The group “water molds,” or oomycetous plant pathogens, is comprised of both foliar and soilborne organisms with the potential to cause great destruction of a number of economically valuable crops when environmental conditions are wet and warm. Water molds are distinguished from true fungi, the classification of most plant pathogenic organisms, by several features including 1) lack of cell walls in hyphae resulting in the coenocytic condition, 2) diploid nuclei of vegetative cells, 3) cell walls composed of beta-1,3 and beta-1,6 glucans rather than chitin in true fungi, and 4) many species produce biflagellated swimming spores termed zoospores in structures called sporangia (3). The distinguishing features of water molds make their control on agricultural crops a challenge unique from that of true fungi. On vegetable and potato crops, the water molds which threaten the greatest crop losses include Phytophthora infestans (causal agent of late blight on potatoes and tomatoes), Phytophthora capsici (causal agent of Phytophthora crown and fruit rot on tomatoes, peppers, squash, and cucumbers), and Pseudoperonospora cubensis (causal agent of downy mildew on cucumbers).
Soil Science Extension
University of Wisconsin Madison