Abstract

Abstract A description is provided for Peronospora trifoliorum . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Medicago sativa subsp. saliva, M. sativa subsp. falcata . DISEASE: Downy mildew of lucerne (alfalfa). Infected leaflets, which tend to be near the top of the stem, are light green or yellow. The affected areas can vary from small localized spots to larger areas of infected leaf or a systemic infection where the entire shoot may become yellow, stunted and swollen. GEOGRAPHICAL DISTRIBUTION: Worldwide, wherever lucerne is grown. CMI Map No. 343 includes records on clover and other hosts. TRANSMISSION: Mycelium is reported to overwinter in the crown bud (42, 617). Conidia are produced the following spring and in damp and humid conditions soon spread the disease. Oospores occur but reports on their frequency under field conditions are so few that it is difficult to assess their importance in overwintering the disease. Seed transmission, though suspected by Eriksson (1930) and also reported from Italy in a sample of seed from Argentina (Campbell, 1922), is thought to be unimportant (Richardson, 1979).

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