Rsv3 is one of three genetic loci conferring strain-specific resistance to Soybean mosaic virus (SMV). The Rsv3 locus has been mapped to a 154-kb region on chromosome 14, containing a cluster of five nucleotide-binding leucine-rich repeat (NB-LRR) resistance genes. High sequence similarity between the Rsv3 candidate genes challenges fine mapping of the locus. Among the five, Glyma14g38533 showed the highest transcript abundance in 1 to 3 h of SMV-G7 inoculation. Comparative sequence analyses were conducted with the five Rsv3 candidate NB-LRR genes from susceptible (rsv-type) soybean [Glycine max (L.) Merr.] cultivar Williams 82, resistant (Rsv3-type) cultivar Hwangkeum, and resistant lines L29 and RRR. Sequence comparisons revealed that Glyma14g38533 had far more polymorphisms than the other candidate genes. Interestingly, Glyma14g38533 gene from Rsv3-type lines exhibited 150 single-nucleotide polymorphism (SNP and six insertion-deletion (InDel) markers relative to rsv-type line, Furthermore, the polymorphisms identified in three Rsv3-type lines were highly conserved. Several polymorphisms were validated in 18 Rsv3-type resistant and six rsv-type susceptible lines and were found associated with their disease response. The majority of the polymorphisms were located in LRR domain encoding region, which is involved in pathogen recognition via protein-protein interactions. These findings associating Glyma14g38533 with Rsv3-type resistance to SMV suggest it is the most likely candidate gene for Rsv3.

In this study, four recombinant inbred line (RIL) soybean populations were screened for their response to infection by Pythium sylvaticum, Pythium irregulare, Pythium oopapillum, and Pythium torulosum. The parents, PI 424237A, PI 424237B, PI 408097, and PI 408029, had higher levels of resistance to these species in a preliminary screening and were crossed with “Williams,” a susceptible cultivar. A modified seed rot assay was used to evaluate RIL populations for their response to specific Pythium species selected for a particular population based on preliminary screenings. Over 2500 single-nucleotide polymorphism (SNP) markers were used to construct chromosomal maps to identify regions associated with resistance to Pythium species. Several minor and large effect quantitative disease resistance loci (QDRL) were identified including one large effect QDRL on chromosome 8 in the population of PI 408097 × Williams. It was identified by two different disease reaction traits in P. sylvaticum, P. irregulare, and P. torulosum. Another large effect QDRL was identified on chromosome 6 in the population of PI 408029 × Williams, and conferred resistance to P. sylvaticum and P. irregulare. These large effect QDRL will contribute toward the development of improved soybean cultivars with higher levels of resistance to these common soil-borne pathogens.

Widespread use of resistance to race 0 of Phytophthora nicotianae in flue-cured tobacco (Nicotiana tabacum) has increased problems with race 1 in commercial fields. The RAPD marker UBC30, tightly linked to the Ph gene for resistance to race 0, was used to clarify the presence of the Ph gene in specific cultivars to enable farmers to more appropriately match cultivar resistance to the pathogen races predominating in their fields. The marker UBC30 was present in 20 of the 31 flue-cured tobacco cultivars tested, including CC 27, GL 350, NC 196, SP 220, SP 225, SP 227, and NC 810. These cultivars were previously thought to not possess the Ph gene. Presence of UBC30 was highly correlated (r = 0.93; P ≤ 0.001) with survival in fields infested primarily with race 0, and with greater survival in fields infested primarily with race 0 versus race 1 of the pathogen (r = 0.76; P ≤ 0.001). The likely presence of the Ph gene in so many currently grown flue-cured tobacco cultivars may limit farmers' ability to shift pathogen populations back to race 0 from race 1 via the recommended cultivar rotation strategy. Accepted for publication 27 February 2008. Published 18 June 2008.

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