Brown Spots in Lawn Probably "Frogeye" Disease

Q. I have some large dead areas in my lawn. I don’t think it’s an insect problem. Watering is at 2 AM and 7 AM. This was my first lawn problem in 16 years!

Unusual pattern for dead grass. Too random for irrigation I think.

This picture you start to see the "frogeye" disease pattern. Looks like it could be a chemical spill.

Here is where "frogeye" starts to make an impression. But its over. 

A. I looked at the pictures you sent, but the last picture was the most “telling” to me. I think your lawn had a disease problem that is now finished. Don’t do anything now! Let me tell you why I came to that conclusion.

            Insect problems here are not like those “rolling back like a carpet” problems described in books and online. Those descriptions apply to Kentucky bluegrass damaged by white grubs. We have grubs, but we don’t have much bluegrass anymore. Our lawns are 95% tall fescue and tall fescue reacts differently to insect damage. It pulls out like loose hair from an old animal skin. Plus, the pattern of dying grass does not resemble insect damage.

Rolls Back Like a Carpet

            I first thought the brown spots were an irrigation problem, but the pattern doesn't fit an irrigation problem. With irrigation problems, dead or damaged areas are in a pattern that relates to the location of sprinkler heads. I didn’t know where the sprinkler heads were, but the damage was irregular, so I ruled out irrigation.

Irrigation problems are nearly always easy to see how they connect to the sprinklers.

            The pattern does, however, resemble a chemical spill flowing downhill and damaging the grass in its path. It was a possibility, but the last picture looked like advanced stages of a disease problem. We used to call this disease Fusarium blight or “frogeye”. This disease has since been renamed Necrotic Ring Spot. I thought “frogeye” was much more descriptive.

Seldom will you see it this obvious but this is frogeye. See how it got its name? In advanced stages these dead areas can become one but you still look for little patches of green grass that would still be there. Sometimes its hard to see them.

            What led me in the direction toward disease were the small patches of green grass still alive but surrounded by dead grass. These small green patches of grass are how “frogeye”, now Necrotic Ring Spot, got its name.

            From the pictures, it looks like the disease has run its course and the grass that’s alive is healthy. I say this because the grass surrounding the dead areas appears healthy. This is the way lawn diseases usually work. They have a window of opportunity, when the weather is right for the disease to spread, and it takes off. The weather changes and the disease stops.

            You would be wasting your money to apply a fungicide now that it’s over. But, don’t disturb the brown, dead grass and leave it alone until fall. If you remove it now, it opens the soil to invasion by Bermudagrass. Bermudagrass loves sunshine, bare soil and water. It hates shade. Wait until the weather cools in late September or October, remove the dead grass and either seed or sod these areas.

Controlling Frogeye Disease in Lawns

Q. Can you tell me why lawn grass gets frog eyes and the best way to prevent it? Also why does some of the grass look so yellow?

Summer patch, frequently called frog-eye or frogeye patch.

A. This is the time of year we usually see frogeye disease, or Summer Patch, in lawns. This used to be called Fusarium many years ago, but no longer. The yellowing may be due to a lack of nitrogen or iron fertilizer or both.

This disease is a hot weather disease on tall fescue in the Mojave Desert. It occurs when air humidity increases in the summer months or if we water our lawns early in the evenings.

Consider the disease organism to be present on all tall fescue lawns. The symptoms of the disease appear during hot, humid weather or during a rainy period. The worst scenario is if it rains in the afternoon or early evening and keeps the lawn wet during the night.

If our lawns stay wet for at least six hours at night in July and August, this tends to promote this particular disease. The disease will take about 3 to 4 days to appear when conditions are right.

Apply a preventive fungicide if your lawn has been susceptible to this disease in the past. If it has, you need to plan that it will happen again. Purchase a lawn fungicide that prevents frogeye disease, a.k.a. Fusarium or summer patch and states so on the label.

Apply it to susceptible areas 2 to 3 days after summer rains occur. Follow label directions for reapplication of the fungicide. Lawn fungicides aimed primarily at disease prevention and seldom cure diseases once they start.

Fungicides will stop a disease from spreading once applied but seldom cure it.

Nonchemical control includes aerification of the lawn in spring or fall months. Increase the mowing height or make sure lawns are mowed at 2 1/2 inches or higher. Make sure the irrigation has head-to-head coverage and prevent it from getting water stressed during the heat.

Use organic fertilizers on the lawn including composts and bagged manure products. Compost applied as a fertilizer has been shown to reduce many lawn diseases. Compost should be applied monthly during the growing season. Use mulching mowers and leave the mulched clippings to decompose in the lawn and on top of the soil.
Those of you living in Las Vegas can get compost for top dressing lawns in bulk at Viragrow.
Viragrow website

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