There are a variety of diseases that can affect turf. They all require favorable conditions, the presence of the disease, and a susceptible host plant. Here is a chart, courtesy of Purdue University, that generally describes when certain disease are active. Also, you will find comprehensive descriptions and fact sheets at the following address:

• Purdue University Weed Publications: Disease and Insect Control

On individual plants, symptoms may appear as small, circular, tan-colored lesions surrounded by brown or purple borders (leaf spotting); as yellow, red, or tan blotches over most or all of the leaf blade (blighting); stunting; wilting; or as a brown or black rot on the crowns and roots. Download this comprehensive .pdf as a reference guide to several common diseases...