SPATIAL AGGREGATION OF PATHOTYPES OF BARLEY POWDERY MILDEW

Aggregation in the distribution of pathotypes of Erysiphe graminis f.s p. hordei, the barley powdery mildew pathogen, was investigated in fie ld plots of 'Golden Promise', 'Proctor' and 'Tyra'. 'Golden Promise' a nd 'Proctor' have no effective mildew resistance alleles, whereas 'Tyr a' has Mla1, which was only effective against a proportion of the mild ew population. Isolates of mildew were sampled according to a grid sam pling scheme and their virulence spectra ascertained in order to group them according to pathotype. The populations were very diverse, and e vidence for aggregation (quantified using join counts) was found only in the 'Tyra' plots, at distances of up to 1m. This aggregation was re duced in a subsequent sample. The results are consistent with a model in which mildew epidemics are started by a large number of initial inf ections, which then form diffuse, overlapping aggregations of clones. These aggregations then become more diffuse, so that the amount of agg regation reduces with time. The greater amount of aggregation seen in the 'Tyra' plots might have been caused by there being less initial in oculum with virulence towards that cultivar.