(Megan Kennelly, KSU Plant Pathology)
If you manage perennial ryegrass you are probably well aware of the risks of gray leaf spot.I have not seen it, but I just got my first question about it for the year.
In this region the most common time for GLS symptoms to first appear is early to late August. In some years, epidemics can be severe. In other years, there is little to no GLS. The fungus is picky, with 82-90 degrees as its favorite temperature range. The fungus needs wet conditions as well. The fungus requires certain durations of leaf wetness at different temperatures. At that prime window of 82-90 degrees the infection can occur quickly.
Damage in a rye fairway, a few years ago.
GLS spores in the microscope, a few years back.
For more details and photos you can check out this page for general information:
And, here is some detailed fungicide information (scroll to page 13):