Effects of temperature on germination of sporangia, infection and protein secretion by Phytophthora kernoviae

Phytophthora kernoviae is a pathogen on a wide range of plants, but little is known of optimal infection conditions. Rhododendron ponticum leaves were inoculated with six different isolates of P. kernoviae sporangia and incubated at different temperatures from 10 to 28 °C. After 1 week, lesion development and pathogen recovery were only observed from all isolates at 15 and 20 °C and a few isolates at 10 °C. In an experiment with temperatures ranging from 20 to 25 °C, lesion development and pathogen recovery on R. ponticum , Magnolia stellata and Viburnum tinus occurred consistently at 20 and 21 °C, was limited at 22 °C, and did not occur at 23 °C and above. There was no difference in sporangia and zoospore germination at 20–25 °C. In a temperature fluctuation experiment, the necrotic area of inoculated R. ponticum leaves increased with longer incubation at 20 °C and decreased with longer incubation at 24 °C. Crude extracts of secreted proteins from P. kernoviae cultures grown at 20 and 24 °C were compared to determine any effects of temperature on pathogenicity. When spot tested on R. ponticum leaves, crude protein suspensions from cultures grown at 20 °C induced necrosis, while proteins from cultures grown at 24 °C did not. Proteomic analysis confirmed that a 10 kDa protein secreted at both 20 and 24 °C shared sequence homology to the conserved domains of known elicitins of other Phytophthora spp. The protein secreted at 20 °C that was responsible for necrosis has not been identified.