VIEW ARTICLE

Ecology and Epidemiology

Germination of Oospores of Phytophthora megasperma f. sp. glycinea in the Presence of Soil. Benjamin Jimenez, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824, Present address of senior author: Escuela de Agronomia, Universidad Centro Occidental, Barquisimeto, Estado Lara, Venezuela; J. L. Lockwood, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Phytopathology 72:662-666. Accepted for publication 15 September 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-662.

Oospores of Phytophthora megasperma f. sp. glycinea were produced in cultures grown 4–6 wk in darkness. Oospores were separated from mycelium by grinding for 10–20 min in a Sorvall Omni-Mixer at 5 C, collecting the spores on a 74-μm-mesh sieve, and washing them by 2–4 brief (15 sec each) centrifugations at 325 g. Residual mycelium was killed either with the enzyme complex beta-glucuronidase/aryl sulfatase, or by freezing at –10 C. The oospores were germinated in flooded soil smears, soybean root exudates, or other substrates. Germination in natural or autoclaved soil was high (50–60%) compared with that in deionized water (~10%). Sporangia produced in the presence of unsterilized field soil seldom germinated or released zoospores. Light stimulated germination of oospores incubated in the presence of soil. However, germination of dark-grown oospores under dark conditions was also substantial (20–25%). Light was inhibitory to sporangium production, but soybean seedlings reversed the inhibition. The optimum termperature for activation (thinning of oospore wall), germ tube production, and sporangium production was 20–24 C. Soybean seedlings and other plant tissues stimulated rapid and high percent germination of oospores incubated in soil extract in either light or darkness. Oospore germination was inhibited by glucose (0.5 mg/ml) and by a concentrated extract of sterile soil, but not by that of unsterile field soil.