PENETRATION OF PHYTOPHTHORA-CINNAMOMI INTO DISEASE TOLERANT AND SUSCEPTIBLE EUCALYPTS

The mechanisms of penetration of P. cinnamomi Rands into seedling eucalypt roots were studied by light microscopy and EM. Culture grown seedlings of root-rot tolerant Eucalyptus st johnii and root-rot susceptible E. obliqua were inoculated with both zoospores and mycelium. Zoospores encysted on roots of both species and the germ tubes penetrated without the formation of appressoria. Swellings, previously described as appressoria, were formed when the germ tube was slow to enter the host by intracellular penetration. Vegetative hyphae penetrated both intercellularly an intracellularly into the zones of root elongation and differentiation, often through root hairs. Evidence of hydrolysis of the host cell wall at the point of penetration was observed in electron micrographs. Several hours after the germ tube penetrated the epidermis, a thick plug of amorphous material formed in the germ tube slightly below the level of the outer walls of the epidermal cells, sealing off the hypha within the root. Behavior of zoospores and germ tubes and the mechanism of penetration were similar on both hosts. Micrographs do not suggest any kind of a hypersensitive reaction by the host cells during the early stages of infection.