Downy mildew of alfalfa is caused by the fungus Peronospora trifoliorum. It is an obligate parasite that requires living plant tissue to grow. The disease occurs primarily in early spring and fall. It overwinters as mycelium in crowns and surviving shoots of infected plants. It also can overwinter as thick-walled spores called oospores.
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Early in the spring when vegetative growth in alfalfa stands resumes, mycelium of the downy mildew fungus in infected plants grows and forms sporangia (spores or conidia). The sporangia are borne on branches of mycelia called sporangiophores. When mature, the sporangia are released and infect young leaves. They are spread by wind and splashing rain and can germinate and cause infections only in the presence of free moisture. Sporangia production and plant infection are favored by cool, wet conditions (100% relative humidity, free moisture, and 50o to 65oF). Because these conditions prevail in early spring, yield losses due to downy mildew can occur in the first cutting. Favorable conditions for disease development also can occur in the fall. Newly seeded alfalfa is most severely affected. Infected plants weakened by the disease are more susceptible to winterkill than healthy plants.
Symptoms
Downy mildew begins as chlorotic (yellow) spots on the leaflets. Under favorable conditions for disease development, these spots enlarge and coalesce, causing entire leaflets to become yellow. Alfalfa downy mildew can be recognized by the distinctive pale green or yellow color on the leaflets (Figure 1). Downward curling and twisting of leaflets also may occur. Stunting can result from early infection of spring growth. Seedling death can result if infection occurs soon after emergence. Infected stems are thickened and produce a bunchy, rosette-like growth at their tips. When relative humidity is high, patches of white to gray downy growth (signs of the fungus) can be seen on lower surfaces of leaflets. The downy growth consists of sporangia and sporangiophores. In the field, downy mildew of alfalfa can be distinguished from other fungal diseases by the initial appearance of symptoms on the younger, upper leaves. This is because young, succulent tissue is most susceptible to the fungus. Other fungal diseases start on the lower leaves and progress up the plant.
Management The most effective and economical means of managing alfalfa downy mildew is planting resistant varieties. When disease in the spring is severe, early harvest of the first cutting is desirable as it will remove inoculum (sporangia and mycelia) as well as young susceptible leaflets. Early harvest also will reduce relative humidity provided by the plant canopy. As seasonal temperatures increase, the chances of downy mildew occurring in the regrowth following the first cutting are considerably reduced. Fungicides for control of downy mildew are available; however, a fungicide application may not be economically feasible. Planting certified seed treated with systemic fungicides for control of seedling damping off can reduce losses from downy mildew.
Stephen N. Wegulo
Extension Plant Pathologist, Lincoln
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