Diseases can be a problem on Kentucky Bluegrass lawns in the South since most of them prefer the same temperature and moisture as your grass. Fungal diseases like leaf spot, powdery mildew, and rust can be prevalent under the right environmental conditions. While individual diseases appear at different times of the year depending on weather, it is almost impossible to predict when your lawn will be afflicted. In many cases, a disease will come and go without you ever knowing it. Since fungal diseases are very difficult to identify, it is important to keep a sharp watch for anything abnormal.
In general, fungal disease appear as brown or damaged patches of grass with clearly defined outlines. If you find a lawn area that fits this description, don’t run out and spray the entire lawn with fungicide. Wait to see if the patch of damage grass expands over the next day or two. It is common for the fungal disease to run its course without any treatment. An ‘active’ patch of diseased turf will grow and expand concentrically over a couple of days. Grass plants along the outline of the damaged patch may have discolored, wilted, or spotted leaves. If you find one of these problems, treat the infected area with a lawn fungicide approved for use on Kentucky Bluegrass.
Many fungal diseases are exacerbated by water. This is especially true with fungal diseases like leaf spot that are prevalent in the spring and fall when daytime air temperatures are 60-80° and nighttime air temperatures are around 60°. During these critical times, do not irrigate unless it is necessary. When needed, water on a sunny day when moisture will evaporate quickly from grass leaves. If there is a daytime watering ban in effect for your area, irrigate immediately before dawn.