Turf won’t standup so I can mow it
Am I doing something wrong with fertilizer?
Clinton County Michigan turfgrass
Hard to know from pictures. Appears to be fungal activity but also could be burn from too much fertilizer. If fungal, fertilization usually helps greatly to correct with time. Diagnostic Services could help to diagnose if it is disease from a sample you send and for a small fee. https://pestid.msu.edu/
Otherwise consider involving a professional lawn care company for at least an