HOW DOES SEXUAL REPRODUCTION INFLUENCE POPULATION DYNAMICS OF WHEAT AND BARLEY POWDERY MlLDEW F.G. FELSENSTEIN EpiLogic GmbH & Institute of Agronomy and Plant Breeding, D - 85350 Freising-Weihenstephan Background and objectives. Local selection and pathogen migration by wind are commonly considered to be the main responsible factors for regional differentiation and population dynamics in virulence and fungicide sensitivity of cereal mildews. Sexual reproduction is often neglected. This mainly results from investigations in barley mildew. But, obvious differences between the two pathogens, noticeable by comparing the available data spp.[1], indicate that the sexual phase is of greater importance for wheat than for barley mildew. Results and conclusions Observations in the field: Throughout the year, the asexual reproduction of barley mildew has often a relatively continuous 'green bridge' of host plants available. Thus, the pathogen does not need to form cleistothecia in order to survive the summer and is therefore able to maintain and multiply specific pathotypes. For wheat mildew, however, the green bridge does not exist in such a complete form in most European regions. There is a lack of host plants for several weeks during summertime. Its survival has therefore to rely more on the formation of cleistothecia and ascospores. Variability and frequency of pathotypes, virulence associations: Wheat mildew shows a high variability within its regional populations, combined with an absence of dominating pathotypes. The observed frequencies of pathotypes are, in general, around the expected values. This shows the importance of genetic recombination within the annual reproduction cycle. In contrast, dominance of single pathotypes, apart from a large number of pathotypes with low frequencies, has often been reported for barley mildew, and positive or negative associations of specific virulence genes or between virulence and fungicide sensitivity have been described. Frequency of unnecessary virulence genes: Virulence frequency in a wheat mildew population is not determined by the occurrence of one or a small number of dominating pathotypes and the selective elimination of one specific pathotype has only little influence on that size. On the other hand, selected pathotypes quickly mate with the rest of the population, thus transferring necessary and unnecessary virulence genes to a large number of genotypes. In contrast, the predominant pathotypes of barley mildew may quickly loose their high proportion within the population via selection pressure. This often affects the frequency of unnecessary virulence genes, as the 'success' of a virulence gene is rather more a function of the genotype, in which it is present. Cleistothecia as an element conserving the local virulence patterns: A more intensive cleistothecia -phase of the wheat mildew during summer results in a high level of conservation in local virulence patterns. As cleistothecia are not subjected to dispersal by wind, they act as a kind of stationary stage of the fungus. This factor is reflected by the results on different migration dynamics of the two pathogens. A complete exchange of populations which has been described for barley mildew has never been detected for wheat mildew. Evolution of resistance towards DMIs: The polygenic control of resistance evolution towards DMI compounds (demethylation inhibitors) combined with a sizeable proportion of recombination of the fungus results in a stabilising factor concerning the resistance level of the pathogen population. The more resistant the pathogen becomes, the more an ideal combination of responsible genes is necessary. By regular recombination of genes responsible for DMI resistance, maintenance and uninhibited multiplication of phenotypes with relatively high resistance is not easily possible over the years. At some point this results in a kind of balance between the regional selection pressure and the factor of multigenic control of DMI resistance combined with sexual recombination. Due to the different importance of sexual reproduction in the two pathogens, mean resistance factors of wheat mildew populations towards DMI compounds are generally lower than those obtained for barley mildew. References Felsenstein FG, 1996. Proceedings Integrated Control of Cereal Midews and Rusts: Towards Co-ordination of Research across Europe, EUR 16884 EN, pp.91-96.