VIRULENCE PATTERNS OF UROMYCES-APPENDICULATUS FROM DIFFERENT GEOGRAPHICAL AREAS AND IMPLICATIONS FOR FINDING DURABLE RESISTANCE TO RUST OF COMMON BEAN

Single-uredinium cultures of Uromyces appendiculatus (Pers) Unger from Honduras, Dominican Republic, Puerto Rico, Tanzania and USA were eval uated for virulence on 19 standard bean (Phaseolus vulgaris) rust diff erential lines/cultivars that had been previously selected on the basi s of resistance to bean rust. A disease rating scale, based on uredini um diameter, was used to study pathogenic variability of the bean rust fungus. Virulence of the cultures was measured by the vector of disea se score (mds) on all 19 differentials and the nearest centroid sortin g algorithm was used to cluster the cultures according to virulence si milarity. A total of 373 different virulence patterns was identified f rom more than 2000 cultures and were grouped into four virulence clust ers. None of the cultures produced a susceptible reaction on all the d ifferentials. Cultures that produced uredinia > 0.5 mm in diameter in 15 differentials were considered to have high virulence. About 59, 15, 20, 5 and 0%, respectively, of the Honduran, Dominican, Puerto Rican, USA and Tanzanian cultures were highly virulent. The lines/cultivars Ecuador 299, Mexico 235, 51051, NEP-2, Mexico 309, California Small Wh ite 643, A x S 37, and Aurora had a high degree of resistance to most cultures and can be used as sources of broad resistance. Virulence pat terns of the differentials indicated redundancy in some host parasite interactions and the need to modify the composition of the differentia l lines/cultivars.