PERONOSPORA VICIAE ON PISUM-SATIVUM - MORPHOLOGY OF ASEXUAL AND SEXUAL REPRODUCTIVE STRUCTURES

Asexual and sexual reproductive structures of Peronospora viciae, whic h causes downy mildew of pea (Pisum sativum), were studied using low t emperature scanning electron microscopy. Sporangiophores first appeare d as outgrowths from stomata on abaxial leaflet surfaces 4-6 h after i nfected, glass-house-grown plants were exposed to high humidity. Spora ngiophores continued to develop over the next 6 h, first as simple elo ngating hyphae, then branching from a single axis (monopodially) to pr oduce multiple, terminal sporangia, which developed synchronously and polyblastically on each sporangiophore. Sporangia had smooth surfaces during development, were finely echinulate when mature, and were delim ited by septa at the ends of terminal sporangiophore branches. Gametan gia (oogonia and antheridia) developed extensively on inner surfaces o f field-grown pea pods from smooth, bulbous hyphae adhering to the hos t epidermis. Each oogonium was surrounded by several antheridia. Oospo res within pod tissue of field-grown plants were enclosed by oogonial membranes. Each oospore had a heavily reticulate outer wall enclosing cytoplasm and liquid, possibly lipid. Cryofixation and low temperature scanning electron microscopy have provided new insights into the morp hology of reproduction in P. viciae.