Fungus and bacteria in soil and plants.
Susceptible grasses and pathogens (usually fungi) are present in all lawns. In most cases, the pathogens exist in a dormant or saprophytic (feeding on dead or decaying substances) state and do not attack living plants. Diseases in turfgrass and other plants develop from an interaction among a susceptible plant, a disease-producing organism (pathogen), and an environment favorable for disease development. Diseases occur when environmental conditions (weather, management, and/or site conditions) become favorable for the buildup of pathogen populations and/or cause an increase in the susceptibility of the plant. When this happens, turfgrass loss can occur.
There are a group of turfgrass diseases for nearly every environmental condition that may arise. Different types of grass develop certain diseases pertaining to that grass. Some diseases develop during the frigid months of winter under a blanket of snow, whereas others occur only during the hottest and most humid conditions of summer. Some diseases appear more frequently in wet soils and following applications of high rates of fertilizer, while others are triggered by drought and low fertility. Some types of diseases include: (click on links below to find out more)
Powdery Mildew Leaf Spot Yellow Patch Dollar Spot Red Thread Pink Snow Mold Brown Patch Rust Diseases