Grower-Shipper Association of Central California (GSA) — Implementing effective disease and pest management strategies is a continual challenge for farmers. While these challenges are not new in agriculture, they impact more than just farmers. Lower yields due to disease and pest pressure affects farm employees and harvesting crews too and can also inhibit our ability to provide a steady supply of affordable and healthy produce to consumers.
Unfortunately, farmers of leafy greens are now dealing with the re-emergence of a damaging disease called thrips-vectored Impatiens Necrotic Spot Virus or INSV. According to local farm advisors this increase in INSV is often accompanied by Pythium wilt infections, which is a relatively new problem in the region.
Lettuce fields are infected by INSV via thrips migrating in from infected host plants in the early spring. Fields infected by INSV cause stunting, yellowing, wilting of the outer leaves and eventual death for leafy greens.Currently, there are no effective treatments for INSV and Pythium wilt on organic and conventional lettuce farms.
To support farmers as they battle INSV, GSA created a new task force to identify major research questions, examine treatment strategies, develop treatment efficacy trials and build a grower education program specific to these diseases.
Mary Zischke, former Executive Director of the California Leafy Greens Research Board, was hired by GSA to lead and coordinate Task Force activities. In addition, to assist local farm advisors and help advance University of California cooperative research efforts, GSA added Jasmine Rodriguez to its team. Rodriguez is currently working toward her biology degree at California State University, Monterey Bay and also serves as a laboratory assistant for entomology research projects at the UC Cooperative Extension in Monterey County.
“Mary and Jasmine will provide our industry with the additional personnel resources and experience as we combat this complex disease and pest control problem,” says Christopher Valadez, GSA president.
To further expand resources and personnel, GSA has applied for a California Department of Food and Agriculture Specialty Block Grant. The grant would allow scientists to conduct multi-year field research trials assessing the effects of common crop rotation sequences, monitor Pythium wilt and INSV occurrence in commercial fields, conduct greenhouse studies to characterize the interactions between Pythium wilt and INSV in lettuce to understand their impacts on overall plant health and disease management.
If awarded the grant, the GSA team will work with researchers from California State University, Monterey Bay and U.S. Department of Ag, Agricultural Research Service in Salinas. Grant outcomes would include knowledge of the impact of crop rotation on Pythium wilt and thrips populations as well as the interplay between Pythium wilt and INSV and how to jointly manage them.
While GSA is hopeful the grant provides future resources, we will continue moving needed work forward to uncover potential answers and treatment strategies so farmers in our region can minimize losses. Farming equates to surmounting challenges and GSA is committed to supporting farmers as they face those challenges.
It is never too early to start planning for next produce season, particularly with the threat of Impatiens Necrotic Sprot Virus (INSV) looming. As many of you know, INSV was found in several Yuma locations this spring, particularly in the Tacna area. Overall, INSV incidence was very low (~1% incidence), but there were a few fields where incidence was estimates at > 25% (see Chart 1 below).
With desert lettuce production now finished, the question remains; will INSV in these infested areas survive the summer in the absence of lettuce. Of course, this depends on the availability of host plants in the cropping landscape to serve as reservoirs for INSV in the next 4 months, as well as the abundance of western flower thrips on these hosts. A team of applied scientists at the UA Yuma Ag Center –Bindu Poudel, Stephanie Slinski, Barry Tickes, Marco Pena and myself (with the assistance of Daniel Hasegawa, USDA-Salinas) – are currently trying to determine this. First, we know that many of the common weeds currently found in drainages, field borders, and ditch banks serve as reproductive hosts for western flower thrips. What this means is that thrips are capable of colonizing these weed hosts. Immigrating adults can transmit viruses and larvae can acquire viruses that can be transmitted after they emerge as adults. Recent areawide monitoring of crops and weeds in Tacna and Yuma Valley showed thrips adults and larvae were abundant on nettle leaf goosefoot and cheeseweed (see Chart 2 below), and more importantly, tissue samples recently collected from weeds near previously infected lettuce fields in Tacna tested positive for INSV in nettleleaf goosefoot, cheeseweed, and purslane. It appears that these weeds could provide a green bridge for the virus and the vector from now till September. The threat of INSV surviving within our desert cropping system in the absence of lettuce this summer is real.
So, weed management this summer is going to be very important if we hope to eliminate the local presence and spread of INSV. Growers should be extremely vigilant in keeping ditch banks and low-lying drainages adjacent to fields free of weeds. When upcoming wheat harvests are completed, all weeds along field edges need to be thoroughly disked under along with the stubble. Even a few remaining goosefoot or purslane plants on a field edge can potentially harbor INSV and a significant number of thrips vectors. Similarly, periodic disking of alfalfa borders and field edges should help minimize weed regrowth. Same goes for cotton and Sudan grass fields. When spring melons are harvested, growers should promptly disk under plant residue. Any PCA watching melons this spring can attest to the attractiveness of melon flowers to thrips. Finally, as growers start to flat water fields in preparation for fall lettuce planting, it will be as important as ever to promptly control weeds emerging in those fallow blocks. Remember, recent weed sampling showed purslane tested positive for INSV, and a ten-acre field infested with purslane can potentially generate a lot of thrips. So can volunteer melons. Thus, summer sanitation is as important as ever. Bottom line: anything growers can do to reduce weed abundance in and around their fields can only help. For more information on this crop sanitation, please visit Weed Interference with Insect Management in Desert Crops.The other critical question we are working on is whether our summer crops (alfalfa, cotton, melons) serve as hosts and reservoirs for INSV. We do not know yet, but our team is working on it. We’ll let you know when we know. — John Palumbo, University of Arizona Extension
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.
Effective control of western flower thrips (WFT) to prevent cosmetic scarring and contamination is important in spring lettuce crops, and now that INSV has been found infecting plants in Yuma lettuce, management becomes even more important. For most of the growing season, WFT numbers have been below average based on sticky trap counts, field reports and population densities here at YAC. However, in the past few weeks WFT populations have increased in most growing areas. This is not surprising as we typically observe “bioconcentration” of WFT during March and April on late lettuce as surrounding produce acreage declines. Each time a lettuce field is harvested, and disked, adult thrips disperse from these areas into the next available lettuce field. As the number of lettuce acres becomes reduced near the end of the season, this creates a bottleneck effect that concentrates high numbers of thrips adults on the remaining fields under production. This can often make chemical control of WFT very difficult, particularly in March, as adults can continually re-infest fields following spray applications. So, what management approaches can you take to manage thrips and hopefully reduce the potential incidence of INSV. The first line of defense should be sanitation. Growers should disc under produce crops immediately following harvest. The longer a harvested crop remains above ground, the more insects that can build up and move to adjacent lettuce fields, especially with the dry, windy, and warm weather we typically have in March. If only light INSV incidence is present in pre-harvest fields, PCAs should consider rogueing and removing suspect plants from the field. Ultimately, this may curb secondary infection within the field.
Controlling WFT with insecticides is the best approach to minimizing cosmetic feeding damage and may reduce spread of INSV within fields. For adults, which can easily be found on the leaf surface, your best choices of insecticide are methomyl (Lannate) or acephate at a high label rate. They are the most efficacious products against adult WFT. Radiant is not as consistently efficacious against adults, but a 7 oz or higher rate will provide the best knockdown and residual control of WFT larvae. Remember, the key to preventing cosmetic damage by WFT is to maintain larval populations at low levels. The cryptic or thigmotactic behavior of thrips often makes them difficult to find on lettuce plants. Research has shown that if you can see a few adults and larvae on the plant, it means that there are likely 10-fold more thrips actually on the plant (hiding near the base of the plant between midribs). This behavior also means that spray coverage is important, particularly with contact insecticides like Lannate. There are other insecticides such as Torac, Minecto Pro, Exirel, Movento and Assail that can provide suppression (50% control or less) against WFT larvae, but don’t expect much activity against adults. For more information on the identification, biology, ecology, and management of thrips on desert produce please visit Western Flower Thrips Management on Desert Produce. — By John Palumbo, University of Arizona Extension
In 2020 the incidence of Pythium wilt (caused by Pythium uncinulatum) of lettuce has increased in severity and in the number of affected fields. Pythium infections in lettuce fields have been observed frequently, but not always, occurring with INSV infection. As a result, there has been confusion distinguishing between these two diseases and the role of each of them in causing the problems in fields. In this blog we will discuss these two diseases and explain from our current state of knowledge about the disease dynamics occurring in affected fields.
INSV has been a production problem on lettuce in the Salinas and surrounding valleys for a number of years and in 2020 it continues to be a serious production issue. Pythium wilt of lettuce is a relatively new problem and was first discussed in a blog entry in October 2015 by Steve Koike (https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=19327 ). However, in 2019 and 2020 we have seen an increase in the number of acres affected by Pythium wilt as well as the severity within fields. Given that Pythium is a relatively new problem and the extent of the problem suddenly increased, some growers and PCAs are confronting this problem for the first time. To add to the confusion, at times INSV and Pythium infections are occurring together on the same plants which has caused confusion and has led to much speculation about the role of each disease in the observed damage.
Symptoms of INSV
Issues with INSV infections on head and leaf lettuce types are not a new occurrence in the Salinas Valley and many growers and PCAs are familiar with the symptoms and the patterns of infection in the field, especially on romaine. In general, INSV on lettuce causes characteristic patterns of chlorosis and necrosis on the inner leaves of the plant, as well as significant stunting (Photo 1). However, INSV can cause significant necrosis and lesions on and at the base of the ribs of lettuce plants (Photo 2). It should be mentioned that Tomato Bushy stunt virus (TBSV) can cause symptoms that can be confused with INSV and Pythium wilt; however, lettuce dieback symptoms are always seen on the outer, older leaves and the TBSV pathogen is commonly restricted to low-lying areas along the river. In addition, head lettuce varieties and some romaine varieties are resistant to this virus. When in doubt, it is important to have a sample tested. That said, INSV is the overwhelming virus issue facing growers and PCAs in 2020.
Moderate to severe symptoms of INSV on ribs of romaine.
Viral vs. Fungal Symptoms
One important detail about lettuce plants infected with only INSV is that they do not exhibit wilting of the outer leaves of the plant or show root rot or root discoloration. This is important to note because in 2019 and 2020 we have visited many fields where the plants exhibit symptoms of INSV and have wilting older leaves. In these situations, the roots and crowns of the plants should be examined for symptoms caused by soilborne pathogens such as the wilt pathogens (Fusarium and Verticillium), Sclerotinia, and Pythium. Fusarium and Verticillium do not cause rot on the fine roots or crown. However, they always cause characteristic vascular discoloration in the taproot and crown of the plant. Distinguishing these two pathogens without a laboratory evaluation is not advised, but in general, Fusarium occurs earlier in the crop cycle and often causes a red-to-brown discoloration internally along the taproot and at the base of the crown. Symptoms of Verticillium on the above ground parts of head lettuce become obvious close to harvest; the taproot and crown tissue of infected plants have dark brown-to-black discolorations. Plants with INSV can also be infected with Sclerotinia (S. minor) which is recognized by the characteristic rotting of the crown tissue of the plant and the presence of white, cottony growth and small blacksclerotia (Photo 3). Plants infected with Sclerotinia easily break off at the soil line when you gently tug on them. However, if the plants do not break off at the soil line and do not show any rot on the crown tissue but do exhibit rot on the fine feeder roots or lower down on the taproot, then Pythium wilt is suspected and can be verified by laboratory evaluation.
Sclerotinia infection on lettuce. Note that it infects and rots crown tissue of the plant.
Biology and Symptoms of Pythium
Pythium wilt is caused by the water mold, Pythiumuncinulatum. It infects lettuce roots with swimming spores (zoospores) that move to the roots within the water film in the soil. Additionally, it produces a second type of spore (oospore) that allows the pathogen to survive in the soil in the absence of a host plant. Previous studies have reported P. uncinulatum is almost exclusively a pathogen of lettuce and does not cause disease on other vegetable crops. However, it remains unknown whether other crops may contribute to a build-up of the pathogen in the soil. Affected plants will exhibit rotting of the fine and tap roots (Photo 4) and frequently dark discoloration of the inner core of the main root (Photo 5). Symptoms of the above ground parts of the plant include stunting, yellowing, and wilting of the outer leaves and eventual death (Photo 6). Sometimes the plants have a characteristic look where the younger leaves remain upright, but the older leaves are totally wilted down to the soil (Photo 7). This year, we frequently observed fields where plants are infected with Pythium wilt but are also infected with INSV. These mixed infections are confusing and make it more difficult to distinguish what is the cause of the damage. In our experience to date, plants that show foliar symptoms of INSV and that have wilting older leaves are typically infected with both INSV and, in many cases, Pythium wilt. It should be mentioned that we have also observed plants infected with INSV as well as Fusarium.
Pythium wilt infection of fine lettuce roots.Pythium wilt infection on lettuce taproot.Mini romaine infected with Pythium wilt.Romaine infected with INSV and Pythium wilt. Note that the older leaves are wilted and lay on the ground.
The distribution of Pythium wilt in a lettuce field can be variable. Earlier in the summer, fields with this disease typically were infected along the upper or lower ends of the field indicating that the disease may be responding to irrigation or drainage issues. It is possible that there may be a difference in the level of infection between sprinkler and drip irrigated fields, but we cannot say anything definitive at this time. The disease has been found from King City to Castroville. There is a significant difference in the susceptibility of varieties. In fields with multiple leaf type lettuce, we have observed significant differences in susceptibility among varieties with red types being less susceptible (Photo 8). Recently, there have been severe losses in some fields. It is not clear as of this writing, but it is possible that the incidences occurred in response to the heat spells. It is likely that diseased plants were not able to withstand the weather stress due to damaged roots or that extra water applied to address the heat may have stimulated the development of Pythium wilt. Another observation we have made is that at times Pythium mostly infects the fine roots higher up on the root system and in other situations it is more severe at the bottom of the taproot (Photo 9) which may indicate disease initiated farther down on the root system. Given that the disease needs a period of soil saturation for the swimming spores to travel to the roots, issues with soil preparation, drainage and irrigation management may affect the severity of the disease.
Difference in susceptibility of two green leaf lettuces and a green leaf and red leaf lettuce to Pythium.Plant with Pythium wilt infection lower down on the tap root (note the plant on the top with healthy fine roots higher up and infected tip of the tap root).
Research Efforts
Daniel Hasegawa is conducting research on the epidemiology and spread of thrips and INSV. JP Dundore Arias is working on a project with the California Leafy Greens Research Board monitoring the occurrence of Pythium wilt in the Salinas Valley. He is also characterizing isolates of this disease to better understand the organism and will be conducting preliminary evaluations of the sensitivity of the organism to fungicides. Given the rapid onset of severe damage of Pythium and the continued severity of INSV, we are trying to better understand these diseases and how they may interact. We are interested in receiving samples of Pythium wilt. Please contact Richard (rifsmith@ucdavis.edu) or JP (jdundorearias@csumb.edu) to submit samples.
Authors: Richard Smith, JP Dundore Arias, Daniel Hasegawa and Steve Koike
Farm Advisor, UCCE Monterey; Plant Pathology Professor, Cal State Monterey Bay; Research Entomologist, USDA ARS; Director, TriCal Diagnostics, respectively
