what is the causal organism of Downy Mildew of Alfalfa?
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The disease is primarily a problem on the first cutting and
in fall-seeded stands. The causal organism is Peronospora
trifoliorum de Bary. The fungus has been divided into
pathogenic races based on cultivar reaction. It is an
obligate parasite that is pathogenic on Medicago spp. The
fungus overwinters as mycelium in systemically infected
plants, residing in crown buds, the cortex of crown
branches and in surviving shoots. Researches also suspect
that oospores found in crop residue may serve as
overwintering structures. The infection cycle is initiated
in the spring when conidia are formed from the mycelium in
infected shoots and systemically infected spring growth.
The spores are produced in the dark when the humidity is
near 100%. Spores are wind blown or water splashed to
susceptible young leaf tissue. Older tissue is resistant to
infection. Spores germinate within in a temperature range
from 4oC to 29oC, with 18oC being optimum. Free moisture
must be present for germination to occur. Germ tubes form
an appressorium and commonly invade the host directly
through the epidermis. The fungus may also penetrate
through stomata. Conidia are formed on newly infected
tissue with infection cycles occurring every 5 days under
favorable conditions. Oospores may also be produced in
infected tissue.
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