Going against the gray

Gray leaf spot, a fast-moving disease that is hard to diagnose in the field because of its resemblance to other foliar diseases and stress caused by heat and/or drought, can cause extensive damage to delicate golf course turfgrass.

Nathaniel Guldseth of International Country Club in Fairfax, Va., said the disease never arrives at his course before July, but can be active through October. He has been told by researchers that the fungal spores get blown into his region from tropical storms coming from the Deep South.

“It looks similar to other leaf spots, but its lesion does acquire an hourglass shape that helps to ID it,” Guldseth said of diagnosing gray leaf spot. “We always bring in a specialist if we have a true concern though, just to be safe.”

Guldseth has a standard strategy in combating the disease, he said.

“Chemical treatment for us starts with a strobilurin fungicide (Heritage), right before the 4th of July, followed three to four weeks later by a DMI fungicide (Tourney for us) or a spray of thiophanate-methyl (Cleary’s 3336) and then another three to four weeks later by another strobilurin (Insignia),” Guldseth said. “Resistance management is very important for gray leaf spot. The ‘strobies’ are the best, but if you use them consecutively you run the risk of resistance. 

“We also always include chlorothalonil in these sprays (Daconil), which is a multi-site fungicide with very good contact control of gray leaf spot and low chance of resistance,” he added. “As with all diseases, we shorten the window between sprays if conditions are ripe. For gray leaf spot, that would be humid days and nights with temperatures ranging from 80 to 90 degrees.”
 
Gray leaf spot generally starts in tall grass and moves toward the lower heights of cut, according to Guldseth. 

“This means that we actually keep our fairways relatively low, about .375,” he said. “For us, raising the mowing height is only an option when we really thin out for some reason. We also use very little nitrogen in the summer. This helps keep many diseases in check - gray leaf and Pythium mostly for ryegrass. We are also always trying to improve drainage and keep the leaves as dry as possible by either dragging them or blowing them off.”

While Guldseth said avoiding the disease requires a chemical strategy, he added that it helps to use gray leaf spot resistant varieties of ryegrass.

“We use mostly Paragon here and it works great,” he said of International. “If you do not have the money to spray the fungicides, then you should definitely seed to a GLS-resistant variety.”

Having a strategy of cultivation practices and fungicides gives the best coverage, Guldseth said. Start by using a resistant variety of turf, and spray at least one application of Heritage as well as Daconil through the summer. Avoid overwatering and use low nitrogen rates during the hot months. Also, never hold off on getting the turf tested.

“Bring in a disease specialist if you have any doubts,” he said. “Then you don’t waste money on a useless spray.”