Effects of physical and chemical factors on the germination of oospores of Phytophthora capsici in vitro

Oospores were produced by crossing two isolates of opposite mating type of Phytophthora capsici obtained from pepper on clarified V8 agar. Cultures were incubated in the dark at 24 C for 2 mo. Germination of oospores produced in the dark at 24 C was reduced by 78 or 90% when cultures were exposed to light (cool-white fluorescent, 40 micromole m(-2)s(-1) or incubated at 30 C for 1 wk before oospore collection and germination. Exposure of oospores to light during germination did not significantly affect the percentage of oospores that germinated, regardless of pregermination conditions. Germination of oospores in the dark was reduced by exposure to continuous light during oospore formation. Germination of oospores in sterile distilled water or soil extract showed a quadratic response to temperature, with maximum germination at 24 C. The mode and rate of germination of oospores was affected by incubation media. The predominant mode of oospore germination was via formation of sporangia. After 5 days, sporangia developed on germ tubes from approximately 94% of the oospores that germinated in soil extract, whereas sporangia developed on 33 or 8% of the oospores that germinated in root extract or sterile distilled water, respectively. Oospores incubated in root extract and distilled water formed germ tubes that continued to elongate for several days before sporangia were formed. However, after 12 days, sporangia had formed on germ tubes from 82, 78, or 94% of the oospores that germinated in distilled water, root extract, or soil extract, respectively. The total percentage of oospores that germinated after 12 days was 29, 39, or 36% in distilled water, root extract, or soil extract, respectively. Treatment of oospores with Novozym effectively removed sporangia and mycelial fragments from suspensions, but it either increased or did not affect germination in two experiments, and decreased germination in two additional experiments. The total percentage of oospores that germinated differed among experiments using oospores of similar age produced under identical conditions. Germination was greatest (51%) when oospores were produced in the dark and germinated in soil extract at 24 C in the dark for 12 days. Oospores germinated predominantly by production of sporangia in all experiments.