VIEW ARTICLE

Ecology and Epidemiology

Soil Matric Potential Effects on Changes in Wall Morphology, Germination, and Lysis of Oospores of Pythium ultimum. L. F. Johnson, Professor emeritus, Department of Entomology and Plant Pathology, University of Tennessee, Knoxville 37901-1071; P. Qian(2), and R. S. Ferriss(3). (2)Research associate, Department of Entomology and Plant Pathology, University of Tennessee, Knoxville 37901-1071; (3)Associate professor, Department of Plant Pathology, University of Kentucky, Lexington 40546-0091. Phytopathology 80:1357-1361. Accepted for publication 10 July 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-1357.

Thick-walled oospores of Pythium ultimum in agar films on glass slides were incubated in soil for 84 days. In soil with matric potentials of ?0.03 to ?0.30 MPa, almost all of the thick-walled oospores converted to thin-walled oospores, and 91% of these lysed. In soil near saturation (0 MPa) and in much dryer soil (?1.50 MPa), 82 and 62% of the oospores remained thick-walled, respectively. Occasionally, oospores with attached germ tubes were observed but not in soil with matric potentials of 0, ?0.7, or ?1.5 MPa. Oospores were germinated in corn meal agar films on glass slides and then were killed by fumigation with propylene oxide, aerated, and placed in soil. After 16 days of incubation, 98% of the germ tubes had disintegrated and were no longer visible. Based on rate of visual disappearance in soil of germ tubes killed with propylene oxide, calculations were made for the quantity of oospores buried in soil at different moisture levels that lysed directly and for those that germinated before lysis. As an average for soil with moisture levels of ?0.03 to ?0.30 MPa and soil with fluctuating moisture levels, it was calculated that 89.5% of the oospores lysed directly and 1.5% germinated.