Leaf spots are most destructive during cold, wet, overcast weather in spring and fall. Look for gradual browning and thinning of grass. Small, dark-brown, purplish, or purplish-red colored spots appear on leaves from early spring to late fall. As these lesions increase in size their centers may fade to a straw or light-brown color. The spots are usually surrounded by narrow dark reddish-brown to purplish-black borders.
As the disease progresses in favorable weather, the spots run together and girdle the leaf blades. As the weather warms, the crowns, rhizomes, and roots may rot. Plants lack vigor and wilt from moisture stress during the middle of the day.
- Proper mowing. Maintain lawn at maximum height and remove no more than a third of the plant when mowing.
- Keep thatch less than 1 inch.
- Don't overfertilize. Avoid applications before late May or early June. Avoid excess nitrogen, especially in spring.
- Avoid frequent light waterings. If you water during dry spells, apply enough to soak in 6 to 8 inches deep.
- Choose resistant varieties. Resistant varieties of Kentucky bluegrass and fescues are available.