SPOTS on leaves at first greenish, later yellow to brown, usually small, vein-limited, frequently surrounded with a darker border. Spots on shoots and siligues violet, sometimes covering the whole infected fruit; higly affected siligues are frequently deformed.
CONIDIOPHORES WITH CONIDIA formed on the under surface of cotyledons, leaves, shoots, petioles, and siliques, white, dense, covering the whole under side of cotyledons, less dense and diffused on the under leaf surface.
CONIDIOPHORES (cp) 250-640 x 7-18 µm, not or lightly thickened, up to 20 µm, at the base, branched 5-8 times.
Branches (b) straight; final branches pincers-like curved, rarely forked or beaklike, slanted at acute or right angles, up to 30 µm long, 2-3 µm wide at the base.
CONIDIA (c) widely elliptical, egg-shaped or subglobose, rounded the both tops, colourless, 22-30 x 18-22(-27) µm.
OOGONIA 35-50 µm, with a persistent wall.
OOSPORES 25-30 µm diam, yellowish to pale brown, globose.
PLANT HOST AND DISTRIBUTION. Peronospora brassicae affects different plant species of the genera Brassica, Raphanus, and Sinapsis (Kochman and Majewski 1970).
The fungus has a worldwide distribution (Kochman and Majewski 1970).
NOTES. Peronospora brassicae causes downy mildew of plants of the genera Brassica, Raphanus, and Sinapsis (Kochman and Majewski 1970). The most important cultivated plants affected by this fungus are B. napus L., B. oleracea L., and R. sativus L.
Peronospora brassicae attacks booth seedlings and plants at the end of their vegetation. Early affected seedling decline and die.
Peronospora brassicae is distributed by conidia. The conditions favouring the formation and germination of conidia, as well as the penetration of their germ tubes into leaf tissues are low temperatures and a high air humidity. In contrast, in the plant host tissues, the parasite best develops at higher temperatures (ca. 24oC). The germ tubes of P. brassicae conidia penetrate into leaf tissues through cells of epidermis, not through stomata.
Kochman J., Majewski T. 1970. Grzyby (Mycota) IV. Glonowce (Phycomycetes), Wroslikowce (Peronosporales). Warszawa, 308 pp.
Brooks F. T. 1953. Plant diseases. Geoffrey Cumberlege. Oxford University Press. London, New York, Toronto.