ULTRASTRUCTURE OF ONION ROOTS INFECTED WITH PYRENOCHAETA TERRESTRIS, A FUNGUS PARASITE

Onion roots of varieties ‘951’ and ‘Excel’ were infected under artificial conditions by a virulent isolate of the onion pink‐root fungus, Pyrenochaeta terrestris . Roots of variety ‘951’ were penetrated initially at the tip while roots of variety ‘Excel’ were penetrated initially 2 cm or more back from the tip. Ultrastructural observations of infected roots of both varieties indicated that the fungus penetrated primarily or entirely by dissolution of host cell walls, rather than by pressure of fungal cells. In both varieties the fungus caused cell disruption one to several cells in advance of the fungus. In addition, the same isolate of the pink‐root fungus was grown on paper consisting primarily of gymnosperm cells. The remnants of cell walls were dissolved by the fungus with no evidence of physical pressure by the fungus.