Peronospora viciae on Pisum sativum: morphology of asexual and sexual reproductive structures

Abstract

Asexual and sexual reproductive structures of Peronospora viciae, which causes downy mildew of pea (Pisum sativum), were studied using low temperature scanning electron microscopy. Sporangiophores first appeared as outgrowths from stomata on abaxial leaflet surfaces 4–6 h after infected, glasshouse-grown plants were exposed to high humidity. Sporangiophores continued to develop over the next 6 h, first as simple elongating hyphae, then branching from a single axis (monopodially) to produce multiple, terminal sporangia, which developed synchronously and polyblastically on each sporangiophore. Sporangia had smooth surfaces during development, were finely echinulate when mature, and were delimited by septa at the ends of terminal sporangiophore branches. Gametangia (oogonia and antheridia) developed extensively on inner surfaces of field-grown pea pods from smooth, bulbous hyphae adhering to the host epidermis. Each oogonium was surrounded by several antheridia. Oospores 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 morphology of reproduction in P. viciae.

Key Words:

• antheridia
• downy mildew
• low temperature scanning electron microscopy
• morphology
• oogonia
• oospores
• pea
• Peronospora viciae
• Pisum sativum
• sporangia
• sporangiophores