Bacteria?
Twenty years ago, there was a bentgrass variety called Toronto or C-15, which had been used on many courses in the 1960's and 1970's. All of a sudden, in the mid-1980's, it just started puking and dying, and the researchers had no idea why. They called the problem "bacterial wilt", but I don't know if they ever really identified a pathogen or not, and the problem was limited to that one cultivar of bentgrass. So, eventually, all of the courses affected re-grassed their greens with newer varieties.
That episode is the reason some of us are reluctant to use new cultivars of grass for our greens ... many of them haven't been in existence very long, and who knows when another such problem will arise in a new grass after a few years' time?