Diseases of lawns
Fairy Ring
Fairy ring is caused by a fungus which forms circles and arches of darker green grass. Mushrooms appear in the circles in damp weather. Dense threads (mycelium) form a mat just below the ground surface that water cannot penetrate, so the center eventually dies. Fairy ring grows more quickly in poor soil and unhealthy lawns, where it isn't competing with strong grass growth.
- Cultural control - aerate the area deeply, to allow water to reach the roots of the affected grass, so it resumes growth.
- Fertilize appropriately, but more is not better. Clean tools! Fairy ring is often spread on aerating tools by well-meaning gardeners. Fairy ring disappears when it reaches a flower bed, sidewalk, or another ring. It also disappears when soil is mixed with soil from another fairy ring.
- Chemical control - none. Formaldehyde is effective but illegal. Garden and domestic fungicides do not work.
Snow Mold
Snow mold is a fungus that creates a pink or grey fuzz on grass as snow melts. It grows in low temperatures on wet soil, and the blades turn yellow or brown in patches. It is worst in areas where snow lasts the longest or is packed down by traffic.
- Cultural control - remove fallen leaves before snowfall, break up snow where it is packed and spread it out in the spring so it melts more quickly, prune shrubs to allow light to penetrate shady areas.
- Chemical control - lawn fungicide (chlorothalonil) in the fall in expected areas.
Dog Urine
Dog urine is toxic to lawns, creating dead patches in the spring from the concentration of urine over the winter (no dilution from watering). While urine is acidic, that isn't the only problem - kidneys collect and excrete toxins. Adding lime or baking soda is risky, as it is impossible to tell the pH and doesn't solve the problem of other toxins. The safest and quickest method is to dig out the areas, removing a couple of inches of soil beneath the grass, add some new soil (purchased in bags) and cut a piece of sod to fit. Water it in, and you have instant lawn the dog and kids can walk on. Seeding takes two weeks to germinate and another few weeks before it can tolerate traffic.