Phytophthora kernoviae oospore maturity, germination, and infection

Limited information is known on the basic biology of the recently described Phytophthora kernoviae that produces homothallic oospores. In this study, different P. kernoviae isolates were used to investigate oospore maturity, germination, and infection. All isolates produced oospores in V8 broth at 20 °C in the dark by 6 d. Oospores also formed at 10 and 15 °C, but did not form at 25 and 28 °C. Continuous light inhibited oospore production of some isolates but had no negative effect on others. Maturation time of the oospores, as noted by germination and staining with tetrazolium bromide, was not much different among the isolates between 2 and 14 weeks. Oospore germination was optimal at 18 and 20 °C, and did not occur at 5, 25, and 30 °C. Oospore germination under continuous light was higher than in the dark, but individual isolates showed variable results. Rhododendron leaf disks inoculated with oospores and maintained in the dark at 20 °C were necrotic after 1 week, while those kept under continuous light did not develop necrosis. The percentage of leaf disks infected with P. kernoviae was lower in the leaves exposed to continuous light (40 %) compared to those kept in the dark (100 %).