Races ofPhytophthora megaspermaf. sp.glycineain Wisconsin

Abstract

Studies were undertaken to determine the range of physiologic races of P. megasperma f. sp. glycinea in Wisconsin in 1978 and 1980 and to investigate the relationship between race frequency and cultivar selection. In 1978, race 3 was isolated at the highest frequency, whereas races 1, 4, 7, 8 and 9 were isolated less frequently. The use of virulence formulae (effective/ineffective host [soybean] genes) was necessary in 1980 to describe the virulence of the isolates because the variation encountered could not be described under the current numeric classification scheme. Previously undescribed races were found more frequently than were races 3, 9, 1, 5 and 8 and were detected in decreasing frequency on cultivars containing race-specific resistance conferred by the Rps1c allele, the Rps1 allele, and those with no race-specific resistance, respectively. The results suggest that selection pressure exerted by cultivars containing race-specific resistance has resulted in increased virulence in the pathogen population and cast doubt on the long-term effectiveness of race-specific resistance.