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Circles and Patches in Lawns
Question. I reseeded my lawn last fall with Kentucky bluegrass and it looked beautiful until early June, when I started noticing circular spots that turned brown, then gray and then black. It spread and approximately 3/4 of my lawn is now completely dead (not dormant). I have done a considerable amount of research and know that Kentucky bluegrass is difficult to grow here because of the harsh conditions for lawns. Although I have not had my grass tested, I'm fairly confident it has a fungus based on my research (although I realize it could be other things such as insects, poor drainage, etc.) But all I want to know is whether it is possible to completely eliminate fungus in the soil and, if so, how. Thanks,
    ---Matt in Arlington, VA
Answer. Well, it may be all you want to know, Matt, but it may not be the answer to your problem. To quote our turf grass advisor, Dr. Nick Christians of Iowa State University: "Lawn diseases can be difficult to identify. Many—especially the patch diseases—look much the same." And, as you note, the symptoms are often the same as those caused by things like insect damage and poor lawn care. So any 'confidence' you have in a fungal villain might not be shared by experts.

Now, there are several good organically approved disease-fighters available (like the naturally occurring soil organism Bacillus subtilis and products derived from Harpin proteins, but nothing will ever {quote} 'completely eliminate fungus in the soil'. First, a lot of soil fungi are beneficial to plants and you do NOT want to chase THEM away. And two, when a bad disease is the problem, the organism responsible tends to be ubiquitous in the local environment. So if something you're doing—hint, hint—is weakening the grass—hint, hint—and making it susceptible, there's always going to be a felonious fungal spore nearby to take advantage of it.

(Oh—and your bluegrass is no harder to grow than any other turf in the DC area. Your location—in the heart of The Transition Zone—is just a tough place to have a nice lawn, period. That makes proper lawn care even more essential, as the weather WILL stress your turf; and anything you do to stress it further could push it over the edge into illness.)

Now, let's take a look at some of the diseases that cause discolored circles or patches to appear in turf—with an eye towards the lawn care practices that can help prevent your ever having to look at little crop circles and play 'guess the disease'.

Leaf spot (several kinds): Grass blades turn brown (light, dark and/or reddish brown) in irregular patches. Close inspection of the blades may reveal distinctive little spots. Worst on Bermuda grass and 'common' (i.e., cheap) Kentucky bluegrass. (Newer, named varieties of bluegrass are often specifically designed to be resistant). Biggest causes: Overfeeding, thatch (which is redundant, as thatch is caused by overfeeding) and watering in the evening.

The patch diseases: These include the delightfully named necrotic ring spot, yellow patch, summer patch and fusarium blight. All display a 'frog's eye' appearance: rings of dead grass with an 'eye' of healthy turf in the middle of each ring. Biggest causes: compacted soil and thatch.

Brown patch: Lawns develop somewhat circular patches of dead grass that can be several inches to several feet across; they often seem to have a dark halo around them in the morning. Biggest causes: Overfeeding (especially with quick-release high-nitrogen chemical fertilizers), feeding a cool-season grass ANYTHING in the summer time, thatch, over watering and evening watering.

Snow molds: Pink snow mold first appears as yellowish-green spots after winter snows melt, then the spots change to a pinkish hue. Biggest cause: excessive fertilizer in the fall. Gray snow mold also starts out yellowish-green, then turns grayish-white as the grass becomes covered with the my celia of the fungus responsible. Biggest causes: Overfeeding in the fall, letting leaves lay on the lawn over winter and our old friend thatch.

Notice any patterns here? Although all of these turf diseases are caused by specific organisms (you can look these up online via their common names—much better than listening to me badly mispronounce them), be assured that each hard-to-spell organism is everywhere in your environment. The only real way to prevent them is to have a strong lawn.