The Distribution and Frequency of Virulence Genes in Geographically Separated Populations ofLeptosphaeria nodorum

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Abstract
Virulence frequency of 33 isolates of Leptosphaeria nodorum from eight countries were evaluated on 38 wheat and triticale cultivars. Isolate .times. cultivar interactions indicate that resistance and virulence have some specific effects. A computer program that estimates the number of genes, assuming a gene-for-gene relationship, was used to analyze the interacting matrix. The analysis hypothesized 21 different interacting genes in the 33-isolate .times. 38-cultivar matrix. The winter bread wheat, Red Chief, was assigned 19 hypothetical genes for resistance; cultivars Yamhill, 81UWWMN 2095, and MT 71-8-10 also were assigned large numbers of hypothetical genes. Isolates of L. nodorum from South America (Brazil, Chile, and Ecuador) expressed high relative virulence with those from Canadian and United States populations following. Among the United States isolates of L. nodorum, those form Louisiana and Florida exhibited the highest relative virulence. Some wheat and triticale accessions exhibited a low present necrosis in response to populations of both L. nodorum and Mycosphaerella graminicola that have wide virulence spectra. The cultivars most resistant to populations of the two pathogens were the winter hexaploid wheat cultivars JCR-979 (CI 16906), Red Chief (CI 12109), 81UWWMN 2095 (Maris Huntsman//VPM/Moisson), and triticale accession DU-75. A Bobwhite "S" CIMMYT line expressed the highest level of resistance of both pathogens among the spring hexaploid wheats. The implications of these findings on the deployment of germplasm and the accumulation of resistance to both pathogens is discussed.