Morphology and ecology of Phialocephala fortinii in roots of Rhododendron brachycarpum

Abstract

The morphology of the interaction of three strains of Phialocephala fortinii with the roots of Rhododendron brachycarpum was examined by light, scanning, and transmission electron microscopy using material grown in axenic Petri dish cultures and in nonsterile pot cultures. In Petri dish cultures, Phialocephala fortinii quickly overwhelmed root tips of young seedlings, causing distortion of cells of the root apex and inhibition of elongation. In older roots, mature, vacuolated, and moribund epidermal cells were penetrated. In pot-cultured plants the epidermis was heavily colonized by hyphae growing parallel to the long axis of the root. Microsclerotia formed within individual cells, between the epidermal and the subepidermal layers, and within cells surrounding the point of emergence of lateral roots. Hyphae did not invade the stele. The effects of two strains of Phialocephala fortinii on shoot dry weight accumulation of R. brachycarpum grown in pot culture were examined and compared with the effects of Hymenoscyphus ericae, Myxotrichum stipitatum, Oidiodendron echinulatum, and Pseudogymnoascus roseus. One strain of Phialocephala fortinii had a significant negative effect on dry weight accumulation, whereas a second had no effect. Hymenoscyphus ericae had a positive effect. The remaining fungi had no effect. One strain of Phialocephala fortinii gave rise to what appeared to be sterile discocarps on the surface of soil in 4-month-old pots of R. brachycarpum. Key words: Phialocephala fortinii, Hymenoscyphus ericae, microsclerotia, Rhododendron brachycarpum, ericoid mycorrhizas, Mycelium radicis atrovirens.