Early blight (EB) is one of the dreadful diseases of tomato caused by several species of Alternaria including Alternaria linariae (which includes A. solani and A. tomatophila), as well as A. alternata. In some instances, annual economic yield losses due to EB have been estimated at 79%. Alternaria are known only to reproduce asexually, but a highly-virulent isolate has the potential to overcome existing resistance genes. Currently, cultural practices and fungicide applications are employed for the management of EB due to the lack of strong resistant cultivars. Resistance sources have been identified in wild species of tomato; some breeding lines and cultivars with moderate resistance have been developed through conventional breeding methods. Polygenic inheritance of EB resistance, insufficient resistance in cultivated species and the association of EB resistance with undesirable horticultural traits have thwarted the effective breeding of EB resistance in tomato. Several quantitative trait loci (QTL) conferring EB resistance have been detected in the populations derived from different wild species including Solanum habrochaites, Solanum arcanum and S. pimpinellifolium, but none of them could be used in EB resistance breeding due to low individual QTL effects. Pyramiding of those QTLs would provide strong resistance. More research is needed to identify additional sources of useful resistance, to incorporate resistant QTLs into breeding lines through marker-assisted selection (MAS) and to develop resistant cultivars with desirable horticultural traits including high yielding potential and early maturity. This paper will review the current understanding of causal agents of EB of tomato, resistance genetics and breeding, problems associated with breeding and future prospects.
Background: Exserohilum turcicum is an important pathogen of both sorghum and maize, causing sorghum leaf blight and northern corn leaf blight. Because the same pathogen can infect and cause major losses for two of the most important grain crops, it is an ideal pathosystem to study plant-pathogen evolution and investigate shared resistance mechanisms between the two plant species. To identify sorghum genes involved in the E. turcicum response, we conducted a genome-wide association study (GWAS). Results: Using the sorghum conversion panel evaluated across three environments, we identified a total of 216 significant markers. Based on physical linkage with the significant markers, we detected a total of 113 unique candidate genes, some with known roles in plant defense. Also, we compared maize genes known to play a role in resistance to E. turcicum with the association mapping results and found evidence of genes conferring resistance in both crops, providing evidence of shared resistance between maize and sorghum. Conclusions: Using a genetics approach, we identified shared genetic regions conferring resistance to E. turcicum in both maize and sorghum. We identified several promising candidate genes for resistance to leaf blight in sorghum, including genes related to R-gene mediated resistance. We present significant advancements in the understanding of host resistance to E. turcicum, which is crucial to reduce losses due to this important pathogen.
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.
Bacterial spot caused by Xanthomonas spp. is one of the most devastating diseases of tomato in North Carolina (NC). In total, 290 strains of Xanthomonas spp. from tomato in NC collected over 2 years (2015 and 2016) were analyzed for phenotypic and genetic diversity. In vitro copper and streptomycin sensitivity assays revealed that >95% (n = 290) of the strains were copper tolerant in both years, whereas 25% (n = 127) and 46% (n = 163) were streptomycin tolerant in 2016 and 2015, respectively. Using BOX repetitive element PCR assay, fingerprint patterns showed four haplotypes (H1, H2, H3, and H4) among the strains analyzed. The multiplex real-time quantitative PCR on a subset of representative strains (n = 45) targeting the highly conserved hrcN gene identified Xanthomonas strains from tomato in NC that belonged to X. perforans. Race profiling of the representative strains (n = 45) on tomato and pepper differentials confirmed that ∼9 and 91% of strains are tomato races T3 and T4, respectively. Additionally, PCR assays and sequence alignments confirmed that the copL, copA, copB (copLAB copper tolerance gene cluster), and avrXv4 genes are present in the strains analyzed. Phylogenetic and comparative sequence analyses of six genomic regions (elongation factor G [fusA], glyceraldehyde-3-phosphate dehydrogenase A [gapA], citrate synthase [gltA], gyrase subunit B [gyrB], ABC transporter sugar permease [lacF], and GTP binding protein [lepA]) suggested that 13 and 74% of X. perforans strains from NC were genetically similar to races T3 and T4 from Florida, respectively. Our results provide insights that bacterial spot management practices in tomato should focus on deploying resistance genes to combat emerging pathogenic races of X. perforans and overcome the challenges currently posed by intense use of copper-based bactericides.
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