First Report of a Resistance-Breaking Strain of Tomato spotted wilt virus Infecting Tomatoes With the Sw-5 Tospovirus-Resistance Gene in California

Tomato spotted wilt virus (TSWV) is a highly destructive pathogen of fresh market and processing tomato (Solanum lycopersicum) in the Central Valley of California, U.S.A. In April 2016, severe tospovirus-like disease symptoms (i.e., stunting; leaf, stem, and petiole necrosis; and concentric rings on fruits) occurred in fields of fresh market tomato cultivars with the Sw-5 gene, which confers resistance to TSWV in Fresno County. Disease incidences of 30 to 50% were observed in some fields, and later reached 50 to 80%. Leaves with these symptoms were collected from four fresh market fields in Cantua Creek and one in Firebaugh, as well as from nearby processing tomato fields (n = 75). These plants were all positive for infection by TSWV when tested with AgDia immunostrips (Elkhart, IN), and RT-PCR with TSWV-specific N gene primers (Pappu et al. 1998). RT-PCR tests for Alfalfa mosaic, Tobacco/Tomato mosaic, Tomato chlorotic spot, and Groundnut ringspot viruses as well as ilar- and torradoviruses, were negative. Presence of the Sw-5 gene was confirmed by PCR with gene-specific primers (Dianese et al. 2010; Shi et al. 2011). To assess for resistance-breaking (RB) TSWV strains in these plants, RT-PCR and sequencing of the NSm was used to look for the amino acid substitutions, C118Y and T120N, previously associated with a Sw-5 RB TSWV strain in Spain (Lopez et al. 2011). The complete NSm gene was amplified from TSWV isolates in all 75 samples and each was directly sequenced in both directions. Sequence analysis showed that the complete NSm sequences were 99% identical to each other (e.g., KX898453 and KX898454), and revealed the C118Y but not the T120N substitution in the predicted NSm amino acid sequence from all TSWV-infected Sw-5 tomato plants tested (n = 45). Neither of these substitutions was present in the NSm of TSWV infecting processing non-Sw-5 tomato plants (n = 30). To assess infectivity, putative RB (C118Y substitution) isolates from Cantua Creek and Firebaugh and one wild-type isolate from Yolo County were mechanically inoculated onto three Sw-5 and one non-Sw-5 fresh market cultivars, and five Sw-5 and two non-Sw-5 processing tomato cultivars. In these experiments, the TSWV RB strains from California induced typical and severe symptoms of TSWV infection, similar to those observed in the field, 10 to 14 days post inoculation (dpi) in Sw-5 cultivars, whereas less severe symptoms appeared ∼30 dpi in a smaller number of plants of the non-Sw-5 cultivars. Disease incidence in the mechanically inoculated cultivars ranged from 10 to 100%. For the RB strains, RT-PCR and sequence analysis confirmed the presence of the C118Y substitution in the NSm of all isolates infecting Sw-5 cultivars and most (88%) infecting non-Sw-5 cultivars. None of the Sw-5 cultivars inoculated with wild-type TSWV developed disease symptoms by 30 dpi. PCR analysis confirmed presence/absence of Sw-5 in these cultivars as expected. Taken together, these results indicate that a TSWV RB strain has emerged in California, which can infect and cause typical symptoms in commercial fresh market and processing tomato cultivars with the Sw-5 gene. In field surveys, the TSWV RB strain was detected in Fresno but not in Yolo, Solano, or San Joaquin counties. This strain likely arose following a mutation, as Sw-5 cultivars have been widely grown in California over the past 5 years (e.g., ∼50% of processing cultivars grown in 2016). © 2017, American Phytopathological Society. All Rights Reserved.