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ISSUE 5  May 29, 2003


The dinner plate size patches of dead grass in your yard may be an indication of a patch disease. , Leptosphaeria korrae, a fungus which causes the disease Necrotic ring spot, attacks the roots and crowns of grass plants, causing plants to wilt and eventually die. Typical symptoms begin as circular or crescent patches of dead grass that are usually 8-12 inches in diameter. These dead patches usually grow together causing larger and larger areas of affected lawn unless the problem is addressed. Conditions that favor development of this lawn disease include drought-stressed lawns, compacted soils, and excessive thatch buildup. The thatch layer is the section of organic material that accumulates on the soil surface. Approximately Ĺ inch of this thatch is healthy in a lawn, however when this layer is excessively thick, the roots of the grass do not reach the soil beneath and so donít have access to available nutrients and moisture. This results in a double-whammy of sorts - excessive thatch is a favorable environment for the necrotic ring spot pathogen and it is a stressful environment for the lawn making it more susceptible to attack from any soilborne pathogen.

Excessive thatch can be caused by routine use of herbicides and fungicides 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 do not decompose as readily and 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 the patch disease necrotic ring spot will require judicious cultural practices, and may include fungicide applications. Generally, regular aeration will provide huge benefits. This 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 stratfied soil layers. In any area where there is a heavy concentration of clay in the soil and average activity on the grass (kids, dogs, outdoor activity), annual core aeration will be beneficial to lawn. In a lawn where patch disease is diagnosed, aerating in the both the spring and the fall may be necessary for a couple years. In addition, low nitrogen applications (even in the spring, to minimize fast, lush growth) will help decrease a favorable disease climate. This does not mean not fertilizing, rather just using a balanced fertilizer such as 10-10-10 instead of a 30-4-10 formulation for example. The other important cultural control measure is to water 2-3 times/week, in the heat of the day while the weather is hot (greater than about 85 degrees F). This ensures that the grass is not drought stressed since the thatch layer tends to dry out quickly. It is beneficial to cut grass no shorter than about 3-3 Ĺ 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. 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). This last product 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.

Cheryl Biller
NDSU Plant Diagnostic Lab

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