Infectivity and sporulation potential of Phytophthora kernoviae to select North American native plants

Phytophthora kernoviae exhibits comparable epidemiology to Phytophthora ramorum in invaded UK woodlands. Because both pathogens have an overlapping geographic range in the UK and often concurrently invade the same site, it is speculated that P.similar to kernoviae may also invade North American (NA) forests threatened by P.similar to ramorum, the cause of Sudden Oak Death. This paper addresses the susceptibility of select NA plants to P.similar to kernoviae, including measures of disease incidence and severity on wounded and unwounded foliage. The potential for pathogen transmission and survival was investigated by assessing sporangia and oospore production in infected tissues. Detached leaves of Rhododendron macrophyllum, Rhododendron occidentale and Umbellularia californica, and excised roots of U.similar to californica and R.similar to occidentale were inoculated with P.similar to kernoviae and percent lesion area was determined after 6 days. Leaves were then surface sterilized and misted to stimulate sporulation and after 24 h sporangia production was assessed. The incidence of symptomless infections and sporulation were recorded. All NA native plants tested were susceptible to P.similar to kernoviae and supported sporangia production; roots of U.similar to californica and R.similar to occidentale were both susceptible to P.similar to kernoviae and supported sporangia production. Oospore production was also observed in U.similar to californica roots. The results highlight the vulnerability of select NA native plants to infection by P.similar to kernoviae, suggest that symptomless infections may thwart pathogen detection, and underscore the importance of implementing a proactive and adaptive biosecurity plan.