Roots of Norway spruce (Picea abies), at different stages of development, were infected by an S strain of Heterobasidion annosum for periods up to 20 days. Sectioned roots were studied using light and rransmission electron microscopy, whereas intact or cleaved roots were analysed using scanning electron microscopy. In nonsuberized roots all tissues were infected, and the stele was preferentially digested implying subsequent root abortion. In suberized and woody roots infection was restricted to the cortex and the rhytidome, respectively. This inferred that the endodermis and periderm(s) are natural barriers to infection. The formation of a necrose ring in the outer cortex of nonwoody roots was an induced resistance reaction, in order to delimit the infection. Other general resistance reactions, induced or enhanced in these roots, comprised secondary thickening of cell walls, papillae formation opposite to adjacent or penetrating hyphae, and accumulation of vacuolar phenolic deposits. Hyphae entrapped in these deposits showed signs of degeneration. The presence of light regions around such hyphae and around hyphae penetrating cell walls was an indication of fungal enzymatic activity. Analysis of suberized and woody roots after longer incubation periods than 20 days will provide further information on host/pathogen interactions and manifestation of resistance or susceptibility.
