All host plants are introduced, three being important weed species. Fry (1958) showed that the occurrence of this fungus in roots of lettuce was associated with the occurrence of lettuce big vein virus. He observed Olpidium in roots of the three weed species, which were growing with infected lettuce. The relationship of this fungus with the big vein virus has now been firmly established (Fry and Grogan, 1964). Fry (1958) stated that soil treatment with organic fungicides reduced the incidence of the disease. It is common in winter months in the Auckland area. Brandenburg and Hurndell (1960) showed that it is also common in the Christchurch area.