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Bacterial blight resistance gene B 5 has received little attention since it was first described in 1950. A near-isogenic line (NIL) of Gossypium hirsutum cotton, Ac B 5 , was generated in an otherwise bacterial-blight-susceptible 'Acala 44' background. The introgressed locus B 5 in Ac B 5 conferred strong and broad-spectrum resistance to bacterial blight. Segregation patterns of test crosses under Oklahoma field conditions indicated that Ac B 5 is likely homozygous for resistance at two loci with partial dominance gene action. In controlled-environment conditions, two of the four copies of B 5 were required for effective resistance. Contrary to expectations of gene-for-gene theory, Ac B 5 conferred high resistance toward isogenic strains of Xanthomonas citri subsp. malvacearum carrying cloned avirulence genes avrB4 , avrb7 , avrBIn , avrB101 , and avrB102 , respectively, and weaker resistance toward the strain carrying cloned avrb6 . The hypothesis that each B gene, in the absence of a polygenic complex, triggers sesquiterpenoid phytoalexin production was tested by measurement of cadalene and lacinilene phytoalexins during resistant responses in five NILs carrying different B genes, four other lines carrying multiple resistance genes, as well as susceptible Ac44E. Phytoalexin production was an obvious, but variable, response in all nine resistant lines. Ac B 5 accumulated an order of magnitude more of all four phytoalexins than any of the other resistant NILs. Its total levels were comparable to those detected in OK1.2, a highly resistant line that possesses several B genes in a polygenic background., Competing Interests: The author(s) declare no conflict of interest.

Cotton bacterial blight (CBB), caused by Xanthomonas citri pv. malvacearum , was a major disease of cotton in the United States in the early part of the twentieth century. The reemergence of CBB revealed many gaps in our understanding of this important disease. In this study, we employed a wild-type (WT) field isolate of X. citri pv. malvacearum from Georgia (U.S.A.) to generate a nonpathogenic hrcV mutant lacking a functional type-III secretion system (T3SS-). We tagged the WT and T3SS- strains with an auto-bioluminescent Tn 7 reporter and compared colonization patterns of CBB-susceptible and CBB-resistant cotton seedlings using macroscopic image analysis and bacterial load enumeration. WT and T3SS- X. citri pv. malvacearum strains colonized cotton cotyledons of CBB-resistant and CBB-susceptible cotton cultivars. However, X. citri pv. malvacearum populations were significantly higher in CBB-susceptible seedlings inoculated with the WT strain. Additionally, WT and T3SS- X. citri pv. malvacearum strains systemically colonized true leaves, although at different rates. Finally, we observed that seed-to-seedling transmission of X. citri pv. malvacearum may involve systemic spread through the vascular tissue of cotton plants. These findings yield novel insights into potential X. citri pv. malvacearum reservoirs for CBB outbreaks.