Effect of pH on germination and infection by Phytophthora kernoviae oospores

The pathogen Phytophthora kernoviae has a wide host range and produces large numbers of oospores, which are resistant to adverse conditions and can be easily transported to new locations in plants or soil debris. To better understand the conditions under which P. kernoviae oospores will germinate, oospores were incubated in solutions over a range of pH levels (3.6, 5.4, 6.9, 9.3, 11.9). Germination differed by pH (P < 0.0001), with the lowest germination (20%) occurring at pH 3.6. Consistently high germination (80.8 to 84.3%) was observed at pH 6.8, 9.3, and 11.9. To determine the effect of soil type on infection and detection of P. kernoviae, three different soils (pH 4.2, 5.2, and 7.4) were infested with oospores (1, 10, 100, and 1000 oospores / cc soil) and baited with Rhododendron ponticum leaves. Disease development, as measured by proportion of symptomatic leaf baits, differed based on soil under inoculum concentrations of 1, 100, and 1000 oospores / cc soil (P < 0.05). No leaf disks became infected in the acidic soil (pH 4.2) at any of the inoculum concentrations tested. Infection in the slightly alkaline soil (pH 7.4) ranged from 0 to 89.2% across the inoculum concentrations, and 3.3 to 100% in the pH 5.2 soil. However, when calcium carbonate was added to the acidic soil, increasing the pH to 6.1, 90% of the leaf discs became infected, showing that pH alone can suppress infection. This study provides useful information on the biology of P. kernoviae oospores, which can play an important role in the long-distance spread of this pathogen. The suppression of germination and infection under acidic conditions will determine which habitats the pathogen is most likely to colonize and could lead to the development of treatments for oospore deactivation.