Hello Richard,

What you are seeing is likely a fungus. We have had favorable conditions for fungal growth for several weeks now.

It is nearly impossible to identify a fungal infection from photos alone. From what I can see, I would guess a patch disease.

In order to identify what organism is doing the damage, you may want to send a sample to the MSU Diagnostic Lab for evaluation. They could provide a positive identification of the disease that is affecting your lawn. Information about the Diagnostic Services and fee schedule can be found at: https://www.canr.msu.edu/pestid/ Suggestions and tips about how to take a good sample can be found at: https://www.canr.msu.edu/pestid/submit-samples/ If you choose to send a sample for identification, I would suggest taking a sod sample including infected and unaffected grass plants. This allows the lab to compare the healthy grass and the diseased grass.

From your picture, the condition of the grass is consistent with brown patch disease. This is caused by a fungus and generally affects the grass blades and the tillers (spreading mechanisms) but does not damage the crown of the plant. Once the conditions for fungus growth are no longer optimal, the grass plants will regrow, but the affected area is left thin and sparse.

Here is a good article about patch diseases. Homeowner action is summarized at the end of the article. https://www.extension.purdue.edu/extmedia/bp/bp-106-w.pdf

The best cultural care is to avoid fertilization during the summer months and water early in the day so that the grass plants are dry by dark.

Having the disease mechanism identified by the Diagnostic Lab will give you the best information to develop a treatment plan. If you have the pathogen identified and would like specific treatment plans, please write back once you have an identification.

Hope this helps.