26/07/2025
ICYMI: Latest turf news: A model of evolution, gray leaf spot could be coming soon to a course near you: It is not uncommon for those who kill grass for a living to have a certain fondness for turfgrass diseases.
Nathaniel Mitkowski, Ph.D., professor of plant pathology at the University of Rhode Island, is no exception. If Mitkowski had to pick a favorite it would be gray leaf spot. And, as a model of evolution, it soon could be coming to a golf course near you.
"Frankly, it's my favorite disease, because it's the easiest one to identify," Mitkowski said during July 24 in a TurfNet webinar on managing and preventing summer stress.
"I said some diseases are killers and some diseases are just aesthetic. Well, this one is a killer. It moves very quickly."
Caused by the fungal pathogen Pyricularia grisea, gray leaf spot is a foliar disease that affects perennial ryegrass and tall fescue, and also can persist in some warm-season grasses.
The disease, which often presents in late summer or early fall, first infects and kills leaf blades and can spread to the crown, resulting in plant death, according to Purdue University research. Severe cases can cover wide areas of turf.
It can spread quickly under hot, humid conditions like those that have dominated this summer, Mitkowski said.
"At this point, I have not seen any gray leaf spot in New England or the Northeast, but I am sure it is coming because we are looking at some hot weather," he said. "We are looking at some humid weather.
"Gray leaf spot rolls through just about every summer."
Early symptoms often resemble drought or heat stress and can manifest as small, water-soaked lesions which become necrotic, according to the University of Massachusetts.
Gray leaf spot infects and kills leaf blades before moving on to the crown and killing the plant. It is found most often in perennial ryegrass and tall fescue as well as in some warm-season grasses.
Affected leaves can become twisted and covered with grayish spores. Disease can develop rapidly under hot, humid conditions.
Mitkowski said outbreaks of gray leaf spot almost routinely accompanied hurricane season in the Northeast.
"Within three days of the first hurricane rolling through, gray leaf spot would explode, and that was usually September," he said.
Warmer winters have changed that, he said. According to the National Weather Service, the 10 warmest years on record in the U.S. (compared to the average temperature from 1900 to 1999) all have occurred since 1998.
"Because we have warmer winters, we have warmer growing seasons," Mitkowski said. "This is a disease that has taken advantage of warmer climates. It now overwinters. The winters don't usually kill it. It survives the winter, which means it's gonna start earlier."
With turf diseases it is always better to prevent them than treat them, and because gray leaf spot is so destructive it is no exception.
Mitkowski recommends preventive applications of DMI fungicides. However, not all DMIs are created equally.
"I would avoid the strobilurins, because resistance is pretty much 100 percent widespread through the populations of gray leaf spot. DMIs are your best option," he said.
"There are a lot of DMI fungicides out there. They are not all the same though. So, if you are going to go and manage your gray leaf spot, if you are going to put out a preventative material, make sure that the DMI is labeled for it, because not all of these DMIs will work against gray leaf spot, which is unusual because we typically think a DMI is a DMI is a DMI, but that's absolutely not true. Every DMI is a little different and some work better on some things and others work better on other things."
Researchers often suggest regrassing susceptible or affected areas with a turfgrass variety that is naturally resistant to gray leaf spot.
"Plant resistant varieties, because they're the best way to solve the problem," he said.
"If you are growing ryegrass, you should be growing gray leaf spot-resistant ryegrass. There is a lot of it available, and you should never in the Northeast grow anything that is not GLS-resistant. . . . If it (is resistant), you don't have to worry about it. It works."
It is not uncommon for those who kill grass for a living to have a certain fondness for turfgrass diseases. Nathaniel Mitkowski, Ph.D., professor of plant pathology at the University of Rhode Island, is no exception. If Mitkowski had to pick a favorite it would be gray leaf spot. And, as a model ...
