ISSUE 9 June 27, 2002
LEAF SPOT DISEASE ON LAWNS
Weather conditions this year have been favorable for a lawn disease we donít usually see as a serious problem in ND yards. Helminthosporium leaf spot is a warm weather disease that can become severe when there are prolonged periods of leaf wetness or high relative humidity and temperatures remain above 70 degrees. Kentucky bluegrass and perennial ryegrass, two commonly planted grasses in ND, are the most susceptible to this disease. We rarely have the combination of sustained high humidity with overnight temperatures above 70 degrees and daytime temperatures at 80-90 degrees that facilitate rapid and widespread disease development.
As a general rule, most lawns show very minimal levels of infection with this pathogen that never progress to the point of causing damage to a lawn. However, with the favorable weather this summer, newer lawns especially may be at risk for some injury from this disease. The most susceptible lawns will be those newly planted and those where heavy fertility programs are in place, particularly where high nitrogen applications have been made.
The initial symptoms of helminthosporium leaf spot are small, purplish spots on the leaves. As they increase in size, the center will become tan to brown with a purple halo. As infection progresses, irregularly shaped areas of grass will appear to dry out and the leaves will become a light straw color. If left unchecked during favorable conditions, a crown and root phase of the disease may develop that is not treatable.
Good cultural strategies applied to lawn maintenance should be followed to maximize lawn vigor and health. These include mowing to a recommended height (1.5 to 2 inches unless temperatures are in the high 80's and 90's, then at about 2.5 to 3 inches), watering in the early morning hours to minimize the time of leaf wetness, adequate but not heavy nitrogen fertilizer, and avoid excessive thatch buildup (anything more than 0.5 inches is excessive) by aerating and/or de-thatching regularly. Fungicides may be necessary to keep this disease from progressing to the more severe melting out phase. Products containing the following active ingredients are effective (some of the products that contain these ingredients): captan; chlorothalonil (Gordonís Multipurpose Fungicide, Daconil); iprodione (Chipco 26019); mancozeb (Protect T/O), myclobutanil (Spectracide Immunox, Eagle, Scottís Golden Eagle); propiconazole (FertiLome Liquid Systemic Fungicide, Ortho Lawn Disease Control Banner Maxx); PCNB, and vinclozolin (Curlan and Vorlan). Always follow label instructions and recommendations when using pesticides.
NDSU Plant Diagnostic Lab