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

Seasonal changes in susceptibility to ink disease, caused by Phytophth ora 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, ba rk strips were removed from the trunk of each tree, inoculated with P. cinnamomi, and incubated in standardized conditions. Analysis of vari ance showed that, both on unexcised and excised tissues, lesion develo pment significantly changed depending on the date of inoculation. An e ffect of temperature on lesion development in situ was observed; no le sions were induced from December to February, and the greatest suscept ibility of red oaks was observed in June. This seasonal pattern of sus ceptibility 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. Th roughout 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 vari ed during the year. These changes could not be related to climatologic al data, but the lesion linear extension in vitro was significantly re lated to this parameter, which could reflect physiological changes in the living bark. Analysis of variance also demonstrated tree-to-tree v ariability of red oak susceptibility. This could be at least partly ex plained by differences in tree phenology, because the correlation betw een phenology and bark susceptibility in early spring was significant and positive.