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Emergence of a resistance breaking TSWV strain in tomato in California

Ozgur Batuman: University of Florida IFAS

<div><em>Tomato spotted wilt virus</em> (TSWV) is a highly destructive pathogen of tomato in the Central Valley of California, USA. During the 2016 tomato growing season, unusually early and severe symptoms of TSWV occurred in fields of TSWV-resistant (Sw-5) fresh market tomato cultivars in Fresno County. High disease incidences (50-80%) were observed in some fields. The result of biological and molecular (sequence) analyses of TSWV isolates from these symptomatic Sw-5 tomato plants collectively revealed that these infections were caused by a newly emerging resistance-breaking (RB) TSWV strain in California. Furthermore, in mechanical inoculation assays in a greenhouse, it was established that RB TSWV strains infected fresh market and processing tomato cultivars with and without the Sw-5 gene. The emergence of RB TSWV strains in California occurred 5-6 years after ever-increasing planting of Sw-5 cultivars. The widespread use of Sw-5 cultivars, especially in ‘hotspot’ areas, is an important component of an effective integrated pest management (IPM) strategy, which was developed in California and helped reduce TSWV incidences in processing tomatoes for over a decade now. The new RB strain appears to have evolved following a mutation, which was associated with the C118Y substitution in the NSm gene of a local wild-type TSWV strain. With the emergence of the RB strain, the IPM program needs to be revisited and revised.</div>

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