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

Germination by Blumeria graminis. DC Speer ff. spp. avenae, hordei and trit ici, was greatly suppressed when conidia fell within colonies of ff. spp. a venae 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 germina ted within established barley mildew colonies. Within barley mildew colonie s, 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 penetrat ed epidermal cells subtending established barley mildew colonies. Thus, col ony, 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 tub es. Since they did not form appressoria, and ere thus Unable to attempt pen etration, it was impossible to determine whether oat mildew colonies induce d accessibility of underlying oat epidermal cells. However, when superficia l structures of established colonies were removed, germlings of all ff. spp . formed appressoria freely,, on cells containing oat mildew colony haustor ia. Furthermore, these cells showed high level induced accessibility not on ly to f. sp. avenae but also to the normally non-pathogenic ff, spp. This i ndicated that factors disrupting germination and further development by con idia lying within oat mildew colonies were produced from the Superficial co lony structures and not by haustorium containing plant cells. The factors a ppear to have limited mobility. (C) 2001 Academic Press.