Diseases of alfalfa (
Downy mildew
Downy mildew of alfalfa is caused by the fungal-like organism Peronospora trifoliorum. Downy mildew is a foliar disease that occurs in the winter when the plant canopy stays moist from rain or dew. Although closely related to other downy mildew organisms that infect other plants, P. trifoliorum infects only alfalfa. It produces air borne spores that infect the leaves of susceptible alfalfa varieties, and survives in the plant as mycelium in the crown buds and shoots. Spore production is most prevalent on the underside of leaves in the lower canopy and appears downy and violet; the corresponding upper side of the leaf is yellow and bleached (photo 1). Leaf infections may result in defoliation, especially in very susceptible varieties.
Control of disease depends on the use of tolerant varieties of alfalfa and on timely harvesting. Harvesting reduces the dense canopy that favors growth of downy mildew and removes spores and mycelium resulting in a reduction of inoculum in the alfalfa stand.
1 
July 13 , 2006