Genetic variation of aggressiveness in individual field populations of Fusarium graminearum and Fusarium culmorum tested on young plants of winter rye

Fusarium graminearum and F. culmorum are capable of infecting winter cereals at all growth stages. From natural field epidemics of wheat head blight and rye foot rot, three fungal populations were collected with 21, 38 and 54 isolates, respectively; their aggressiveness was analyzed in comparison to collections of F, graminearum (25 isolates) and F culmorum (70 isolates) that represent a wide range of geographical locations and host species. All isolates were tested for aggressiveness on young plants of winter rye in the greenhouse and scored for disease severity on a 1-9 scale. Disease ratings of individual isolates ranged from 1.5 to 5.7 indicating quantitative variation of aggressiveness. Genotypic variance was highest in the two Fusarium collections. No substantial difference was found in the amount of genotypic variation between F, graminearum and F culmorum. Individual field populations revealed 57-66% of the total genotypic variation of the collections. This implies a high degree of diversity of aggressiveness within single field populations of F. graminearum and F. culmorum causing natural epidemics.