Rutgers University | New Jersey Agricultural Experiment Station | Pesticide User Responsibility

September 13, 2024

Resistance-breaking Tomato Spotted Wilt Virus present in tomato in New Jersey

Tomato spotted wilt virus (TSWV) has caused significant problems for some fresh-market tomato and pepper growers in New Jersey the past few growing seasons. Although not uncommon, economic losses these past few years have been extensive on some farms.

TSWV is vectored by various species of thrips, a common vegetable insect pest that seems to be on the incline and very difficult to control in vegetable production throughout the state. The most important vector of TSWV is the western flower thrips, Frankliniella occidentalis. It’s mode of transmission is persistent propagative, meaning that thrips nymphs have to feed on an infected plant, whereby followed by a short incubation period (lasting from hours to days), the virus is then persistently transmitted throughout the rest of the insect’s life span.

TSWV cannot be passed from infected females to eggs; and TSWV is not transmitted in seed.

The western flower thrips and TSWV both have a wide host range! Western flower thrips host range includes: tomato, pepper, onion, celery, cucumber, lettuce, potato, basil, strawberry; a wide range of herbaceous ornamentals (e.g., impatiens, geranium, marigold, petunia, dahlia, gerbera daisy, carnation) as well as many common weeds (e.g., pigweed, chickweed, lambs quarter, thistle, galinsoga). TSMV can infect over 1,000 plant species from more than 90 plant families.

In California and other tomato production regions, resistance-breaking (RB) strains (C118Y, C118F, T120N) of TSWV have recently been discovered that can overcome the single gene resistance (Sw-5b) bred into widely-grown processing and fresh-market tomato varieties. Recent research in TX has also shown that RB TSWV strains may increase western flower thrips fitness (ability to reproduce) by prolonging the adult period and increasing fecundity (i.e., a measure of an insect’s reproductive success, often expressed as the number of eggs or offspring produced by an insect) compared to non-RB and non-viruliferous controls. Unfortunately, the breakdown of genetic resistance along with the potential increase in TSWV-infected thrips reproduction rates, may lead to significant problems in thrips and TSWV control in New Jersey and elsewhere.

Recent work in Texas has shown that RB-TSWV can break resistance (conferred by the commonly deployed TSW-mediated single gene resistance) in TSWV-resistant pepper (Capsicum annuum) with varying levels of symptom development in both resistant and susceptible cultivars; with none being completely immune.

In October 2022, samples of a TSWV-infected fresh market tomato variety with Sw-5b resistance were collected in southern New Jersey and sent for analysis. Results determined that RB (C118Y) TSWV was present in the state; and the strain found in New Jersey was similar to the RB TSWV found in fresh-market tomato from Mexico and processing tomato in California suggesting a high potential for its widespread movement.

So, where do we go from here? Based on the isolated report of TSWV being found on resistant fresh-market tomato in New Jersey in 2022, and the more recent reports of it this year, it does not appear that RB TSWV is currently widespread throughout the state. However, this may likely change.

All vegetable growers, those who produce their own transplants or bring them in, need to carefully evaluate their thrips monitoring and mitigation programs this winter.

  1. Start fresh. Prior to the transplant production season, clean and disinfect the greenhouse or any other structure where you might be holding transplants. Remove any weeds within and around the structure. Use sticky cards to monitor the potential carryover thrips population during the winter months, especially if you have any plant material in the greenhouse during those months.
  2. Never produce or keep tomato or pepper transplants you start yourself or bring in, in the same greenhouse with any ornamental plants.
  3. Segregate any transplants that are brought into your operation from your own transplants, as well as segregate different source of transplants as best you can.
  4. Evaluate all incoming transplants for thrips damage.
  5. Treat all incoming transplants with an insecticide immediately.
  6. Use yellow sticky cards to continually monitor for thrips populations in the greenhouse from the start of the transplant season until the end.
  7. Consider using biological or natural control(s) in the greenhouse.
  8. The use of silver reflective mulches have been shown to reduce thrips populations in fields.
  9. Develop a season-long insecticide program prior to the production season; from applying an insecticide at transplanting through cover sprays until harvest.
  10. Monitor thrips populations and feeding damage in the field with regular scouting and sticky cards.
  11. Closely monitor thrips feeding injury on pepper and tomato fruit during the production season.
  12. Proper weed control is essential since many weeds may harbor the virus or infected thrips. This includes areas around the production field.
  13. Rogue out any suspicious looking plant(s) prior to transplanting, or any suspicious looking plant early in the production season (e.g., any plant that starts to stunt out early) to help mitigate the within field spread.

Growers should continue to utilize TSWV-resistant tomato and pepper varieties realizing the effectiveness of those in limiting TSWV is becoming compromised. All growers need to continue to follow best management practices (such as those listed above) and pay careful attention to current weaknesses in their production practices and thrips control programs and adjust their management practices heading into 2025 growing season.

by: Andy Wyenandt and Kris Holmstrom

References:

Macedo MA, Melgarejo T, Cespedes M, Rojas M, Lazicki P, Turini T, et al. (2024) An all-out assault on a dominant resistance gene: Local emergence, establishment, and spread of strains of tomato spotted wilt orthotospovirus (TSWV) that overcome Sw-5b-mediated resistance in fresh market and processing tomatoes in California. PLoS ONE 19(7): e0305402.

Tomato spotted wilt virus on pepper and tomato. Inga Meadows and Andy Cooper, NCSU 2024

Gautam et al., 2022. First report of a resistance-breaking strain of tomato spotted wilt orthotospovirus infecting Capsicum annuum with Tsw resistance gene in Texas. Plant Dis. 107:1958.

 

Filed Under: Organic Production, vegetable, Vegetable Crops Tagged With: ,
June 21, 2024

The Curious Case of the Virus Infected Tomato Fruit

Original Article published on 6 June 2024 by Dr. Jerry Brust, University of Maryland Extension

Ben Beale, educator from St. Mary’s County found an odd thing last week. He found in a grower’s high tunnel, tomato fruit that had the symptoms of a virus infection, but there were no foliar symptoms on any of the plants. Some fruit on a cluster had symptoms while other fruit on the same cluster looked perfectly fine. Ben had the fruit tested and got a fast response from Jill Pollok at the University of Delaware Diagnostic Clinic and it was Tomato Spotted Wilt Virus. The cultivar in question Big Beef Plus has TSWV resistance. There could be one or two possibilities for TSWV symptoms showing up in a resistant cultivar and for fruit symptoms but not any foliar symptoms of the virus.

We will look at how tomatoes can get infected by the TSWV (if you already know how this works skip down to the next paragraph). Tomato spotted wilt virus (TSWV) is an obligate parasite, i.e., it must have a living host and must be moved from one plant to another by thrips or through cuttings or possibly seed. This disease can affect tomato and other Solanaceae crops as well as lettuce, beans and cucumber. TSWV may occur in the field but tends to affect greenhouse and high tunnel crops more severely. The virus is transmitted most efficiently by Western flower thrips (WFT) (Frankliniella occidentalis), and less so by Onion thrips (Thrips tabaci), Tobacco thrips (Frankliniella fusca) and several other thrips species.  It is not transmitted by Eastern flower thrips (Frankliniella tritici). Only immature thrips can acquire the virus, which they can acquire within 15 minutes of feeding, but adults are just about the only stage able to transmit the virus. Adults can transmit the virus for weeks. It may take 2 – 4 weeks from when the adult thrips first feeds on a plant until initial symptoms are observed. Because of this TSWV appears to worsen in plantings over time.

Why are we seeing fruit symptoms but little if any foliar symptoms? The most likely explanation is that TSW viruliferous thrips fed on the flowers or very young fruit. The resistance to the TSWV is expressed (active if you will) in the non-reproductive parts of the plant but not expressed to any extent in the reproductive parts (flowers and fruit) of the plant. Even though the flower and fruit can act as an entry point into the plant the resistant tomato does a good job of limiting the virus into any other areas of the plant. Thus, under low to moderate feeding pressure (1-4 thrips/flower) only the fed upon fruit or possibly a few other fruits on the same cluster become infected. The other possibility is that the TSWV resistance in Big Beef Plus is intermediate and under environmental stress it may not be ‘complete enough’ to protect all of the plant.

The first question above as to why is the virus showing up at all in a resistant cultivar can be explained by the above paragraph. But there may be another possibility although not in this particular case, that we will have to watch out for in the coming years. And that is resistance- breaking virus variants. The Sw-5b gene (Sw-5) is the most widely used resistance gene for TSWV in tomato. It not only provides resistance to TSWV but also resistance to a several associated viruses including Tomato chlorotic spot virus (TCSV) and Impatiens necrotic spot virus (INSV). The presence of the Sw-5 gene in tomato plants confers resistance to TSWV by a hypersensitive defense response that causes local lesions on the leaf, preventing the spread of the virus from the infection site through the plant. In 2022, symptoms of TSWV were observed in North Carolina tomato fields on cultivars that had the gene for TSWV resistance (Sw-5). The cultivars from different farms had symptomatic foliage and fruit. Samples were collected from both farms and subjected to sequencing to identify and confirm the presence of resistance-breaking variants, which they did. I mention this because NC is very close by, many growers get their tomato transplants from NC and because the resistance-breaking occurred in the last year or so. These resistance-breaking variants may start to show up in our fields and I am guessing that when they do, we will see 20-40% viral foliar infection and possibly worse in the fruit of resistant plants. For now, we should keep using the resistant cultivars, but watch closely for any breakdown of the resistance.

Resources: 2023. First Report of Resistance-Breaking Variants of Tomato Spotted Wilt Virus (TSWV) Infecting Tomatoes with the Sw-5 Resistance Gene in North Carolina. K. Lahre, R. Shekasteband, I. Meadows, A. E. Whitfield, and D. Rotenberg. Plant Disease vol. 107.

This article appears in Volume 15, Issue 4 of the Vegetable and Fruit News.

Filed Under: Vegetable Crops Tagged With: , ,