic label through enforcement, education and outreach.
If you are in possession of AGRO GOLD WS and seek additional information, please contact the Fertilizing Materials Inspection Program at FMIP@cdfa.ca.gov. Any appeal of the determination that this product violates the Food and Agricultural Code must be filed with the Fertilizing Materials Inspection Program no later than 15 days from receipt of the Stop Use notice and statewide quarantine. See Food and Agricultural Code section 14659.
Author: Richard Smith and Daniel Hasegawa, UCCE Monterey County and USDA ARS, Salinas
Impatiens necrotic spot virus (INSV) caused significant crop loss in 2020. The disease was most severe north of Gonzales, but later in the production season it was observed south to Greenfield, as well as in the Hollister, Gilroy, and Watsonville areas. The disease is caused by a tospovirus that is spread by the insect vector, western flower thrips (Frankliniella occidentalis). Tomato spotted wilt virus (TSWV) is another tospovirus that is transmitted by thrips, and can also infect lettuce on the Central Coast, displaying nearly identical symptoms as INSV. However, occurrence of TSWV in the region is much more limited, and was documented in Hollister, Gilroy, and Soledad during the 2020 lettuce season. Lettuce fields are infected by INSV by thrips migrating in from infected host plants in the early spring. During the production season, infected lettuce fields can be the principle source of INSV. However, at the end of the lettuce production season in November, INSV in the weeds and other host plants (e.g. ornaments) becomes the over wintering habitat. These plants serve as the bridge for the virus to survive during the winter fallow months and then as the source of the virus for the coming lettuce production season (Photo 1).
Key weed species that are good hosts for INSV include: malva, short pod mustard, sow thistle, lambsquarters, shepherd's purse, nettleleaf goosefoot, mares tail, nettle, field bind weed, purslane, flax leaf fleabane and the nightshades (see photos 2-13). These weed hosts need to be controlled in critical areas such as cropped and non-crop areas, fallow fields, roadsides, waste areas, banks, equipment yards and ditches. The first storms have occurred in the north end of the Salinas Valley and winter weeds have begun to germinate and will need attention from November to March to cut off the virus reservoirs. It is important to note that grasses, willows, giant reed (Arundo sp.) and coyote bush are not hosts of INSV which may reduce the concern about some wild areas, that are not also infested with weedy host plants.
The Agricultural Commissioner is notifying growers of their responsibilities to control weeds and has authority to enforce removal of weeds deemed a nuisance. Cal Trans has agreed to increase mowing of the median strip and areas adjacent to highway 101 twice per year (rather than the use once per year). It is very helpful to the lettuce industry to have the cooperation of these agencies.
Given the severity of the losses in 2020, the industry is requesting the cooperation of all growers this winter to make a special effort to reduce weed populations in all the usual areas as well as areas that may not have received as much attention in the past. It is hoped that these efforts may prove successful in reducing the source of INSV, similar to the successful efforts of the lettuce free period to reduce the incidence of Lettuce Mosaic Virus (LMV) in years past.
Photo 1. Graphic illustration of the role of host weed as over wintering reservoirs of INSV and their role as serving as the source for production fields infections in the spring.
Photo 2. Little mallow (Malva) a key host of INSV
Photo 3. Short pod mustard (germinates in the winter and the most common summer mustard along roadsides)
Photo 4. Sow thistle seedling (common in crop and non-crop areas)
Photo 5. Lambsquarter seedling (common warm season weed that germinates in late spring)
Photo 6. Shepherd's Purse seeding (common year-round weed in production fields and ditches)
Photo 7. Nettleleaf Goosefoot (common summer weed that can grow in the winter as well)
Photo 8. Mare's Tail (typical dense infestation of mature plants)
Photo 9. Burning Nettle
Photo 10. Field Bindweed (perennial around fields; infected plants can begin the season infected)
Photo 11. Purslane (common summer weed)
Photo 12. Hairy Fleabane (common summer annual that germinates in the winter)
Photo 13. Hairy Nightshade (common summer annual that can survive into the winter)
Strategies to Control Pythium Wilt of Lettuce
Author: Richard Smith and JP Dundore-Arias
Farm Advisor, UCCE Monterey; and Plant Pathology Professor, Cal State Monterey Bay
In 2020 Pythium wilt of lettuce (Pythium uncinulatum) caused up to 100% yield loss in some fields in the Salinas Valley. Although, Pythium wilt has been present in the Salinas Valley for nearly a decade, the widespread severity of the outbreaks this year were devastating and unprecedented.
Symptoms of the above ground parts of the plant include stunting, yellowing, and wilting of the outer leaves and eventual death (Photo 1). Sometimes the plants have a characteristic look where the younger leaves remain upright, but the older leaves wilt down (Photo 2). Upon examination, the roots of Pythium wilt infected plants, exhibit rot on the feeder (Photo 3) and/or the tap roots (Photo 4). The crown of the plant does not rot (however, in advanced infections the whole length of the root may be rotted) and this is one way to distinguish Pythium wilt from Sclerotinia. For instance, when you gently tug on wilting plants, Pythium infected plants do not break off at the soil line, whereas plants infected with Sclerotinia readily break off at the soil line and exhibit characteristic white mycelia and black sclerotia. The presence of rot on the roots distinguishes Pythium wilt from Verticillium and Fusarium wilts which cause internal vascular discoloration, but not external rot. If you are unsure about the cause of wilting/dying plants, it is best to have them tested.
Pythium wilt infects lettuce roots with swimming spores (zoospores) that move to the roots in soil water films. Additionally, it produces a second type of spore (oospore) that allow long-term survival in the soil. Moreover, Pythium species are generally known as good soil saprophytes, characterized for their ability to grow and survive in the soil even in the absence of a host plant by living on organic matter. Previous studies have reported P. uncinulatum is a pathogen of lettuce and does not cause disease on other vegetable crops. However, it remains unknown whether rotations with other crops may contribute to a build-up of the pathogen in the soil.
Devastating outbreaks of Pythium wilt mostly occurred on the east side of the Salinas Valley north of Gonzales on decomposed granite soils, but outbreaks also occurred in clay loam and sandy loam soils along the river. Given that Pythium wilt is a water mold, it was often most severe in wetter parts of the field with slower drainage, such as the bottom end of the field. However, in severe infestations Pythium wilt occurred the entire length of the field, particularly on susceptible varieties.
It is not clear what might have triggered the disease outbreaks, but widespread infections occurred following heat spells in mid-August and early-September. It is not clear why the heat may have triggered the outbreaks, but it is possible that additional irrigation water applied to cope with the heat could have imposed additional stress on plants with already unhealthy root systems. However, this explanation is not entirely satisfactory as smaller outbreaks were also observed in June and July prior to heat spells.
Pythium wilt infections frequently occurred in association with Impatiens Necrotic Spot Virus (INSV) infections. In trial 5, 8.4% of the plants had Pythium wilt and nearly all of them,7.9%, were also infected with INSV (Table 2, Photo 5). However, this was not always the case; in trial 4, plants with symptoms of Pythium wilt occurred on 22.6% of the plants, but only 0.8% were infected with INSV. Coinfection with Sclerotinia, Fusarium and Verticillium also commonly occurred.
In late September, we conducted Pythium wilt control trials in commercial fields. Each trial consisted of areas treated with 2-pints/A of Ridomil Gold which was compared with an untreated control. Ridomil Gold is registered for use on lettuce for at-planting soil applications to control damping off caused by Pythium sp. These trials were initiated too late in the planting season to include at-planting treatments, so Ridomil was applied at the following timings: 2 true leaf stage (one trial), at thinning (2 trials) and at the rosette stage (3 trials) (See Table 1 for more details).
In trials 2 and 3 we measured a significant reduction in the number of plants per plot that showed symptoms of Pythium wilt (Tables 3 and 4); no significant reduction in infected lettuce plants was observed in the other four trials. These trials showed that Ridomil applied at thinning and at the rosette stages could provide limited control of Pythium wilt. The best control was observed in the at-thinning application in Trial 3. It is not clear why Ridomil did not provide better control in the other four trials. We did not have the opportunity to examine at-planting applications in this year's trials, but they will be examined in 2021. Pythium wilt affects feeder roots on lettuce plants which are higher up in the soil profile, but it has also been commonly observed to attack that tap root of lettuce indicating that the infections can start deeper in the soil profile. Ridomil is mobile in soil which should allow it to distribute throughout the root zone. Questions remain regarding improving the efficacy of Ridomil: what are the effective rates, what is the most efficacious timing of application and can split applications improve efficacy?
There were significant differences in the susceptibility of varieties to Pythium wilt. Table 5 shows the results of ratings of varieties in several commercial production fields and one seed company variety trial. The evaluations were made at or near crop maturity and the lower numbers indicate less infection. None of the varieties were completely resistant to Pythium wilt, but several had greater tolerance. In general, the red leaf types were more tolerant than green leaf types, but there were some green leaf types that were tolerant as well (Photo 4). There were several head types that were also tolerant, but no tolerant romaine lettuce types were observed.
It is unclear if Pythium wilt will be a recurring threat to lettuce production in 2021. As mentioned, in 2020 the widespread incidence of the disease occurred in late summer. Pythium wilt was observed to a lesser extent in the spring of 2020 and the question whether it will cause widespread infection in the cooler months of 2021 is unknown. Another remaining question is whether disease occurrence and first appearance of symptoms might be accelerated, or aggravated, in soils where high disease severity was observed in 2020. The disease was mostly found north of Gonzales and there is a question whether it will extend its range in the future. There is a great deal that we do not know about managing Pythium wilt of lettuce, and we'll have to be watchful in 2021 using the few lessons and tools from 2020 that we gained.
Photo 1. Healthy plant on left, Pythium wilt infected plant on right.
Photo 2. Younger leaves remain upright while older leaves wilt down.
Photo 3. Pythium infection on fine roots.
Photo 4. Pythium infection on tap root.
Photo 5. Pythium wilt infected plant with lesions from INSV evident on older leaves.
Photo 6. Tolerant variety on the left and susceptible on the right.
Table 1. Trial details of Pythium wilt (PW) control trials.
1 – Spray = material sprayed over the top of the plants and incorporated by subsequent sprinkler irrigation (within 24 hrs)
2 – SL = seed lines
Table 2. Percent of plants infected with Pythium wilt and other diseases
Table 3. Number of plants infected with Pythium wilt in each plot in trials 1 and 2 on evaluation dates.
Table 4. Number of plants infected with Pythium wilt in each plot in trials 3-6 on evaluation dates.
Table 5. Informal evaluations of varieties for Pythium wilt of lettuce.
* Scale for Pythium wilt infection: 0 = plants all healthy to 10 plants all dead.
Up-coming on-line meeting on November 10th, 2020 from 12:50pm- 4:00pm.
This will be a Zoom event.
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Agenda for this meeting is below.
Up-coming on line meeting on December 1st, 2020 from 8:50am-11:30am
This will be a Zoom event.
If interested in participating, please register at:
Agenda for this meeting is below.