Downy Mildew in Alfalfa
Downy mildew (Peronospora aestivalis) was found in Cache Valley late spring. The pathogen likes cool wet weather which we had plenty of this past spring. The symptoms you see consist of chlorotic or yellow blotches on the leaf surface. On the underside, the leaves are covered in a white to pale violet colored fungal growth. Those are the spores and fruiting structures that emerge from the leaves.
Once plants get systemically infected, they can have stems that have a wider diameter and a rosette-like growth at the tip. The pathogen overwinters in the cortex of the crown as well as in bud tissue. During the following spring the downy mildew colonizes the infected buds and shoots as they grow and spores develop on the new leaves. The spores are then blown by wind to neighboring alfalfa plants and infect the leaves. Only young leaf tissue is susceptible to infection. Once the tissue has matured the pathogen can no longer colonize it. Therefore, usually only the first cut of alfalfa is negatively affected.
If downy mildew has been a problem in the past, varieties more resistant to it should be used. The cutting should take place as soon as possible because inoculum as well as susceptible host tissue is removed. The downy mildew spores are very short lived and will be dead before new susceptible tissue has grown. When planting new alfalfa in fields with a history of downy mildew, a seed treatment with metalaxyl or mefenoxam can minimize infection.
For more information, please visit:
Dr. Claudia Nischwitz (Extension Plant Pathologist)