Non-suberized seedling roots of Norway spruce (Picea abies) were challenged in vitro under sterile conditions with oidiospores from the saprotrophic basidiomycete Phlebiopsis gigantea. The root surface supported spore germination and mycelial growth. Samples were taken at defined time intervals for histochemical. SEM and TEM observations. Infective structures developed 24-48 h post inoculation (p.i). Appressoria appeared to be induced by ridges or groove-like structures on the root surface and by simlar structures created by mycelial mats. Penetration into epidermal cells occurred 3 d p.i. with cortical and meristematic colonization observed 4-5 d p.i. Browning reactions of the root surface occurred within 5 d p.i. at high spore concentrations but were delayed at lower inoculum concentrations. The inoculum threshold for penetration of epidermal cells and coloization within 5 d p.i. was 10 hoch 4 spores ml-Ü. Invasion and disruption of root vascular regions were not observed with any inoculum. At the TEM level, tissue colonization was prismarily intercellular by cell to cell hyphal development within middle lamellae. The presence of a translucent layer ahead of fungal hyphae during cell wall penetration was probably caused by the action of hydrolytic enzymes secreted by the invading fungi. Host responses were mostly marked by encasing of hyphae within an electron dense layer, formation zones and retraction of the plasmalemma of cortical cells. However, the majority of inoculated seedling roots examined were not rotted in contrast to spruce roots inoculated with a common pathogenic basidiomycete (Heterobasidion annosum).
