Pathotypic variation of Phakopsora pachyrhizi isolates from Uruguay

Abstract

Soybean rust, caused by Phakopsora pachyrhizi, is one of the most important foliar diseases threatening soybean production in Uruguay. Knowledge of pathogenic variation among Uruguayan rust populations is necessary to guide development of resistant soybean cultivars in national breeding programs. To assess pathogenic variation,12 P. pachyrhizi isolates were collected from fields across the country over a four-year period and were inoculated onto a set of 12 differential soybean genotypes. All Uruguayan rust isolates (URPs) were highly virulent on differential soybean plants carrying resistance genes Rpp1, Rpp3, and Rpp4. Conversely, all isolates showed resistant reactions on soybean differentials with Rpp1-b and on a line carrying the genes Rpp2, Rpp4, and a Rpp5 allele. The pathogenic variation of the URPs was compared to that of a collection of 157 P. pachyrhizi isolates from other Latin American countries and Japan. All URPs clustered together and with other fungal isolates from South America. Of the seven different pathotypes that were found, four shared identical virulence patterns with isolates from South America, and three were associated with unique virulence patterns, which mainly resulted from the reactions they caused on plants carrying Rpp3, Rpp4 and especially Rpp6. The results indicate that the URPs can overcome the resistance of a larger number of Rpp genes than P. pachyrhizi isolates from neighboring countries. The resistant genes Rpp1-b and Rpp5 proved to be effective against the pathogen and will be introgressed into local soybean lines.