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

Abstract

A study of the temporal and spatial relationships between the downy mildew, Peronosclerospora noblei and its host, Sorghum leiocladum (wild sorghum) has been undertaken. Hyphae of P. noblei perennate in the tiller bases of wild sorghum when the grass is dormant over the winter months. In early summer, hyphae grow into some tiller buds but not into others. Tillers that develop from invaded buds become systemically invaded by the hyphae, in most cases remain vegetative and bear the anamorph and teleomorph of the downy mildew. Tillers that develop from buds that are not invaded become short vegetative tillers or tall flowering tillers. On infected tillers symptom development is delayed until at least the third leaf, as a direct result of hyphal activity in the developing tiller. The location and activity of hyphae of P. noblei in tiller bases and developing tillers are responsible for the perennation of the pathogen and for the expression of symptoms in infected tillers of S. leiocladum. There may be similar relationships between hyphal location and activity and symptom development in other systemic pathogens of perennial grasses.