Controlling Bentgrass Dead Spot


If you’re looking for some good news in the new year, look to bentgrass dead spot. If you’re looking for an authority on the turfgrass disease, look to John Kaminski, an assistant professor of turfgrass science at Penn State University.

Kaminski was a graduate student at the University of Maryland when the first samples of a strange fungus came into the diagnostic lab there. He started grad school on August 20, 1998, and the first sample came in on August 21. His advisor, Peter Dernoeden, became the official discoverer. This fungus became Kaminski’s primary focus for the next six years, because it came out of nowhere and was an immediate problem.

Bentgrass dead spot can kill a golf course green with a thousand spots, such as these seen with a golf tee for scale.
Photos courtesy of John Kaminski.
Although dead spot is a grass killer, it is now considered rare and exclusive to new greens in North America.

“It had never been found, either the organism or the disease,” Kaminski recalls. The disease first appeared in 1998 on 10 golf courses in six different states, all at about the same time. It was found only on new bentgrass putting greens, so he assumed it came in on seeds. “Actually, we never figured out where it came from.” However, the USDA found the fungus on some plants used as packing materials from Indonesia, so it was thought to be nonnative.

The fungus, Ophiosphaerella agrostis, was deadly, however. It comes from a family of fungi that cause turfgrass diseases such as spring deadspot, but it produced reproductive stages that made it difficult to kill. Turf managers were suddenly finding what appeared to be brown ball marks on creeping bentgrass greens that had just been planted. The dime-sized spots of dead turf grew to only 2 inches in diameter, but within a season there were hundreds of them on some greens.

The disease cycle took 10 to 12 days to repeat, Kaminski says, so early attempts to kill the fungus with fungicides on a 14-day schedule failed miserably; some fungicides didn’t work at all. Symptoms appeared as early as May in the mid-Atlantic region, and by the middle of summer the disease was at its most active. The brown spots first appeared on sunny, exposed surfaces and spread across large greens very quickly.

It looked like this was going to be a permanent and deadly new bentgrass disease, but it occurred only on brand-new putting greens. It also only showed up on sand-based systems, not native-soil foundations.

Kaminski and other turfgrass scientists began running tests, and over the course of a few years discovered that the fungus was vulnerable on a couple of fronts. Knowing that other fungal diseases don’t like acidic soils being fertilized with ammonium sulfate, he discovered that using ammonium sulfate as a primary source of nitrogen on new bentgrass greens was an excellent way to prevent or slow the growth of the disease once it was established. The normal fertility rate of 2.5 to 3 pounds of ammonium sulfate per year per 1,000 square feet worked fine.

“You spoon-feed it in the summer and apply it as normal in the spring and fall,” Kaminski says, recommending that the applications not be lumped all at once. Soil testing is necessary, because if the soil is too acidic—below a pH of 5.2—it would not be healthy for the plants.

Fungicides are still necessary once the fungus is established. Thiophanate-methyl (Cleary’s 3336 Plus) is Kaminski’s first recommendation, followed by a tank-mixed application of chlorothalonil (Daconil) and propaconazole (Banner Maxx). But, the timing is almost as important as the material.

Kaminski says that because of the rapid reproductive cycling of the bentgrass dead spot fungus, the thiophanate-methyl should be applied immediately upon discovering the disease in the spring, followed with the chlorothalonil/propaconazole mix within seven to 10 days.

Another good choice for the second application is boscalid (Emerald) or pyraclostrobin (Insignia), which provides a different chemistry. In fact, this is one of the few diseases that boscalid is effective against, the other being dollar spot. As for the strobilurins, pyraclostrobin is the only one in the group that is effective against dead spot.

Don Dale resides in Altadena, Calif., and is a frequent contributor to Turf. He has covered the green industry for more than 10 years.