From the September 21 Turf iNfo, the University of Nebraska’s blog:
We have received numerous reports from golf courses of severely thinning and dead turfgrass following the record rains in early September. The thinning is occurring on stands of perennial ryegrass and older varieties of Kentucky bluegrass. Leaf spot/melting out is the primary cause on the bluegrass while gray leaf spot is the primary culprit on the ryegrass. See this Turf iNfo from August for more information about GLS. Areas that don’t receive regular fungicide applications have been hardest hit. Read more and see pictures here