Interactions between Norway spruce (Picea abies) and Heterobasidion annosum. I. Infection of nonsuberized and young suberized roots

Abstract

Nonsuberized and young suberized roots of Norway spruce (Picea abies) were inoculated with an S-strain of Heterobasidion annosum for 2–20 days. The roots were sectioned for light microscopy and transmission electron microscopy. They were susceptible to infection at both stages of development. The host reaction to delimit the infection was the formation of a necrotic ring barrier in the outer cortex. In cases where the inner cortex also became infected, fungal hyphae accumulated just before the endodermis, which acted as a new barrier. Only in nonsuberized roots did the hyphae succeed in penetrating the stele, and within 3 days after inoculation the stele was almost completely digested. Other resistance reactions included accumulation of phenolic deposits, secondary thickening of cell walls, and formation of papillae. The hyphae were able to grow within and across cell walls. The presence of translucent areas around penetrating hyphae possibly reflected the digestive action of fungal enzymes. The hyphae showed signs of degeneration when entrapped in encompassing cellular deposits. Key words: Norway spruce, Heterobasidion annosum, root infection, nonsuberized roots, young suberized roots, microscopy.