Genetic diversity ofPuccinia kuehnii, the causal agent of orange rust of sugarcane, from Brazil

摘要

The use of resistant genotypes is the preferred method to control orange rust of sugarcane (Saccharumspp) caused byPuccinia kuehnii.This approach has been adopted in Brazil but outbreaks of the disease on previously resistant varieties showed that the efficacy of this method is limited and requires a better understanding of pathogen diversity. Nevertheless, adequate molecular markers for examining pathogen diversity at population level are not available, which limits the success of orange rust control by genetic resistance. Therefore, two independent investigations were conducted to examine genetic diversity ofP. kuehniifrom Sao Paulo state, the most important sugarcane growing state of Brazil. First, simple-sequence repeat (SSR) markers were developed in the present work and genotypic diversity of orange rust isolates from different locations investigated. Second, phenotypic diversity was examined by the single-pustule inoculation technique onP. kuehniiisolates retrieved from three susceptible commercial sugarcane cultivars. A total of 96 SSR markers were generated and tested for this species. Subsequently, 29 isolates ofP. kuehniiwere fingerprinted with nine SSR markers to estimate the genotypic diversity by neighbour-joining and 3D principal coordinates. The 29 isolates of the pathogen clustered into four main groups, which were identified by three SSR markers (NPRL_PK_108a, NPRL_PK_162_spka and NPRL_PK_221_spka). Phenotypic data at 21 days after the single-pustule inoculation showed thatP. kuehniifrom highly susceptible commercial cultivars harboured a small proportion of variants capable of causing disease on resistant cultivars. A differential reaction was demonstrated for the most virulent variant in a repeated experiment confirming the existence of races withinP. kuehniiin Brazil.