Impatiens Necrotic Spot Virus on Lettuce

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» Impatiens necrotic spot virus (INSV) is an increasingly important pathogen of lettuce in the western United Stat 
» INSV infections now result in substantial economic losses in lettuce production.
» The western flower thrips transmit INSV, and controlling thrips is a primary means for helping to reduce infection of lettuce. 

 The impatiens necrotic spot virus (INSV) is an important pathogen of ornamental plants in North America and Europe, especially on greenhouse-grown plants. However, INSV has become an increasingly important pathogen on lettuce in the western U.S.^1,2 INSV was identified as a pathogen on lettuce in Italy in 1997. In 2006 INSV was found infecting lettuce in the Salinas Valley of California. Most likely, the virus was introduced to the region on infected ornamental plants. 

The virus is now well established in the coastal valleys of California and has spread to the lettuce-growing regions of the central valley and the desert-southwest region of California and Arizona.^2,3,4 In many fields where it is present, impatiens necrotic spot (the name of the disease caused by INSV) occurs at incidence levels of 0.5 to 27%. However, the levels of pervasiveness and incidence have been increasing. A study published in 2022 indicated that INSV was detected at incidence levels above 1% in over 750 fields in Monterey County, and incidence levels over 50% were not uncommon. Incidence levels rose from 16 to 80% over a seven-week period in two fields that were part of the study.⁵ INSV now annually causes millions of dollars in losses for the lettuce industry in California and Arizona. 

All types of lettuce (romaine, green leaf, red leaf, butterhead, iceberg) are susceptible to INSV infection. Impatiens necrotic spot occurs in direct seeded and transplanted fields and in conventional and organic production systems^.2,4,6 INSV is a tospovirus, the group that also includes tomato spotted wilt virus (TSWV), which is another important pathogen on lettuce.¹ 

The symptoms of impatiens necrotic spot (INS) include tan to dark-brown spots forming on leaves, often initially associated with leaf veins (Figure 1). Other symptoms include yellowing, deformation, and a downward curling of leaves. The leaf spots expand and coalesce resulting in necrotic areas, initially on the leaf margins (Figure 2). As the necrosis spreads, leaves turn brown and dry out.^1,6,7 

Lettuce plants can be infected at any growth stage. If infected when young, lettuce plants can be stunted and may die (Figure 3). Infected plants often do not produce marketable heads or leaves. INSV and TSWV infection symptoms are identical, and diagnostic assays are required to identify the pathogen accurately^.1,6,7 Some host species other than lettuce can be symptomless when infected by INSV. 

INSV infects a wide range of hosts, including crop plants such as basil, blackberry, celery, faba bean, lettuce, peanut, pepper, potato, radicchio, spinach, tomato, and several ornamental species. Common weed hosts in California include little mallow and shepherd’s purse, with lower detection levels on sowthistle, hairy nightshade, London rocket, and yellow and purple nutsedge^.1,3 

INSV is transmitted primarily by the western flower thrips (Frankliniella occidentalis), and to a lesser extent by the European flower thrips (F. intonsa) and the tobacco thrips (F. fusca)^.1,5,6 The thrips vectors acquire the virus in the larval stage when feeding on INSV-infected plants (including lettuce and other host species). Recent speculation is that INSV infected strawberries are a major source of inoculum for lettuce in the Salinas Valley. The vector does not transmit the virus until it reaches the adult stage. The virus is persistent and propagative within the thrips vector, meaning that the virus can replicate in the insect, and the insect carries the virus for the rest of its life. Viruliferous thrips can transmit the virus after feeding periods as short as five minutes, but transmission efficiency increases with longer feeding times^.3,6,7 INSV can also be transmitted in plant sap, but sap transmission is not as effective as transmission by the thrips vector. INSV is not known to be seed transmitted^.1,3,7 

No commercial lettuce varieties have been found to have complete immunity to INSV infection. However, lettuce varieties vary in susceptibility to INSV, with some varieties showing fairly low rates of infection even under moderate disease pressure.^7,8 Partial resistance has been demonstrated in some lettuce varieties, which are now used in breeding programs focused on INSV resistance. Lettuce breeding lines, plant introductions (PIs), and other Lactuca species are also being evaluated as possible sources of INSV resistance. The Sw-5 gene for TSWV resistance found in tomatoes has also been shown to provide tomatoes with resistance to INSV. It may be possible to develop transgenic varieties of lettuce with this gene as a form of resistance. There have been reports of lettuce plants with complete resistance to both INSV and TSWV, but the seed of this variety has not been made widely available for verification and further testing.⁷ 

Cultural practices, such as planting virus-free transplants and avoiding planting lettuce near potential sources of the virus may help manage INSV infection. Planting new lettuce fields upwind from established planting of INSV hosts and avoiding planting known host species as ground covers or intercropping plants (trap crops or habitat plants for beneficial insects) is also advised.^1,3,6 Managing known weed hosts within and around lettuce plantings may help reduce the rate of INSV infection in the lettuce crop. Weed cultivation should take place before lettuce emerges because the cultivation of weeds after emergence can increase the movement of thrips onto lettuce plants.⁵ Promptly destroy (till in) crop debris after harvest to help limit the spread of INSV to nearby plantings and reservoir hosts.^1,3 

In growing regions where INSV and TSWV are known to be present, lettuce fields should be regularly scouted to check for the presence of western flower thrips. Thrips typically feed on the undersides of leaves, in leaf folds, and at the bases of plants. Examine plants for thrips and the feeding scars they create. Blue and yellow sticky traps can be used to detect adults migrating into the field from nearby vegetation.⁹ 

The primary strategy for managing INSV infection is to monitor and manage the thrips vector through the application of insecticides.^1,6 Applications must be timed carefully to help keep incidence levels of INS below damaging thresholds. It can be difficult to apply insecticides to the plant structures where thrips tend to aggregate. Also, some of the available insecticides have been shown to have limited efficacy for managing western flower thrips, in part because there are thrips populations that have developed resistance to some of the commonly used insecticides. Thrips can also be reintroduced to the field on air currents after the residual activity of the insecticide has declined.³ 

1 Gilbertson, R. and Koike, S. 2017. Impatiens necrotic spot. In “Compendium of Lettuce Diseases and Pests, Second Edition”, Subbarao, K. et al. editors. The American Phytopathological Society, St. Paul. 

2 Koike, S., Kuo, Y., Rojas, M., and Gilbertson, R. 2008. First report of impatiens necrotic spot virus infecting lettuce in California. Plant Disease 92:1248. 

3 Kuo, Y. Gilbertson, R, Turini, T., Brennan, E., Smith, R., and Koike, S. 2014. Characterization and epidemiology of outbreaks of Impatiens necrotic spot virus on lettuce in coastal California. Plant Disease 98:1050-1059. 

4 Hasegawa, D., Hladky, L., Wintermantel, W., Putman, A., Barman, A., Slinski, S., Palumbo, J., Poudel, B. 2022. First report of impatiens necrotic spot virus infecting lettuce in Arizona and southern desert regions of California. Plant Disease 106:2274. 

5 Hasegawa, D. and Del Pozo-Valdivia, A. 2022. Epidemiology and economic impact of impatiens necrotic spot virus: a resurging pathogen affecting lettuce in the Salinas Valley of California. Plant Disease. https://doi.org/10.1094/PDIS-05-22-1248-RE. 

6 Koike, S., and Turini, T. 2017. Tospovirues. UC IPM Pest Management Guidelines: Lettuce. 

UC ANR Publication 3450. 

7 Simko, I., Richardson, C. E., and Wintermantel, W. M. 2018. Variation within Lactuca spp. for Resistance to Impatiens necrotic spot virus. Plant Disease 102:341-348. 

8 Richardson, K., and Nayak, S. 2021. Update on Breeding Lettuce for Resistance to Impatiens Necrotic Spot Virus. Hortscience 56:S103-S104. 

9 Natwick, E., Joseph, S., and Dara, S. 2017.Thrips. UC IPM Pest Management Guidelines: Lettuce. UC ANR Publication 3450. 

Performance may vary from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields. The recommendations in this article are based upon information obtained from the cited sources and should be used as a quick reference for information about vegetable production. The content of this article should not be substituted for the professional opinion of a producer, grower, agronomist, pathologist and similar professional dealing with vegetable crops. 

BAYER GROUP DOES NOT WARRANT THE ACCURACY OF ANY INFORMATION OR TECHNICAL ADVICE PROVIDED HEREIN AND DISCLAIMS ALL LIABILITY FOR ANY CLAIM INVOLVING SUCH INFORMATION OR ADVICE. 

5911_160000 Published 1/3/2023