Typescript (photocopy).Single spore isolates of Colletotrichum graminicola were taken from sorghum or johnsongrass in agricultural, research station and natural populations and compared on a series of sorghum differential varieties in the greenhouse. The isolates taken from johnsongrass and monocultured sorghum hybrids were of simple virulence capability, i.e. avirulent on all but the most susceptible host type. Research station isolates were virulent on numerous sorghum genotypes and therefore had complex virulence capabilities. Populations of C. graminicola were analyzed on two sorghum nurseries in the field. One consisted of a series of genotypes that had differential responses to different races of the fungus. The other nursery consisted of 20 sorghum lines selected for apparent horizontal resistance. These nurseries were planted three years consecutively at research stations in Texas, Puerto Rico and Georgia. Significant variance of rank reaction across all sites showed that the geographically isolated populations have some specific virulence difference(s), confirming allelic divergence. Within sites and across years the rank analysis showed yearly non-significant shifts in virulence punctuated by significant shifts to new virulence patterns in years with propitious environments for disease development. Equal ranking across sites, as determined by non-significant ANOVA, was found in 10 of the 20 suspected horizontally resistant lines. Only five of these 10 lines had low area under disease progress curve values. Analyses of rates of vegetative growth and spore production of four single spore isolates were performed on detached leaf pieces. The effect of initial spore density on the growth rates was studied. Higher initial spore densities greatly reduced secondary spore production (per initial spore) relative to low initial spore density. The effect of competition between isolates was studied by determining rates of vegetative growth and spore production when the isolates were grown in mixtures as compared to when they were grown singly. Rate parameters generated by fitting logistic curves were compared by statistical analysis and the isolates with a significant effect of competition were used in an intraspecific competition model..
