Studies on the genus Mastigosporium: II. Parasitism

Abstract

Summary Penetration of the appropriate host by species of Mastigosporium is achieved by a germ tube growing directly from a spore through the cuticle and outer wall into the cavity of an epidermal cell. The hyphae develop extensively in the epidermal cell and the contents of adjacent mesophyll cells become disorganized. The hyphae grow into the mesophyll, where they cause limited necrosis, but do not extend to the periphery of the affected area. The lesion is delimited laterally by the sclerenchyma of the parallel leaf veins; longitudinal extension of the lesion is limited, not by a mechanical barrier, but probably by accumulation of staling products. Efforts to prepare extracts of fungal toxins have been unsuccessful. There is no evidence of the production of pectinase by these fungi. The specialization of parasitism of these species cannot be accounted for by differential spore germination in liquids on the host surfaces. Inappropriate grass hosts do not seem to contain any extractable substance which is toxic to the fungi which normally do not parasitize them. Each fungus can penetrate a number of inappropriate grass species. Hyphal development is restricted to the epidermal cells, and no effect on the mesophyll tissues is apparent. The reasons for non-lethal attack in most of the inappropriate fungus-host combinations studied appear to lie in the inability of the fungi to secrete toxins sufficient to kill the host protoplasm or to derive adequate nutrients from the tissues.