A year ago, on August 31, 2011, I put up a post warning about Gray Leaf Spot on perennial ryegrass. This disease shows up rather infrequently in Iowa, but it does occur around Labor Day on years when temperatures are unusually high. Last year's Labor Day temperatures were in the 90's and if you check the post from September 19, 2011, you will see that it did occur on several golf courses with perennial ryegrass fairways.
This disease is caused by the fungi Pyricularia grisea. It seems to selectively hit perennial ryegrass on intensely managed areas such as golf course fairways and sports fields. Under the right conditions, it can be more devastating than Pythium blight. When it hits, it can wipe out large areas of turf overnight. Fungicides, such as Banner/Daconil will control it, but they must be applied before the outbreak. Once the disease begins, it is too late to apply.
The reason I bring this up is that we are in the same weather pattern again this year. The disease does not always occur when its hot around Labor Day, but it did last year and I would watch for it this year.
The pictures below were taken in 1998 on Willow Creek Golf Course in Des Moines and last year on Cedar Poine golf course in Boone. The dead grass is perennial rye and living grass is Kentucky bluegrass.
If anyone has an outbreak of Gray Leaf Sport this year, let me know and send some pictures.