Ultrastructure of the fungus Ophiodothella vaccinii in infected leaves of Vaccinium arboreum

Abstract

Light and electron microscopy were used to study the fungus Ophiodothella vaccinii E.S. Boyd in infected leaves of Vaccinium arboreum Marsh. This pathogen produces large lesions on leaves in which host cells are surrounded by intercellular hyphae and penetrated by intracellular hyphae. In this study, we examined details of acervuli that are produced on these lesions, the process of conidiogenesis that took place within acervuli, and the relationships between hyphae and host cells within lesions. Most acervuli developed on the adaxial leaf surface just beneath the epidermis, which also was permeated by fungal hyphae. The epidermis covering an acervulus separated from the underlying host cells as numerous conidia embedded in extracellular material accumulated within an acervulus. The raised portion of the epidermis formed a shield-like clypeus over the acervulus. Mature conidia coated with extracellular material were released through a small pore in the clypeus. A columnar mass of sterile hyphae was present in each acervulus near the pore region. This mass was surrounded by conidia embedded in extracellular material. Conidia arose from a layer of phialides present at the base of an acervulus. Phialides arose from both intercellular hyphae and intracellular hyphae that emerged from host cells and also were surrounded by extracellular material. In older lesions, O. vaccinii clearly functioned as a necrotroph. However, in young lesions, we found evidence indicating that some cells were alive when penetrated by hyphae. Hyphae were highly constricted at host cell wall penetration sites and intracellular hyphae commonly grew completely through infected host cells and directly into adjacent cells. Hyphae in older lesions became extremely thick walled and were packed with large lipid bodies.