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  • Author or Editor: Gary E. Vallad x
  • Journal of the American Society for Horticultural Science x
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Bacterial spot of tomato (Solanum lycopersicum), caused by several Xanthomonas species, is one of the most important diseases of the crop in humid production regions of the world. Conventional breeding approaches for resistance to bacterial spot previously identified race-specific resistances, but current efforts also seek to use quantitative trait loci (QTLs) effecting broad-spectrum resistance. Resistance QTLs and candidate QTLs have been reported on several chromosomes, including a major QTL on chromosome 11. Fusarium wilt (Fusarium oxysporum f. sp. lycopersici) race 3 resistance gene, I-3, is associated with smaller fruit size and has been implicated in other associations with negative characteristics. We evaluated four F2 populations involving the bacterial spot-tolerant breeding lines Fla. 8517, Fla. 8233, and Fla. 8326 across two field seasons to validate and quantify previously identified loci and to test for an effect of I-3 on bacterial spot sensitivity. The chromosome 11 QTL and the I-3 locus were each consistently positively and negatively associated with resistance, respectively, and together explained from 44% to 47% of the variation in each population. The chromosome 11 QTL displayed a dominant to incompletely dominant effect, reducing infection by 14% to 25%. This QTL is distinct from the X. perforans race T3 hypersensitivity loci, Rx-4 and Xv3. The I-3 locus contributed to as much as a 20% increase in infection in I-3/I-3 plants vs. i-3/i-3 plants, and heterozygosity for I-3 generally resulted in an intermediate susceptible response. Significant effects for QTLs on chromosomes 3, 5, and 12 were also observed, but these effects were not consistent in all populations or seasons in which they were segregating. Implications of these findings toward breeding strategies are discussed.

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Tomato (Solanum lycopersicum) is an important vegetable crop and a valuable source of nutrients for the human diet. The southeast is the main fresh market tomato producer of the United States, with much of the production concentrated in Florida. However, production in this region is threatened by plant diseases such as target spot of tomato (TS) caused by Corynespora cassiicola, a multitrophic fungus widely distributed in tropical and subtropical areas. TS can infect foliage and fruit, often resulting in significant yield losses in conductive environments. There are no known TS-resistant cultivars, and control relies entirely on fungicidal sprays. However, several studies have demonstrated that the fungus is developing resistance to commonly used fungicides which further complicates disease management. The objective of this work was to identify sources of resistance to TS from wild Solanum accessions. Initial screens of 83 accessions informed the selection of 24 accessions for a more robust screening in which six diverse C. cassiicola isolates were used for single-isolate inoculation experiments. The results from a broad-sense mixed-model analysis including data from all six experiments demonstrated that all 24 accessions had significantly lower disease severities compared with the susceptible controls, suggesting that all accessions potentially harbor resistance quantitative trait loci (QTLs). Solanum cheesmaniae accession LA0524, S. galapagense accessions LA0483 and LA0532, and S. pimpinellifolium accession LA2093 were among the most resistant accessions tested and may be particularly useful for introgression of resistance into cultivated germplasm and for mapping of TS resistance QTLs.

Open Access