Pythium Blight (Pythium aphanidermatum), this disease begins in mature lawns as small slimy or greasy looking spots. The spots may assume a reddish color if weather conditions become cooler and drier. When the turfgrassis wet or where air circulation is poor, the cottony, white, web-like mycelium of the fungus may be observed. This disease can develop rapidly and extensively in poorly drained areas and is especially prone to following surface drainage channels. Pythiumfungi are "water molds," which produce microscopic swimming spores. These spores are attracted to living plants. Plants that have undergone recent nitrogen fertilization are more susceptible to infection. The fungi grow well in soil and thatch in wet weather and survive as thick-walled resting spores during adverse conditions. Pythiumspecies are present in every soil and will grow rapidly under conducive conditions. The disease occurs most commonly in perennial ryegrass lawns.