Observed and predicted changes over eight years in frequency of barley powdery mildew avirulent to spring barley in France and Denmark

Aerial populations of Blumeria graminis f.sp. hordei were studied in two French and two Danish regions from 1991 to 1999, at a time of year when only winter barley was present. A high frequency of genotypes not able to grow on the spring-sown crop of the previous growing season (denoted 'spring-avirulent') was observed in most years and regions. This frequency increased with increasing proportion of winter barley; it was highest in France and decreased in general over the 8-year period. Most of the spring-avirulent genotypes possessed the V _{a 22} virulence gene, matching a resistance that has never been present in barley cultivars grown in Europe. A hypothetical cropping system, including winter- and spring-sown crops with three resistance genes altogether, was constructed to mimic the utilization of host cultivars in the four regions. Results from a mathematical model simulating changes in the composition of the pathogen population in this system, demonstrated that selection solely due to host resistance genes, i.e. without assuming any cost of virulence, might lead to such results as those observed. The changes in frequency of spring-avirulent genotypes and the frequency of unnecessary virulence genes may be predicted from the proportion of the barley area sown with winter barley, the use of resistance genes in the cultivars, the initial composition of the pathogen population, and hitch-hiking due to gametic disequilibria.