Abstract
Woody roots, 2 – 4 mm in diameter, of Norway spruce (Picea abies) were inoculated 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 several periderms and sometimes remnant cortex cells on the surface. The fungal infection was restricted to the remnant cortex cells and the rhytidome after an incubation period of 20 days. Accumulation of granular materials, most likely phenolic in nature, prevailed in the infected periderm cells. Fungal hyphae enclosed in these materials showed signs of degeneration. Based on these results, we conclude that the rhytidome acts as a successful barrier to infection of the inner parts of the root for at least 20 days following inoculation with H. annosum. Both structural and chemical defence mechanisms are involved. Key words: Norway spruce, Heterobasidion annosum, root infection, woody roots, microscopy.
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