Variation in virulence and pathogenic specialization of Colletotrichum gloeosporioides isolates from Stylosanthes scabra cvv. Fitzroy and Seca

Abstract

A survey of anthracnose of Stylosanthes scabra cvv. Fitzroy and Seca, caused by Colletotrichurn gloeosponoides, at 31 sites in Queensland found a wide range of disease severity. Disease was present at each of the 26 sites surveyed for Fitzroy, and inoculation experiments with an isolate from each site indicated that the isolates varied markedly in their virulence on Fitzroy. Some of the variation in disease severity between sites could be explained by differences In virulence of the isolates. Disease was observed In only one of the five fields of Seca surveyed. When nine of the isolates from Fitzroy were tested on SIX genotypes of S. scabra, there was no evidence of pathogenic specialization, and all isolates were avirulent on Seca. The isolate from Seca, when tested on the same genotypes, was moderately virulent on Seca and highly virulent on Fitzroy, indicating further pathogenic specialization within the Type A forms of C. gloeosporioides in Australia. There was a direct relationship between mean linear growth rate of isolates from Fitzroy in pure culture, and virulence on Fitzroy. The recovery of highly virulent Isolates from pods of a Fitzroy seed crop is discussed in relation to the rapid spread of highly virulent physiologic races.