Early Reports of Downy Mildew
Posted: February 21, 2012
Plasmopara viticola: This fungus can grow and sporulate on all green vegetation and fruit of the grapevine. In the spring symptoms may appear as an "oil-spot" on the upper side of leaf, and in the fall as angular yellow to red lesions.
The downy mildews are relatives of Phytophthora and Pythium and are vastly different from powdery mildews. Downy mildews tend to be favored by cool, wet conditions and are not sensitive to most of the commonly used fungicides.
The downy mildew of roses overwinters in the south and the spores blow here each Spring. Also, there is a downy mildew on impatiens that looks a lot like Botrytis because it form masses of gray spores. Downy mildews can be difficulty to diagnose unless you have a microscope to verify the spores.
With the very mild weather this winter both here and in the South, you should anticipate that downy mildews will be major pathogens this spring.
Gary W. Moorman, Prof. of Plant Pathology
The Pennsylvania State University, Department of Plant Pathology