MORPHOLOGY AND ECOLOGY OF PHIALOCEPHALA-FORTINII IN ROOTS OF RHODODENDRON BRACHYCARPUM

The morphology of the interaction of three strains of Phialocephala fo rtinii with the roots of Rhododendron brachycarpum was examined by lig ht, scanning, and transmission electron microscopy using material grow n in axenic Petri dish cultures and in nonsterile pot cultures. In Pet ri 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 mori bund epidermal cells were penetrated. In pot-cultured plants the epide rmis was heavily colonized by hyphae growing parallel to the long axis of the root. Microsclerotia formed within individual cells, between t he epidermal and the subepidermal layers, and within cells surrounding the point of emergence of lateral roots. Hyphae did not invade the st ele. The effects of two strains of Phialocephala fortinii on shoot dry weight accumulation of R. brachycarpum grown in pot culture were exam ined and compared with the effects of Hymenoscyphus ericae, Myxotrichu m stipitatum, Oidiodendron echinulatum, and Pseudogymnoascus roseus. O ne strain of Phialocephala fortinii had a significant negative effect on dry weight accumulation, whereas a second had no effect. Hymenoscyp hus ericae had a positive effect. The remaining fungi had no effect. O ne strain of Phialocephala fortinii gave rise to what appeared to be s terile discocarps on the surface of soil in 4-month-old pots of R. bra chycarpum.