June 19, 2003

 

What Are Those Dead Circles in Your Lawn?

If you’re noticing dinner plate size patches of dead grass in your yard, your lawn may be suffering from a fungus disease according to Cheryl Biller, North Dakota State University plant diagnostician. "Necrotic ring spot attacks the roots and crowns of the plant causing it to wilt and eventually die."

Typical symptoms begin as circular or crescent patches of dead grass that are usually 8-12 inches in diameter. The dead patches usually grow together causing larger and larger areas of affected lawn. Conditions that favor development of the fungus include drought-stressed lawns, saturated and/or compacted soils and excessive thatch buildup.

"Having about a half inch of thatch on your lawn is healthy," Biller says. "However, when the thatch layer becomes excessively thick, the roots of the grass do not reach the soil beneath and so don’t have access to available nutrients and moisture. It results in a double-whammy of sorts – excessive thatch creates a favorable environment for the necrotic ring spot pathogen and the grass becomes more susceptible to attack from any soil borne pathogen."

Excessive thatch can be caused by routine use of pesticides since they may destroy the activity of the microorganisms necessary to decompose thatch, turning it into usable fertilizer. Wet and compacted soils inhibit microorganism activity as well. Mowing infrequently and leaving large clumps of thick clippings that do not decompose as readily may also contribute to excessive thatch buildup. Installing peat-backed sod on a heavy clay soil creates conditions very favorable to development of excessive thatch and necrotic ring spot.

Management of necrotic ring spot will require judicious cultural practices, and may include fungicide applications. "Generally, regular aeration will provide huge benefits," Biller says. It oxygenates the soil and increases the natural activity of the microorganisms responsible for normal thatch breakdown. Dethatching will also help reduce the thatch layer but may not address potential underlying problems that contributed to the buildup in the first place, usually compacted or stratified soil layers. In any area where there is a heavy concentration of clay in the soil and average activity on the grass such as kids, dogs or other outdoor activity, annual core aeration will be beneficial to the lawn."

In a lawn where patch disease is diagnosed, aerating in the both the spring and the fall may be necessary for a couple of years, Biller says. In addition, low nitrogen applications (even in the spring, to minimize fast, lush growth) will help decrease a favorable disease climate. This means using a balanced fertilizer such as 9-3-6 instead of a 29-3-4 formulation for example.

The other important cultural control measure is to water two to three times a week in the heat of the day while the weather is hot (greater than about 85 degrees). This ensures that the grass is not drought stressed since the thatch layer tends to dry out quickly. It is beneficial to cut the grass no shorter than three to three and a half inches in the heat of the summer to shade the crowns and reduce stress.

Dead patches will need to be re-seeded and using a Fusarium-resistant variety is recommended. Because this is a disease of the roots and crown, getting fungicide to the area where the pathogen was active has proven somewhat challenging, Biller says. Several fungicides are labeled for use against this disease including fenarimol (Twosome Flowable, Rubigan, Patchwork), propiconizole (Banner Maxx), muclobutanil (Golden Eagle, Eagle) thiophanate methyl (Cavalier), and the newest chemistry, azoxystrobin (Heritage). Azoxystrobin has given very good results in studies of fungicide efficacy for this disease. In lawns with extensive disease symptoms or high value turf, use of a fungicide should be integrated into the disease management strategy. In most home lawns, cultural practices are usually sufficient.

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Source: Cheryl Biller, (701) 231-7854, diaglab@ndsuext.nodak.edu
Editor: Rich Mattern, (701) 231-6136, richard.mattern@ndsu.nodak.edu