Location and activity of hyphae of the downy mildew, Peronosclerospora noblei (Family Peronosporaceae), and its relationship to symptom expression onwild sorghum (Sorghum leiocladum)

A study of the temporal and spatial relationships between the downy mildew, Peronosclerospora noblei and its host, Sorghum leiocladum (wild sorghum) h as been undertaken. Hyphae of P. noblei perennate in the tiller bases of wi ld sorghum when the grass is dormant over the winter months. In early summe r, hyphae grow into some tiller buds but not into others. Tillers that deve lop from invaded buds become systemically invaded by the hyphae, in most ca ses remain vegetative and bear the anamorph and teleomorph of the downy mil dew. Tillers that develop from buds that are not invaded become short veget ative tillers or tall flowering tillers. On infected tillers symptom develo pment is delayed until at least the third leaf, as a direct result of hypha l activity in the developing tiller. The location and activity of hyphae of P. noblei in tiller bases and developing tillers are responsible for the p erennation of the pathogen and for the expression of symptoms in infected t illers of S. leiocladum. There may be similar relationships between hyphal location and activity and symptom development in other systemic pathogens o f perennial grasses.