New Virus Strains Overcome Key Resistance in Market and Processing Tomatoes

Greg Howard
11th July, 2024

New Virus Strains Overcome Key Resistance in Market and Processing Tomatoes

The widespread failure of Sw-5b resistance in tomato is visually demonstrated by an unusual, severe symptom phenotype of yellowing and scorching (a–d) caused by emerging resistance-breaking strains of Tomato spotted wilt orthotospovirus (TSWV), which contrasts with typical disease symptoms (e, f).

Image adapted from: Macedo et al. / CC BY (Source)

Key Findings

  • The study by the Federal Institution of Brasília found new TSWV strains in California that can overcome the Sw-5b resistance gene in tomatoes
  • The C118Y strain, first detected in Fresno County in 2016, became dominant in central and northern California until 2023
  • In 2023, a new strain, T120N, became predominant in Colusa, Sutter, and Yolo counties, infecting resistant tomato varieties and causing severe symptoms
Tomato spotted wilt orthotospovirus (TSWV) is a significant threat to tomato production, causing considerable economic losses worldwide. The Sw-5b resistance gene has been widely used to manage this virus. However, a recent study by researchers at the Federal Institution of Brasília[1] has revealed the emergence of resistance-breaking (RB) TSWV strains in California, posing new challenges to tomato growers. Over the past decade, RB strains of TSWV have become increasingly prevalent in tomato production areas in California. The study identified the C118Y RB strain, which emerged in Fresno County in 2016 and quickly became predominant in the central production area. This strain remained dominant until 2021 when it spread to the Northern production area, maintaining its prevalence until 2022. However, in 2023, the C118Y strain was detected in fewer samples from resistant tomato varieties due to the emergence of a new RB strain, T120N, which was previously known to occur in Spain. The T120N strain became predominant in Colusa and Sutter counties, detected in 75-80% of samples, and in about 50% of samples from Yolo County. Pathogenicity tests confirmed that California isolates of the T120N strain could infect Sw-5b tomato varieties and induce severe symptoms. Another RB strain, C118F, was associated with spotted wilt samples from Sw-5 varieties in fresh market tomato production in southern California. This study builds on earlier research that highlighted the challenges posed by TSWV and its vectors. For instance, previous studies have shown that TSWV is transmitted by thrips, with the western flower thrips (WFT) being the primary vector in California. Monitoring of WFT populations and TSWV incidence from 2007 to 2013 revealed that thrips populations and TSWV infection rates were highest in late-planted fields, leading to the development of an integrated pest management (IPM) strategy for TSWV in processing tomatoes[2]. This strategy included planting TSWV- and thrips-free transplants, using resistant varieties, monitoring for TSWV symptoms and thrips, and implementing extensive sanitation after harvest. The emergence of RB strains such as C118Y and T120N underscores the need for ongoing monitoring and the development of new resistance sources. The study by the Federal Institution of Brasília utilized specific RT-PCR tests to track the prevalence of these strains, revealing their rapid spread and predominance in different production areas. Phylogenetic analyses with complete NSm sequences showed that the C118Y and T120N RB strains infecting resistant processing tomatoes in California emerged locally, while those from fresh market production were more closely related to isolates from Mexico. This indicates that the widespread deployment of the Sw-5b resistance gene has driven the local emergence of multiple RB strains. The findings of this study highlight the importance of continued vigilance in monitoring for RB strains and developing new strategies to manage TSWV. The use of advanced genomic tools, such as RenSeq, can facilitate the identification of novel resistance genes and improve the breeding of disease-resistant tomato varieties. RenSeq has been successfully used to reannotate the full complement of NB-LRR resistance genes in tomato and potato genomes, providing breeders with valuable information to identify new disease resistance traits[3]. In conclusion, the emergence of RB TSWV strains in California poses a significant challenge to tomato production. The study by the Federal Institution of Brasília emphasizes the need for ongoing monitoring and the development of new resistance sources to manage these evolving threats. By incorporating advanced genomic tools and building on previous research, scientists and breeders can work together to protect tomato crops from the devastating effects of TSWV.

AgricultureGeneticsPlant Science

References

Main Study

1) 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.

Published 10th July, 2024

https://doi.org/10.1371/journal.pone.0305402

Related Studies

2) Development of an IPM Strategy for Thrips and Tomato spotted wilt virus in Processing Tomatoes in the Central Valley of California.

https://doi.org/10.3390/pathogens9080636

3) Defining the full tomato NB-LRR resistance gene repertoire using genomic and cDNA RenSeq.

https://doi.org/10.1186/1471-2229-14-120


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