Effect of early infection on pathotype frequencies in barley powdery mildew (Blumeria graminis f.sp hordei) populations in field plots

A field experiment with barley powdery mildew (Blumeria graminis f.sp. hord ei) was designed in order to study how the time of arrival of inoculum in t he field influenced pathotype frequencies in the resulting populations. Thr ee isolates belonging to pathotypes that were absent or rare in the local a erial inoculum were used to inoculate field plots of winter barley cv. Plai sant. Two successive inoculations with different combinations of the three isolates were performed with an approximately two-generation delay, and fre quencies of inoculated pathotypes were assessed four and nine generations a fter the first inoculation. Pathotypes of the first inoculated isolates gen erally persisted throughout the period of sampling; this is described as an 'early arrival' effect. During the epidemics the inoculated isolates were not replaced by isolates from the natural airborne inoculum. Pathotype freq uencies depended mainly on the time of arrival of inoculum in the plot, but frequencies also depended on the isolate that had been inoculated. The mos t frequent isolate, GL1, belonged to the clonal lineage dominant in powdery mildew populations on winter barley in the north of France. These results confirmed that the composition of a powdery mildew population in a field is largely determined by the composition of the initial inoculum.