When lawns are stressed from extended periods of hot, humid weather, conditions are right for fungal diseases and the damage they cause. As with weed control, the best way to deal with lawn diseases is through prevention, by promoting a healthy lawn with an extensive root system and allowing for adequate air and water movement. But even the healthiest lawn can come under attack by these invaders, so it's best to know how to identify common diseases and what to do about them.
One of the most common lawn diseases, dollar spot gets its name from its distinct pattern of dead circular patches about the size of a silver dollar. It occurs when moisture is present during the patterns of warm days and cool nights of spring and fall. Excessive thatch, poor drainage, overwatering, and low soil fertility can all contribute to dollar spot.
Most prevalent in tall fescue and ryegrass, brown patch can attack all other cool-season grasses and some species of warm-season grasses. Brown patch appears as irregular, brownish yellow, circular patches that range from six inches to several feet in diameter. It is notable for a distinctive "smoke ring" border that is sometimes visible on the outer edge, giving the patch a "frog-eye" appearance. Close inspection reveals lesions on the leaves that are tan with a dark-brown border. White, cottony, mycelium can be found on dew-covered turf in the early part of the morning.
Brown patch occurs during extended periods of heat and humidity, especially when nighttime temperatures remain above 68 F. Poor air circulation, overwatering, over-fertilizing, and lawn compaction can also exacerbate the problem.
Often found on Kentucky bluegrass lawns located in shady areas with limited air movement, powdery mildew looks like a white dust covering the grass blades. Although it can yellow the turf over time, powdery mildew is not likely to cause widespread damage to a lawn. Extended periods of cloudiness can bring on powdery mildew, but if the problem is not weather-related, conditions may need to be altered for a healthy lawn. Shade-tolerant grass species are not likely to attract powdery mildew, and pruning or thinning trees can improve air circulation and sunlight to the area.
The infamous circles on the lawn, fairy rings appear as dark full or partial circles that may or may not grow mushrooms. Once thought to be portals to elf kingdoms, relatively little is known about their origins or how to get rid of them. There are many stories about digging out fairy rings, but the widespread nature of fungal spores limits the effectiveness of digging. Maintaining a lush green lawn can blend in the darker color, and regular mowing can help control the presence of mushrooms.
Prolonged snow cover in the spring can lead to gray snow mold or pink snow mold, especially if the ground is not quite frozen. It can also occur on over-fertilized lawns and under leaves left over from the fall. Snow mold damage is largely superficial and temporary. The best way to deal with it is to rake it out to help the area dry. New growth will begin, and soon the damage will be gone.
A common and relatively harmless lawn disease, red thread usually appears when the lawn is due for fertilizing. Chances are if red thread is present, it's time to fertilize the lawn. Red thread is noticeable by its distinctive red hairs amongst the turf, giving it a pinkish-red appearance from a distance.
Dog spot is not really a lawn disease but is often mistaken for one. When dog urine is concentrated on the lawn, circles of dead grass appear and may look similar to diseases like dollar spot or brown patch. Training dogs to do their business in certain areas can help, as can motion-activated sprinklers. There are also products available for treating affected areas, but buyer beware: dog spot is just a fact of life.