Initial stages of host-pathogen interaction between Pinus sylvestris seedling roots and the P-, S- and F-intersterility group isolates of Heterobasidion annosum

Prepenetration and penetration phenomena after inoculation of roots of Pinus sylvestris seedlings grown in vitro with the P-, S- and F-group isolates of Heterobasidion annosum were observed using scanning electron microscop. There were no differences in the behaviour of hyphae and in the appearence of mycelia formed by the three IS-group isolates. Four types of the root penetration by hyphae were observed. In the first, the entrace of hypha into root was achieved throught tiny pore formed in walls of the cortical cells. In the second, swellings resembling appressoria were formed by the hyphal tips at the points of contact with the root surface. In the third, the hyphae penetrated cells through holes in eroded areas of roots, and in the fourth by the natural crevices at points of the cortical cell junctions. It is concluded that the different frequencies of the four types of penetration observed after inoculation with isolates of the three IS group should be carefully explained in term of their different pathogenic and saprobic capabilities on pine.