Inhibition of Blumeria graminis germination and germling development within colonies of oat mildew

Germination by Blumeria graminis DC Speer ff. spp. avenae, hordei andtritici , was greatly suppressed when conidia fell within colonies of ff. spp. avenae or hordei established on susceptible oat or barley, respectively. On healthy oat or barley, and when distant from powdery mildew colonies, all ff. spp. formed normal appressoria. This was also true when conidia germinated within established barley mildew colonies. Within barley mildew colonies, appressoria of f. sp. hordei penetrated epidermal cells (formed haustoria) more frequently than appressoria distant from colonies. Similarly, ff. spp. avenae and tritici, normally unable to infect barley, frequently penetrated epidermal cells subtending established barley mildew colonies. Thus, colony establishment induced barley epidermal cell accessibility, even to non-pathogenic ff. spp. In contrast, when all three ff. spp. germinated within established oat mildew colonies, most formed abnormal, hypha-like germ tubes. Since they did not form appressoria, and were thus unable to attempt penetration, it was impossible to determine whether oat mildew colonies induced accessibility of underlying oat epidermal cells. However, when superficial structures of established colonies were removed, germlings of all ff. spp. formed appressoria freely on cells containing oat mildew colony haustoria. Furthermore, these cells showed high level induced accessibility not only to f. sp. avenae but also to the normally non-pathogenic ff. spp. This indicated that factors disrupting germination and further development by conidia lying within oat mildew colonies were produced from the superficial colony structures and not by haustorium-containing plant cells. The factors appear to have limited mobility.