A new strain of fusarium wilt has been detected in lettuce, putting crops in danger. Researchers at UC Riverside are exploring new ways to combat this potentially devastating fungal disease. Alexander Putman spoke at the World Ag Expo in Tulare and with Matthew Malcolm from Malcolm Media Ag Publishing to discuss the issue. Watch this quick video and learn more in California Fruit & Vegetable Magazine.
Please thank this video’s sponsor Simplot for their industry support.
Fusarium wilt of lettuce, caused by Fusarium oxysporum f. sp. lactucae (FOL), is an economically significant disease on the Central Coast of California. We conducted field trials to evaluate 30 iceberg and 21 romaine varieties for tolerance to Fusarium wilt. The trials were located in commercial fields in Greenfield, CA (wet date of May 27, 2024) and Salinas, CA (wet date of June 8, 2024). Foliar disease severity was visually evaluated on July 31-August 2, 2024 at Greenfield and on August 5-7, 2024 at Salinas and converted to a marketability (yes or no) rating. Disease pressure was high at both locations. Iceberg varieties that exceeded 50% marketability at both locations were Powerball and two coded entries from Vilmorin-Mikado. Nine varieties exceeded 50% marketability at Greenfield but not Salinas, and two varieties exceeded the same threshold at Salinas but not Greenfield. For romaine, 19 out of 22 varieties exceeded 90% marketability at Greenfield, but only two of those varieties (Holbrook and Momentus) also exceeded the same threshold at Salinas. In a greenhouse experiment, isolates from Greenfield showed a susceptible reaction on variety Costa Rica #4, which is consistent with the Costa Rica FOL race variant. Although FOL race 1 is suspected to be present at the Salinas location, greenhouse testing is not complete. These trials provide public data on the tolerance of iceberg and romaine varieties to Fusarium wilt.
Methods
Field trials were conducted in Greenfield, CA and Salinas, CA to evaluate both in-slot and out-of-slot varieties (30 iceberg and 21 romaine) for tolerance to Fusarium wilt in commercial fields with disease history. At Greenfield, bed center spacing was 80 inches, and plots were 1 plant line wide by 100 ft. long. Due to space constraints, 4 iceberg and 10 romaine varieties were not included in the Salinas trial. At Salinas, bed center spacing was 40 inches, and plots were 1 plant lines wide by 40 ft. long. Iceberg and romaine varieties were evaluated separately, and plots of each type were arranged in a randomized complete block with four replications. Treatments were direct seeded using the grower-cooperators’ planters at Greenfield and using single-line push planters at Salinas. The wet dates were May 27, 2024 for Greenfield and June 8, 2024 for Salinas. The Greenfield trial was maintained to commercial standards for lettuce production, whereas the Salinas trial was not. After thinning by commercial crews, 50 plants at Greenfield and 30 plants at Salinas in the center of each plot were counted, and the section was marked with stakes. Data were collected from this center section. Evaluations were performed on July 31-August 2 at Greenfield and August 5-7 at Salinas, which was before maturity at the Salinas trial. Foliar disease severity was assessed on a 0 to 4 scale where: 0 = healthy; 1 = wilting or chlorosis of one to three outer leaves; 2 = up to moderate stunting and wilting or chlorosis of <25% of leaf area; 3 = head is severely stunted or absent and between 25% and 75% of leaf area is wilting or chlorotic; and 4 = head is absent and >75% of leaf area is chlorotic and nearly dead, or plant is entirely dead. For analysis, foliar disease severity was converted to marketability, where: disease severity of 0 or 1 = marketable; and disease severity of 2, 3, or 4 = not marketable. Marketability data was analyzed by an analysis of variance (P < 0.05), and variety means were separated using Tukey’s honestly significant difference test.
Results – Race of the FOL pathogen present
Two races of FOL are present on the Central Coast: race 1, and a novel race variant (Nayak et al., 2024). We are using the temporary name “Costa Rica FOL variant” for the novel race variant until it is officially named following completion of the upcoming ring test, which is a collaborative experiment between researchers and seed companies. To determine the FOL race present, isolates from each location were evaluated in a race typing experiment in the greenhouse. Variety Costa Rica #4 showed a susceptible reaction to both Greenfield isolates, which supports the observation that the Costa Rica FOL race variant is present at the Greenfield location. We suspect FOL race 1 is present at the Salinas location. However, greenhouse testing of the Salinas location isolates is ongoing.
Results – Marketability
Disease pressure was high at both locations. Iceberg varieties that exceeded 50% marketability at both locations were Powerball and two coded entries from Vilmorin-Mikado (Table 1). Nine varieties exceeded 50% marketability at Greenfield only: Balboa, Fontinas, Meridian, Paraiso, San Andreas, San Miguel, two coded entries from Salinas Valley Seeds, and one coded entry from Sakata. In contrast, two varieties exceeded the same threshold at Salinas only: Fredonia and a coded entry from Takii. This pattern of some varieties showing large differences in performance between locations whereas others showed similar performance suggests that a different race is present at each location, but this has not yet been confirmed by greenhouse testing.
Of the 21 romaine varieties evaluated, two varieties exceeded 90% marketability at both locations: Holbrook and Momentus (Table 2). A total of 17 out of 21 varieties exceeded the same threshold at Greenfield but not Salinas. At the Salinas location, Holbrook and Momentus were not statistically different from four varieties (Boronda, Copious, Patton, and Solid Heart) with average percent marketability ranging from 77% to 88%.
If you have additional questions about these trials, please contact Alex Putman at 951-522-9556 or aiputman@ucr.edu.
Please Send Us Samples
We are continuing to collect samples of lettuce Fusarium wilt to determine the distribution of races and to monitor the pathogen. To support this research, please contact the person in your region. Your help would be greatly appreciated.
Acknowledgements
We are grateful to D’Arrigo Brothers Co. of California and an anonymous grower for the space and maintenance of the variety field trials. We thank seed producers for providing seed for the trial. Funding for this project was made possible by a grant from the U.S. Department of Agriculture (USDA) Agricultural Marketing Service. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the USDA. Funding was also provided by the California Leafy Greens Research Program.
References
Nayak, S., K.L. Richardson, A.I. Putman, N.R. LeBlanc, F.N. Martin, N. Li, and J.D. McCreight. 2024. Detection of novel pathogenic variants of Fusarium oxysporum f. sp. lactucae in California. Plant Pathology Early View. doi:10.1111/ppa.14019
The Central Coast Regional Water Quality Control Board (CCRWQCB) approved Ag Order 4.0 in April of 2021. This regulatory action requires growers to calculate the quantity of nitrogen (N) that leaves their fields in harvested product. This value is needed to calculate the metric applied (A) minus removed (R) nitrogen (A-R) which indicates the amount of N that remains in the field over the season and is considered the load of N at risk for nitrate leaching to groundwater resources that municipalities rely upon for drinking water. In Table 1 crop removal coefficients are provided for 75 crops and crop products. They include the larger acreage vegetable crops such as lettuce and broccoli as well as smaller acreage crops such as summer squash, green beans and Asian vegetables. Given the large number of species, gerberas (no foliage on the stem) and snap dragons (foliage on the stem) were evaluated as representative flower crops. Growers producing different flower crops can substitute gerberas or snap dragons to supply the N removal estimate of their crops.
The crop removal coefficient is multiplied by the total weight of harvested product to provide an estimate of the pounds of N that are removed from the field. This information can then be used to report N removal from production fields to the CCRWQCB.
For instance, in the case of whole heads of bulk romaine lettuce which in this example has a net* yield of 36,000 lbs/acre:
36,000 lbs harvested product/acre x 0.00149 = 53.6 lbs of N/acre removed
*subtract box weight from gross weight.
N removal coefficients were determined by multiplying the percent moisture content of the harvested product by its total N content. We worked with harvest crews to get freshly harvested products and immediately measured the moisture content. The samples were then dried and sent to the UC Davis Analytical Lab for total N content. We collected 4 to 6 samples per field from at least 10 to 15 production fields with diverse soil types, production practices and growing conditions (see Table 1 for exact number of fields sampled for each commodity), and as a result, there is variability around the mean values which is shown in Table 1.
Coefficients tend to be higher for commodities with high N content and high percent solids. The CCRWQCB allows growers to use different coefficients if they can justify their use; and growers can also develop their own coefficients using CCRWQCB guidelines. However, N removal coefficients for a given crop will tend to fall in certain ranges. For instance, high N content leafy vegetables will have higher coefficients that will tend to vary from 0.005 to 0.006. Head vegetables tended to range from 0.002 to 0.003. So, although coefficients may vary to some degree, they will tend to fall within certain ranges. In the above example if the maximum coefficient of 0.00166 was used for bulk romaine, the increase in the estimate of N removal would only be about 6 lbs N/acre (59.8 lbs N/acre removed). — By Richard Smith, UCCE Monterey County, Michael Cahn, UCCE Monterey County, Aparna Gazula, UCCE Santa Clara County, Daniel Geisseler, UC Davis, and Andre Biscaro, UCCE Ventura County
Table 1. Mean and range of crop coefficients (coeff) developed by this FREP funded project and additional crops evaluated. Factors used to develop the coefficients are also included: percent dry matter (%DM) and percent nitrogen (%N)
Two of the worst pests plaguing lettuce growers in the Salinas Valley area are aphids, specifically lettuce-currant aphids (Nasovonia ribisnigri), and western flower thrips (Frankliniella occidentalis). Lettuce-currant aphid is an invasive pest that sets up shop in the heart of the lettuce plant and will render the crop unsellable when it reaches high enough numbers. Thrips can both cause cosmetic damage to lettuce crops and are also responsible for the spread of Salinas impatiens necrotic spot virus (INSV), the fatal lettuce disease that has driven large losses since the 2020 growing season.
While effective tools exist to control both aphids and thrips, they are almost exclusively chemical. Chemical sprays are increasingly under pressure due to changes in the regulatory framework in California as well as the development of pest resistance and discoveries of key chemistries in area watersheds1,2. The UC Davis FiVE lab biological control research program addresses a growing interest in developing alternative tools for managing both pests that do not rely on chemical applications. Biological control provides an opportunity for the management of thrips and aphids that do not rely on chemical tools.
Biological control is defined as the use of natural enemies to control a target pest. Three general categories of biological control could possibly be used as management practices for lettuce pests in the Salinas Valley area:
• Conservation biological control refers to the establishment and maintenance of resources and conditions favorable to a native or endemic beneficial species. Instead of releasing predators into crop fields, specific types of flowers and other habitats are planted to attract beneficial species that are already a part of the local ecosystem. To date, most efforts on biological control in lettuce have used the conservation biological control approach.
• Inundative biological control involves the release of a beneficial insect species in large numbers with the expectation that the beneficials that are released will only provide control for a short amount of time before eventually dying out. Such releases would need to be repeated at regular intervals for the duration of the growing cycle for a crop.
• Augmentative biological control refers to the use of releases of smaller numbers of beneficials to areas where a smaller population of the species already exists, but not in numbers great enough to provide adequate control of the targeted pest species. The goal of augmentative releases is to bolster already-existent populations of beneficial species so they achieve great enough numbers to provide control of the pest or pests of interest.
Conservation biological control in the Salinas Valley
Syrphid flies
Aphid pests of lettuce have been effectively managed in some lettuce production systems through the planting of sweet alyssum adjacent to and interspersed within crop fields3. Sweet alyssum is a favorite of the Syrphid fly (Diptera: Syrphidae), the primary biological control agent used to control aphid pests in lettuce. Syrphids, also called hoverflies or flower flies, are a family of black and yellow pigmented flies which resemble bees and stinging wasps. The coloration is a protective camouflage; Syrphid flies are harmless to humans. Syrphid adults are frequently seen visiting flowers for their nectar and pollen, which the insect consumes both as an energy source and to support their reproduction.
In exchange the female Syrphid flies will lay eggs in lettuce plants with lettuce aphid infestations, the primary food source for their young. Once the eggs hatch, the syrphid maggots, which are predatory on slow, soft-bodied insects, will feed on the aphids and suppress their population. Syrphid larvae are known to be voracious; some California species have been shown to consume upwards of 100 aphids per day4!
Syrphids are the intended beneficiaries of most conservation biological control in central coast lettuce fields, but other beneficial species take advantage of these resources as well.
Other predatory species love sweet alyssum
Many other biological control agents are supported by insectary plantings5. Ladybird beetles often inhabit lettuce fields and may provide some control of lettuce aphid infestations. Common lacewings (family Chrysopidae) are also found in lettuce fields and insectary plantings. Lacewings, which are only predatory in their immature or larval life stage, can provide biological control services against lettuce aphids and western flower thrips. Minute pirate bug (Orius sp.) and aphid midges (Aphidoletes aphidimyza) have also been observed in and collected from insectary plantings in lettuce fields, but it is not known the extent to which they can suppress populations of lettuce aphid or Western flower thrips.
UC Davis Fi-VE Bug IPM Lab biological control research programs
Including insectary plantings to attract naturally occurring predators has historically been the only efficient way to get beneficial species into crop fields. Newly developed technology using drones as a dispersal tool may provide another option for growers interested in using biological control as part of their pest management programs for aphids and thrips. This technology drastically reduces the time and labor required to conduct large releases of laboratory-reared beneficial insects, making the approach more feasible for growers.
As part of a research program funded by the California Department of Pesticide Regulation (CA DPR) and in collaboration with Daniel Hasegawa at USDA-ARS and with Parabug, we are studying the release of biological control agents using drones for the management of aphid and thrips pests of lettuce crops. Our three experimental programs are as follows:
In-field inundative releases of green lacewing larvae and predatory cucumeris mites to control aphids and thrips in lettuce
Experiments run by former Monterey County IPM Advisor Alejandro Del Pozo-Valdivia found that a single inundative release of green lacewing eggs (Chrysoperla rufilabris) in lettuce fields reduced aphid pressure six weeks after release6. Our experiment builds on Alejandro’s work, examining whether repeated releases of green lacewing eggs throughout the lettuce growing cycle reduce aphid numbers. Additionally, the experiment includes two treatments aimed at suppressing western flower thrips: inundative releases of a species of predatory mite (Amblyseius cucumeris), and a combined release of both predatory mites and green lacewing eggs.
Augmentative releases to bolster non-syrphid predatory species in insectary strips and intercropped alyssum
Other native predators of aphids and thrips are present in the insectary plantings growers use to attract syrphids, but their numbers are too low to provide suppression of thrips and aphids in adjacent crops. These species are reared by commercial insectaries, but using them in an inundative release could prove too costly for growers. Experiments in this program examine the use of smaller releases of these predatory species early in the growing cycle over insectary plantings. The goal is to determine whether the presence of floral resources allows the predators to stick around and build up enough in population to control aphids and thrips in the crop field. Experiments will be conducted with aphid midge (Aphidoletes aphidimyza), an aphid predator, and minute pirate bug (Orius insidiosus), a predator of western flower thrips.
Augmentative releases to manage thrips in non-crop areas
Western flower thrips plague not just vegetable crop fields but also the vegetation surrounding crop areas. In this experiment, we will examine whether releases of cucumeris mites and minute pirate bugs over field edges planted with ice plant will establish these predators in the vegetation and provide long-term suppression of western flower thrips. — By Ian Grettenberger and Addie Abrams, UC Cooperative Extension
Citations
Canada will once again implement additional temporary import requirements for U.S. origin romaine lettuce for Fall 2023. The additional requirements will be in effect from September 28, 2023 to December 20, 2023. As with previous years’ requirements, Canada will require imported romaine lettuce and/or salad mixes containing romaine lettuce originating from the Salinas Valley counties of Santa Clara, Santa Cruz, Monterey, or San Benito to have a negative test for E. coli O157:H7 or a Proof of Origin for romaine and/or salad mixes from other U.S. growing regions.
Since 2019, the Canadian Food Inspection Agency (CFIA) has implemented additional import requirements during the Fall period for U.S. romaine lettuce. Specific testing requirements for E. coli O157:H7 have been implemented since 2020 for romaine lettuce originating from the Salinas Valley counties of Santa Clara, Santa Cruz, Monterey, and San Benito. CFIA is once again implementing requirements for Fall 2023 during the time period of September 28, 2023 to December 20, 2023.
Market Impact
Since 2019, the implementation of additional import requirements for U.S. romaine lettuce during the final quarter of the year has acted as a market disruptor, with Canadian importers and U.S. exporters challenged to implement the new requirements. In November 2019, CFIA issued a notice that imports of romaine lettuce from the United States must not come from lettuce harvested from the counties of Santa Clara, Santa Cruz, Monterey, and San Benito. In October 2020, CFIA’s import requirements permitted imports from the counties of Santa Clara, Santa Cruz, Monterey, and San Benito with the completion of a negative E. coli O157:H7 test. These additional import requirements resulted in logistical challenges around sampling and diagnostic capacity, consumers and wholesalers reported higher prices, and were periodically challenged to source product. With potential threats to a consistent supply, wholesalers began seeking out additional suppliers.
Prior to 2019, the five-year average of fresh lettuce imports into Canada in Q4 was 51,640 MT, with the U.S. market share reaching 99 percent, 62 percent of which came from California. During the past four years, when additional import requirements were in effect, the average Q4 imports were 47,350 MT. During this time, the U.S. market share was down slightly averaging 94 percent with California dropping to 53 percent market share. The average Q4 volume from Mexico almost quadrupled in the 2019-2022 compared to 2014-2018, increasing from one to five percent. Imports of Mexican romaine to Canada in Q4 may be particularly advantaged if additional import requirements for U.S. origin romaine lettuce continue to be burdensome for importers.
Fall 2023 Requirements
During this time, imports to Canada of U.S. romaine lettuce are required to be accompanied by a Proof of Origin confirming that any romaine lettuce and/or salad mixes containing romaine lettuce do not originate from the Salinas Valley counties of Santa Clara, Santa Cruz, Monterey, or San Benito. Should the romaine lettuce and/or salad mixes containing romaine lettuce originating from the Salinas Valley counties of Santa Clara, Santa Cruz, Monterey, or San Benito then CFIA requires testing for E. coli O157:H7 to confirm it is not detected in order for product to be import eligible. Importers will have a temporary condition on their Safe Food for Canadians (SFC) license for these requirements.
The requirements remain similar to the 2022 requirements. For imports originating from the counties of Santa Clara, Santa Cruz, Monterey, or San Benito, importers must provide for a protocol in their preventive control plan outlining how they will implement the required sampling and testing. Each shipment must also have the completed attestation declaring that the appropriate sampling and testing was completed and E. coli O157:H7 was not detected in the shipment (Form CFIA/ACIA 5961). The official Certificate of Analysis for testing must also be included with each shipment.
CFIA provides for two approaches to fulfill the sampling requirements:
1) Finished-product sampling: sampling and testing is to be conducted before product is imported to Canada but is completed after all post-processing and handling steps are finished. A sampling lot is defined as 1 type or product of a size no larger than 1 truckload (maximum 20,400 kg/45,000 lbs). Each sampling lot must have a total sample weight of 1,500 g comprised of 60 randomized individual sample units of 25 g.
2) Pre-harvest sampling: field sampling may be completed no more than 7 d prior to harvest. A sampling lot is defined as a 2 acre field or less. CFIA requires the field to be a homogenous romaine lettuce crop subjected to homogenous agricultural conditions. A total sample weight of 1,500 g comprised of 60 randomized individual sample units of 25 g must be taken from each sample lot. This option is primarily designed to accommodate field-packed product.
The CFIA website provides for additional details on sampling requirements and testing requirements, including accepted methodologies and laboratory accreditation requirements, which are required. — By Alexandrea Watters, USDA Foreign Ag Service
Maeli Melotto and her team at the UC Davis Department of Plant Sciences are looking for strains of lettuce that are genetically stronger at resisting bacteria that can make people sick. Their work has led to the identification of a gene that could play a role in the plant’s susceptibility to E. coli, a bacterium that causes potentially lethal intestinal illness.
If the team could develop lettuce that can fight off the bacterium, that would avoid thousands of cases of sickness each year, cut production costs for farmers and save millions of dollars in public health expenses.
Lettuce “sweats” substances onto the surface of leaves that E. coli and other bacteria can eat. In addition, bacteria can live just below the surface of lettuce leaves. Melotto, a professor specializing in the interactions between plants and microbes, is studying the genetic factors that determine how long bacteria can endure in the leaves. She and her team looked at more than 300 samples of lettuce and how they react to the pathogens, identifying the gene responsible for letting them in.
Their next step is to test whether removing that gene could make lettuce more resistant to bacteria.
In a related study, Melotto’s team is looking at the surface of lettuce leaves and the layer just below the surface, which has tiny spaces where bacteria can live. The researchers want to learn what natural chemicals are in these two areas, and what compounds lettuce releases onto the surface of leaves that bacteria can use as food. Melotto expects to find that some types of lettuce are genetically disposed toward “feeding” bacteria.
Melotto presented her findings at meetings earlier this year of the California Specialty Crops Council and the California Leafy Greens Research Board. Her work is funded in part by a five-year grant from the National Institute of Food and Agriculture.
The FDA is releasing the findings of a sampling assignment for which FDA collected and tested romaine lettuce from commercial coolers in Yuma County, Arizona during February and March 2021. The agency tested the lettuce for Shiga toxin-producing Escherichia coli (STEC), specifically enterohemorrhagic Escherichia coli (EHEC), and Salmonella spp. This assignment was part of the FDA’s ongoing surveillance following multistate E. coli O157:H7 outbreaks of foodborne illness in recent years linked to or potentially linked to romaine lettuce.
The agency’s goal in conducting this assignment was to determine whether the target pathogens and specific strains may be present in romaine lettuce from the Yuma agricultural region, to help prevent foodborne illness when possible. If product that tested positive for EHEC or Salmonella was found, the Agency planned to work with industry and state regulatory partners to identify the cause (e.g., farm follow-up investigation) to inform future regulatory and/or research efforts and to develop strategies that could help preventive additional outbreaks.
The FDA collected 504 romaine samples for EHECs and Salmonella spp., with the testing performed by an independent laboratory on contract, as part of a pilot project. Each sample consisted of 10 subsamples, and each subsample was made up of at least 300 grams of romaine lettuce (whole heads, hearts or individual leaves). Collecting and testing samples composed of multiple subsamples increases the probability of detecting pathogens if present, since microbial hazards may not be uniformly present.
During the assignment the FDA detected E. coli O130:H11 in one sample. The isolate was found to be moderate to high risk and could be capable of causing severe illness in humans, though it was not linked to any known human illnesses, and no product ever reached consumers. The owner of the product did not harvest the remaining crop from the field where it was grown.
In response to the finding, FDA conducted an investigation at the farm to identify possible sources and routes of contamination. The FDA was able to collect romaine lettuce from the field, multiple samples of soil, water, sediment, and animal fecal material. FDA also assessed farm equipment and other surfaces. Only one of the total 24 samples yielded STEC (specifically, E. coli O116:H-). This sample came from the outer leaves of romaine lettuce. The strain was further characterized as low risk to human health, and FDA’s analysis indicated the strain was not linked with any past known foodborne illness outbreaks.
Helping to ensure the microbiological safety of leafy greens continues to be a priority of the FDA. Romaine lettuce and other leafy greens are among the most widely consumed vegetables in the United States and are an important part of a healthy diet. The agency is working on several fronts to help prevent microbial contamination of leafy greens and to prevent outbreaks of foodborne illness. Chief among these efforts is the FDA’s Leafy Greens STEC Action Plan (LGAP), which features public health approaches related to response, prevention and addressing knowledge gaps. The FDA continues to collaborate with industry, states, academia and other stakeholders through activities outlined in the LGAP to address this important public health issue.
The U.S. Food and Drug Administration will be collecting and testing samples of lettuce grown in California’s Salinas Valley from local commercial coolers from May through November 2021. The agency will test the samples for Shiga toxin-producing Escherichia coli (STEC), including E. coli O157:H7, and Salmonella spp. as part of ongoing surveillance efforts following reoccurring outbreaks linked to this region, including most recently in the fall of 2020.
The FDA assignment will direct sampling to be conducted at commercial cooling and cold storage facilities where field heat is removed from harvested lettuce and where product is cold-stored before processing. Sampling may include pre-cooled product (preferred) or post-cooled product. Sample collection at commercial coolers helps the FDA efficiently obtain samples from multiple farms at centralized locations and facilitates prompt traceback and follow-up if contamination is detected.
The agency plans to collect and test a total of approximately 500 post-harvest samples of iceberg, leaf and romaine lettuce. Each sample will consist of 10 subsamples, each made up of one head of lettuce (trimmed, cored and possibly wrapped), or in the case of romaine lettuce, loose leaves or one package of hearts. FDA laboratories will conduct all testing.
During this sampling assignment, the FDA will take extra precautions to help ensure the safety of agency investigators and firm employees during the COVID-19 pandemic. FDA investigators will preannounce their visits to firms per the Agency’s COVID-19 safety practices. They will be outfitted with personal protective equipment (PPE) and will carry out their work while adhering to local, state and applicable CDC guidance.
Helping to ensure the safety of leafy greens remains a high priority of the FDA. This assignment adds to other work underway in collaboration with stakeholders in the California Central Coast growing region to identify where the recurring strain of pathogenic E. coli is persisting and the likely routes of leafy green contamination with STECs. This includes continued implementation of actions identified in the recently updated Leafy Greens Action Plan, including a multi-year longitudinal study to assess the environmental factors impacting the presence of foodborne pathogens in this region. Consistent with the action plan, if the FDA detects a pathogen such as E. coli O157:H7, the agency will conduct a follow-up investigation to identify potential sources and routes of contamination. Such investigations are designed to inform what additional preventive measures may be needed to help prevent outbreaks of foodborne illness.
A recent report of viral disease on lettuce from our neighbor (Yuma, Arizona) caught our attention since this is highly relevant to our production system (please find information on the first link below). The name of the virus is “Impatient Necrotic Sport Virus” (INSV), which is a tospovirus, similar to the virus that attacks tomato to cause tomato spotted wilt virus symptoms. This virus (INSV) was first reported affecting lettuce crops in Salinas Valley of California in 2006. Subsequently, it was reported to cause crop loss in 2012 and 2015 in the same area. This virus is transmitted by western flower thrips (Frankliniella occidentalis), which is very common and abundant in the low desert region. Early symptoms of infection by INSV are brown to dark spots and dead (necrotic) areas on leaves, which is often mistaken as chemical burn as shown in the picture below on the left-hand side (Photo Credit: Steven T. Koike, UCANR). As the disease progress, multiple leaves could be affected and result in distorted, twisted and dwarf plants (picture on the right). Most of the lettuce types are susceptible to this virus. Several weed species also believed to be the hosts of this virus. Thrips, that also feed on the alternate host weeds can facilitate INSV transmission to lettuce and other crops.
The good news is that this virus has not been reported from Imperial Valley to our best knowledge. However, we must keep an eye on anything unusual, especially the symptoms shown in the pictures below. 
If you observe similar symptoms on your lettuce or related crops, please bring to our attention, contact us at (442) 265-7700 or bring the sample to our office, 1050 E Holton Road, Holtville, CA 92250.
For more information:
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=7309
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=17351
-By Apurba Barman & Oli Bachie, UC Cooperative Extension