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 the leaves from the 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. Improper mowing, watering and excessive thatch buildup are all contributing factors.