Infection of tree stems by zoospores of Phytophthora ramorum and P. kernoviae

The invasive Phytophthora ramorum, P. kernoviae, and other aerial Phytophthoras are causing bleeding lesions on the trunks of mature trees, especially beech (Fagus sylvatica), in Cornwall, southwest England. The relationship between the results of host susceptibility tests using wound inoculation of excised logs in the laboratory and field observations in Cornwall has generally been good. However there have been some discrepancies. For example European sycamore (Acer pseudoplatanus) is moderately to highly susceptible to Phytophthora ramorum and P. kernoviae in the lab tests but is rarely susceptible in the field, although frequently exposed to natural inoculum. This suggests the possibility that host resistance is operating at two levels: at the bark surface-resistance to initial zoospore penetration; and in the phloem once the pathogen has gained entry. Little is known about the ability of zoospore inoculum to directly penetrate intact tree bark, although this is the presumed mode of entry of Phytophthoras above ground level. We are investigating this issue by examining the ability of zoospores to penetrate unwounded stems using both laboratory generated and natural zoospore inoculum. We summarize here research in progress.