INTERACTIONS BETWEEN NORWAY SPRUCE (PICEA-ABIES) AND HETEROBASIDION-ANNOSUM .2. INFECTION OF WOODY ROOTS

Woody roots, 2-4 mm in diameter, of Norway spruce (Picea abies) were i noculated with an S strain of Heterobasidion annosum. After 8-20 days the roots were prepared for examination by light microscopy as well as transmission and scanning electron microscopy. The roots had one or s everal periderms and sometimes remnant cortex cells on the surface. Th e fungal infection was restricted to the remnant cortex cells and the rhytidome after an incubation period of 20 days. Accumulation of granu lar materials, most likely phenolic in nature, prevailed in the infect ed periderm cells. Fungal hyphae enclosed in these materials showed si gns of degeneration. Based on these results, we conclude that the rhyt idome acts as a successful barrier to infection of the inner parts of the root for at least 20 days following inoculation with H. annosum. B oth structural and chemical defence mechanisms are involved.