Garden Detective: Treating lawn brown spots

Leaf spot presents as reddish-purple spots on leaves

Leaf spot presents as reddish-purple spots on leaves that create a gradual browning and thinning appearance in the lawn. (Credit: Cornell Cooperative Extension of Suffolk County)

Jessica Damiano

Jessica Damiano, Newsday columnist Jessica Damiano

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DEAR JESSICA: I've been noticing since spring that many lawns appear to have small circles of brown spots. Do you know what the cause and treatment might be?Fran Abbott, Mount Sinai

DEAR FRAN:Several factors can cause brown spots on the lawn, so I can't say with any certainty what the cause and treatment might be for what you're seeing. But I can provide information on some common reasons this could be happening, and then you can do your own sleuthing.

Let's start with the simplest possible culprit and work our way up: Do you have a dog? Households with canine family members, especially female ones, often have brown spots all over their grass, caused by the ammonia content in urine.

My lawn is one of the many on Long Island plagued with such spots, which I've just learned to accept. The spots can be countered by hosing the area immediately after it's sullied or by applying a small dose of lime to the spots each and every time your dog does its business. The most practical solution is to walk the dog on a leash, which I, for one, am not willing to do.

Common turf problems

Brown patch is a disease of rye, tall fescue, fine fescue and Kentucky bluegrass turfs. Symptoms include brown patches that range between six and 20 inches in diameter that may have a purplish-gray "smoke ring" border or "frog-eye" pattern with green grass in the center. This can be exacerbated by overuse of nitrogen fertilizer. Preventive measures include aerating the lawn, watering deeply and infrequently (versus a daily light sprinkling), and ensuring proper drainage.

Dollar spot manifests as silver-dollar size straw-colored spots on a freshly mowed lawn (the shape will appear more irregular on longer grass). Telltale signs include a white fungus that resembles cobwebs early in the morning when dew is present, and straw-colored lesions with reddish-brown borders on grass blades. To help prevent dollar spot, don't mow grass too short; keep it at 3 to 4 inches, and never mow more than one-third its length in one mowing. Water only early in the day and don't overfertilize.

Leaf spot usually rears its ugly head during cool, cloudy, wet springs and falls. It presents as reddish-purple spots on leaves that create a gradual browning and thinning appearance in the lawn. As spots grow bigger, their color fades to light brown or beige, sometimes surrounded with a dark border. To help prevent leaf spot, follow best mowing and watering practices (see above) and avoid overfertilizing.

Pythium blight, also called cottony blight, can appear to come out of nowhere during hot, humid weather. This isn't likely what you noticed in spring, but it can be identified by a telltale slimy or greasy water-soaked lawn and the presence of patches that are less than 6 inches in diameter. Blight-causing spores can hitch a ride on shoes and, worse, on lawn mowers, to infect other parts of your lawn or other lawns entirely. To help prevent pythium blight, avoid mowing grass when it's wet, don't cut too short, water only in the morning and apply a balanced fertilizer (not a high-nitrogen product). Check the soil's pH and maintain a level in the neutral to slightly acidic range, between 6.0 and 7.0.