Some mechanisms contributing to Verticillium-resistance in the hop root

Abstract

Observations on infection of the roots of wilt-sensitive and wilt-tolerant hop cultivars by mild and virulent strains of Verticillium albo-atrum show that initial invasion of the host may be retarded by lignification of cell-walls and occlusion of penetrating hyphae by sheathing deposits in the epidermal and cortical cells to an extent which appears to be determined by virulence of the pathogen. Exclusion of the fungus from the pericycle and vascular tissues seems to depend upon deposition of suberin in the endodermal cell-walls, and this is associated with invasion of the cortex by the pathogen much more frequently in tolerant than in sensitive cultivars. Though the intensity of vascular invasion appears to be regulated by these host-parasite interactions, further evidence will be required before the limits of the ‘determinative’ phase of disease development can be fully defined. The defensive reactions of the host are not peculiar to the hop but are of widespread occurrence in the plant kingdom; it is suggested that their expression is related to the presence of organisms which represent a transitional stage in the evolution of specialized obligate parasites and symbiotic fungi from the primitive and essentially saprophytic forms, a sequence which is associated with a progressive delay in host-tissue necrosis and which may reflect a progressive reduction of the secretory and synthetic potentialities of the organisms.