Abstract
Bacterial spot is a serious disease of tomato caused by at least four species of Xanthomonas. These include X. euvesicatoria (race T1), X. vesicatoria (race T2), X. perforans (races T3 and T4), and X. gardneri, with the distinct geographical distribution of each group. Currently, X. gardneri and X. perforans are two major bacterial pathogens of tomato in North America, with X. perforans (race T4) dominating in east-coast while X. gardneri dominating in the Midwest. The disease causes up to 66% yield loss. Management of this disease is challenging due to the lack of useful chemical control measures and commercial resistant cultivars. Although major genes for resistance (R) and quantitative resistance have been identified, breeding tomato for resistance to bacterial spot has been impeded by multiple factors including the emergence of new races of the pathogen that overcome the resistance, multigenic control of the resistance, linkage drag, non-additive components of the resistance and a low correlation between seedling assays and field resistance. Transgenic tomato with Bs2 and EFR genes was effective against multiple races of Xanthomonas. However, it has not been commercialized because of public concerns and complex regulatory processes. The genomics-assisted breeding, effectors-based genomics breeding, and genome editing technology could be novel approaches to achieve durable resistance to bacterial spot in tomato. The main goal of this paper is to understand the current status of bacterial spot of tomato including its distribution and pathogen diversity, challenges in disease management, disease resistance sources, resistance genetics and breeding, and future prospectives with novel breeding approaches.
Highlights
The cultivated tomato (Solanum lycopersicum L.) is an important vegetable crop in the United States and worldwide
We review the diverse nature and species complex of the pathogen, available host resistance sources to Bacterial spot (BS) disease, the genetic and breeding efforts deployed so far, including transgenic approach, and the challenges involved in BS disease resistance breeding
Host resistant R proteins mediate a response to effectors and activate plant defense responses such as effector-triggered immunity (ETI), which is characterized by localized cell death, termed as a hypersensitive response (HR) [14]
Summary
The cultivated tomato (Solanum lycopersicum L.) is an important vegetable crop in the United States and worldwide. Bacterial spot of tomato is caused by at least four species of Xanthomonas: X. euvesicatoria, X. vesicatoria, X. perforans, and X. gardneri and four pathogenic races (T1, T2, T3, and, T4) are identified so far [6]. Isolation of race T5 under field conditions has not been reported yet These pathogenic races are discovered based on hypersensitive responses (HR) on tomato differentials that include HI 7998 (T1), HI 7981 (T3), and LA716 (T4). No differential tomato germplasm is available showing HR response to race T2 This is why race T2 has been reported in both X. vesicatoria and X. gardneri. Xanthomonas euvesicatoria was the only species, present as race T1, in Florida until 1991 before the X. perforans race T3 strain was reported. Xanthomonas gardneri has been reported from tomato fields in Pennsylvania [13], and was responsible for the epidemics in the Midwest [5]
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