On vegetable and potato crops, the water molds, or fungus-like, oomycetous plant pathogens, which threaten the greatest crop losses include Pseudoperonospora cubensis (causal agent of downy mildew on cucumbers), and Phytophthora infestans (causal agent of late blight on potatoes and tomatoes). Downy mildew and late blight can both be aerially dispersed over long distances and genotypes identified in the region are not known to be soilborne at this time (1, 3). Initial inoculum and infection occurs as the result of movement of spores in the air from diseased fields to healthy, infected seed or transplants, or by overwintering plant tissues harboring the pathogen from the previous year (e.g. volunteers, cull piles, compost piles). In Wisconsin in 2011, both diseases made minor appearance on vegetable crops.
Soil Science Extension
University of Wisconsin Madison