Early stage development of IS-group isolates of Heterobasidion annosum on Abies alba roots - scanning electron microscopical studies

The growth of hyphae and prepenetration phenomena on Abies alba roots after inoculation with the P-, S- and F-group isolates of Heterobasidion annosum were observed using scanning electron microscope. Elongated hyphae emanating from the inocula grew indiscriminately across and along root tips and entered the cortical cells randomly, while in subapical root areas they quite often grew along grooves at points of cell junctions. The ridges, folds and depressions seen on the root surface obviously directed the hyphal growth and their further entrance into roots through natural crevices or cracks. Although hyphae of all the isolates could penetrate the roots directly through small openings, the hyphae of the F isolate penetrated preferably more eroded and older parts of roots. A peculiar habit of the direct penetration, characterized by formation a structure resembling infection peg, was observed only after inoculation with the P and S isolates. It is suggested that specifically frequent penetration the roots in eroded areas by the hyphae of the F isolate may be one of the factors connecting with its lower pathogenic capabilities.