Host/patogen interactions between Scots pine seedlings (Pinus sylvestris L.) and the p-strains of Heterobasidion annosum (Fr.) Bref. in pure culture

Pinus sylvestris L. seedlings representing half-sib families of several trees of known resistance in field conditions were inoculated with three strains of Heterobasidion annosum (Fr.) Bref. representing the P intersterility group. Standardized inoculation procedure and uniform in vitro conditions allowed for evaluation of the genetically controlled host/pathogen interactions. Early and advanced stages of infection after prolonged exposure to the fungus were studied with the help of light and transmission electron microscopy. Abundance of osmiophilic material in vacuoles, necrotic reaction of cortical cells and secondary thickenings of cell walls in root regions in proximity to external mycelium were responsible for transient stagnation of pathogen spread in the cortex of resistant seedlings. Immobilized and degenerating hyphae inside vacuolar deposits in cortical cells of short roots of resistant plants are indicative of elevated toxicity of the protective materials at the initial stage of infection. Structural changes of vacuolar deposits and their subsequent disappearance, which coincided with heavy colonization of cortical cells of moderately susceptible seedlings, preceded hyphal penetration into the stele within 2-6-months after inoculation. Despite severe damage to root caps, there was no further spread of the fungus into meristems of growing roots and short dormant roots. , In susceptible seedlings, lack of a hypersensitive reaction and apparently low levels of osmiophilic material resulted in quick colonization of cortical cells, breakdown of endodermis and root abortion after entering the stele.