Home lawns in southern Idaho are generally not faced with many disease problems because of the dry climate. Improper fertility or irrigation practices are generally the main cause of disease problems. Cool, overcast weather conditions or areas in a lawn with excessive shade also can contribute to disease outbreaks. Generally, however, a properly managed lawn will be resistant to severe disease outbreaks.
Shady areas with Kentucky bluegrass are especially susceptible to powdery mildew. Powdery mildew appears as a white, powdery mass on grass blades and large affected areas appear as if they have been dusted with flour or talcum powder. It is favored by cool, humid, shady conditions and may even appear after cloudy weather. Poor air circulation and heavy fertilization worsen the problem.
Control. If trees are causing the shade and if possible, trim trees to minimize shade and improve air circulation. Powdery mildew is rarely serious enough to warrant control with fungicides. Overseed shady areas with shade tolerant grasses such as fine fescues or tall fescue.
Additional close up pictures and disease descriptions can be found at www.ipm.iastate.edu/
Rust is an orange colored fungus that affects the grass blades. Heavily infested lawns will leave a rust-colored dust on shoes and clothing. Rust is usually a result of under fertilized and/or drought stressed turf, and the disease is favored by cloudy, overcast conditions. Light, frequent watering will keep leaf blades wet and also increase the development of rust.
Dark circles or partial circles of lush green grass in a lawn are caused by mushroom type fungi living in the soil. These dark circles of grass are commonly referred to as fairy rings. Sometimes mushrooms will appear following wet, humid weather. In severe cases, a ring of dead grass will appear inside the ring of green grass.
The fungus that causes fairy ring grows on organic matter buried in the soil. As the fungus breaks down organic matter, nitrogen is released causing the green lush growth. In some cases, the growth of the fungus is so thick in the soil that water cannot penetrate, causing the grass to become drought stressed and die.
Control. Prevention of fairy ring involves removal of tree stumps and other wood materials from a lawn site prior to establishment. If fairy ring still appears, it can be very difficult to remove the fungus entirely. Core aeration and hand watering the affected area will help move water into the affected soil and help introduce other microorganisms that will compete with the fungus. Make sure to clean aeration equipment to avoid introducing the fungus to other areas of the lawn. Proper fertility and watering practices will help reduce the severity of the symptoms and in some cases prevent fairy ring from developing. Fungicide drenches have shown irregular success and are generally not recommended for home lawns. Severe fairy rings with large areas of dead grass may warrant removal of grass and soil from the rings followed by introducing new soil and re-seeding the area.