Etiology of Sclerophthora macrospora in St. Augustinegrass

Abstract

Studies were conducted to determine the causal agent, etiology, and control of the St. Augustinegrass downy mildew. Sporangial development, zoospore production, and size and morphology of asexual reproductive structures and host range demonstrate the causal organism of downy mildew of St. Augustinegrass is Sclerophthora macrospora (Sacc.) Thirum., Shaw and Naras. Grain sorghum (TX 2536) and maize (OH 545) inoculated with the St. Augustinegrass isolate of S. macrospora became infected and exhibited stunting and vegetative proliferation, although "crazy top" was not produced. "Crazy top" on St. Augustinegrass was observed infrequently. Oospores have not been observed in St. Augustinegrass, grain sorghum, or maize inoculated with the St. Augustinegrass isolate of S. macrospora. Histopathological examination of St. Augustinegrass leaf blades revealed extensively mycelial colonization of vascular bundles causing distortion of bundle sheaths, vascular elements, and mesophyll cells. In a newly initiated leaf, mycelial development was noted only in the leaf anterior to the collar. Sporangia were produced on downy mildew infected St. Augustinegrass over a temperature range of 5 to 25 C with an optimum at 15 C. Sporangia germinated by releasing 53 to 89 zoospores following dehiscence of the operculum. Optimum temperature range for zoospore production was determined to be 15 to 20 C with zoospore encystment and germination at 15 to 25 C. Zoospores demonstrated a positive chemotactic response to the abaxial area of newly expanding leaves. Zoospore infection occurred in a temperature range of 10 to 25 C with an optimum range at 15 C. ...