Those of you who have been around since the 90's will remember a major outbreak of Gray Leaf Spot on perennial ryegrass in 1998. This disease is caused by the fungi Pyricularia grisea. It seems to selectively hit perennial rye. Under the right conditions, it can be more devastating than Pythium blight. Like Pythium, it can kill large areas of turf over night.
In 1998, it wiped out perennial rye from Chicago to the Rocky Mountains. It was particularly hard on ryegrass golf course fairways and sports fields.
The conditions that set it off in 1998 were unusually high temperatures around Labor Day. We have not seen a major outbreak of this disease in 13 years, but tomorrow it is predicted to reach near 100 F. That is very hot for Iowa in September. If you have perennial rye that has not been treated with a fungicide, you may wan to be on the look out for this problem. Banner/Daconil was particularly effective against Gray Leaf spot in the 1998 outbreak, but there are other materials labeled for it as well. Check the label before you spray.
The pictures below were taken in 1998 on Willow Creek Golf Course in Des Moines. The dead grass is perennial rye. The only living grass is Kentucy bluegrass that was intermingled in the fairway and some patches had been used to sod around some of the heads.
Let me know if any of you see this in the next few days. Send some pictures if you do.