Lawn diseases are common, although spraying fungicides is not simply because of cost. Those who wish to spend more on their lawns might request a fungicide as a preventative from previous occurrence or cure at time of appearance. Golf courses and some athletic fields must be sprayed as needed. Identifying a disease is required to properly treat it. For a disease to occur, there has to be a pathogen, suitable climate, and a susceptible host. Diseases occur at certain times of the season, so a seasonal chart is very helpful, along with descriptions and photos for proper identification.

Purdue offers the following Turfgrass Disease Profiles free download with a chart and disease descriptions:
https://www.extension.purdue.edu/extmedia/BP/BP-124-W.pdf

and Seasonal Activity of Turfgrass Pathogens in the Midwest:
https://www.extension.purdue.edu/extmedia/BP/BP-125-W.pdf