Resistance-Breaking Variants of TSWV Found on Tomato in NC
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In summer 2022, symptoms of TSWV (Tomato spotted wilt virus) were observed at two tomato farms in Rowan County, NC on several fresh market tomato cultivars with the gene for TSWV resistance (Sw-5). Approximately 10% of plants of ‘Red Mountain’ had symptomatic foliage on one farm with approximately 30% of fruit showing symptoms. At the second farm, up to 50% of plants had symptomatic foliage with approximately 80% of fruit showing symptoms on ‘Mountain Gem’. Visual symptoms included stunting, severe leaf curling and bronzing, necrotic lesions on leaves, petioles and stems, and concentric ring spots on fruit. TSWV ImmunoStrips from Agdia confirmed the virus was present in the field. Samples were collected from both farms and subjected to sequencing to identify and confirm the presence of resistance-breaking strains.
The resulting partial sequences of the virus from the first farm were confirmed to have the mutation that results in overcoming resistance to the Sw-5 gene, as reported in CA in 2016. The sequences from the second farm had a different and novel mutation that was found to also overcome the Sw-5 gene and were different from the variant from the first farm. Therefore, two resistance-breaking variants of TSWV have been confirmed on tomato in North Carolina.
This discovery resulted from a collaboration of growers and scientists. The growers mentioned above initially alerted Dr. Reza Shekasteband, Research Scholar and Associate Tomato Breeder in Horticultural Science at NC State, who was evaluating TSWV on breeding lines and commercial cultivars with Sw-5 on their farms. When TSWV symptoms were observed on resistant cultivars, he alerted NC State plant pathologists, Dr. Dorith Rotenberg, Kirsten Lahre, Inga Meadows, and Dr. Anna Whitfield to verify that this was indeed a a resistance-breaking variant of TSWV. This new discovery and rapid characterization of the new TSWV variant highlight the strong collaborations between NC vegetable growers and scientists from different disciplines and the importance of on-farm research and trials.
Growers and stakeholders are strongly advised to monitor their tomato seedlings and plants for symptoms of TSWV. If you suspect TSWV is occurring on a tomato variety that is resistant, contact your local extension agent and submit samples through the NC State Plant Disease and Insect Clinic. Keep in mind that resistant varieties may still exhibit symptoms on fruit when thrips and virus populations are high because the gene is not expressed in flowers.
Tomato spotted wilt virus is primarily transmitted by the western flower thrips. Symptoms include those described above and pictures in this factsheet. Management relies on an integrated approach including deployment of resistant varieties, rotation of insecticides, and reducing weed populations around greenhouses and fields which can harbor the virus. Identifying resistance-breaking variants of TSWV reduces our management tools and threatens the tomato industry in North Carolina.
This article was prepared by Dr. Dorith Rotenberg, Dr. Anna Whitfield and Inga Meadows.