VIEW ARTICLE

Etiology

Germination of Oospores of Pythium ultimum in the Cotton Rhizosphere. L. F. Johnson, Professor, Department of Entomology and Plant Pathology, University of Tennessee, Knoxville 37996; Teresa Arroyo, professor, Instituto Tecnologico de Costa Rica, San Carlos, Costa Rica. Phytopathology 73:1620-1624. Accepted for publication 15 June 1983. Copyright 1983 American Phytopathological Society. DOI: 10.1094/Phyto-73-1620.

Microscope slides were dipped in agar suspensions of oospores of Pythium ultimum. Slides with hardened agar films with oospores were placed in soil in small plastic pots. Cotton seeds were planted adjacent to the slides, which were positioned so that the roots would grow down and across the agar film. After various incubation periods, soil was removed from the slides and those with small root segments adhering to the agar film were examined microscopically. Decrease in oospore wall thickness was observed after 12 hr in both rhizosphere and nonrhizosphere soil. After 48 hr, 30% of the oospores had thin walls, and after 45 days 85% had thin walls. Rate of conversion to thin walls was similar in sterilized and nonsterilized soil. Walls of 11- or 13-wk-old oospores were converted to thin ones faster than 5- or 9-wk-old oospores. Oospores germinated in the rhizosphere of both primary and secondary roots. Germinated oospores were not found at distances greater than 1.5 mm from the root surface. All germ tubes observed were oriented toward the root. Germination occurred mostly in the root hair zone, but occasionally oospores germinated near root tips. Germ tubes normally made contact with the root epidermis between root hairs or at the bases of root hairs. Rarely were contact and penetration of root hairs observed. Abnormal growth of some germ tubes prior to emergence through the oogonial wall was observed.