| Common bunt, caused by Tilletia caries and T. foetida, is one of the most devastating seedborne diseases of wheat. In conventional agriculture, the disease is managed almost exclusively with chemical seed treatments. However, synthetic chemicals are prohibited in organic agriculture, so growers must rely heavily on resistant cultivars to manage the disease. To facilitate breeding for resistance under organic systems, the resistance genes in 14 winter wheat cultivars were identified by inoculating these with 40 pathogenic races of T. caries. A resistance gene model was constructed based on their disease reaction to the races, and the reaction of 16 bunt differential cultivars. Eight cultivars were postulated to possess new bunt resistance genes or new gene combinations. To test this model, F2 populations of these cultivars were analyzed for goodness of fit to the expected segregation ratio. The F2 population of Lewjain x Elgin showed a 3:1 ratio (resistant: susceptible), indicating that Lewjain has a single major gene for bunt resistance. Wheat microsatellite or simple sequence repeats (SSR) markers were used to map this gene to its chromosomal location. A total of 763 SSR markers were screened on the parents, and 218 polymorphic SSR loci were screened on the mapping population. The marker wmc112, located on chromosome 2D, was linked with the resistance trait with a distance of 36.9 cM. This is an initial indication of the location of the bunt resistance gene in Lewjain. Madsen, Finch and Masami have resistance genes worth investigating further since these could be different from that of Lewjain. For effective deployment of resistance genes, it is imperative that races of the pathogen be identified and monitored, and the genetic diversity of T. caries examined. Towards this goal, races of the pathogen present in Washington, Oregon, California and Idaho were identified by inoculating 12 T. caries collections on 16 common bunt differential cultivars. Results indicate the presence in Washington of a new pathogenic race which has the broadest virulence spectrum compared to known races. Microsatellite loci were also isolated from T. caries , and 12 polymorphic microsatellite markers were developed for population genetics studies. |
