Molecular Genetic Diversity and Variation for Aggressiveness in Populations of Fusarium graminearum and Fusarium culmorum Sampled from Wheat Fields in Different Countries

Fusarium graminearum and Fusarium culmorum are the major pathogenic organisms causing head blight in small-grain cereals. Natural epidemics may result in severe yield losses, reduction in quality, and contamin- ation of the grain by mycotoxins. The genetic diversity of four ®eld populations of F. graminearum from Germany, Hungary, and Canada, and one population of F. culmorum from Russia was investigated by polymerase chain reaction (PCR)-based ®ngerprinting. Additionally, a world-wide collection and two of the F. graminearum populations were analysed for their aggressiveness on young plants of winter rye in the greenhouse. The number of isolates analysed per popu- lation varied from 25 to 70. Signi®cant quantitative variation for aggressiveness was observed within each of the individual ®eld populations amounting to the same range as the world-wide collection. Abundant variation within populations was also revealed by DNA markers. The F. graminearum populations from Hungary and Winnipeg displayed the least genotypic diversity, the two German F. graminearum populations and the Russian F. culmorum population were highly diverse. Population diversity, however, followed no spatial pattern among samples within a German ®eld for aggressiveness or molecular markers. For F. graminearum, sexual recom- bination is the most likely explanation for the large genetic diversity within ®eld populations. Asexual and/ or parasexual recombination, and balancing selection caused by the periodic alternation between the sapro- phytic and parasitic phase might play an additional role and account for the variation within the F. culmorum population. For improving Fusarium resistance, several resistance genes of di€erent sources should be combined to avoid an unspeci®c adaptation of the genetically variable pathogen to an increased resistance level