SEASONAL-CHANGES IN NORTHERN RED OAK SUSCEPTIBILITY TO PHYTOPHTHORA-CINNAMOMI

Seasonal changes in susceptibility to ink disease, caused by Phytophthora cinnamomi, were evaluated in northern red oaks (Quercus rubra). At monthly intervals, direct trunk inoculations were performed in mature trees, and disease severity was assessed 1 mo later. Concurrently, bark strips were removed from the trunk of each tree, inoculated with P. cinnamomi, and incubated in standardized conditions. Analysis of variance showed that, both on unexcised and excised tissues, lesion development significantly changed depending on the date of inoculation. An effect of temperature on lesion development in situ was observed; no lesions were induced from December to February, and the greatest susceptibility of red oaks was observed in June. This seasonal pattern of susceptibility could not be accounted for simply by a climatic effect on the fungus development, because the lesion development in excised bark tissues was least from October to January and increased until May. Throughout the experiment, the relative water content (RWC) of the bark was measured. Collar bark tissues were more resistant to P. cinnamomi and more moist than tissues removed at a 2-m height. The RWC also varied during the year. These changes could not be related to climatological data, but the lesion linear extension in vitro was significantly related to this parameter, which could reflect physiological changes in the living bark. Analysis of variance also demonstrated tree-to-tree variability of red oak susceptibility. This could be at least partly explained by differences in tree phenology, because the correlation between phenology and bark susceptibility in early spring was significant and positive.