Whether warm-season or cool-season, many grasses are susceptible to the group of fungus that creates leaf spot diseases.
In warm-season turfgrasses, the disease is more of a cosmetic issue than a real health danger, and, to a lesser degree, the same is true in cool-season grasses, says Austin Hagan, professor and extension plant pathologist at Auburn University. But once the dark brown spots take over a leaf, it can cause it to die and eventually thin the turf. "The problem you run into when you thin the turf is you tend to have invasion of weeds, so that creates another problem," Hagan says.
Lawn & Landscape spoke to Hagan about how to identify, prevent and control leaf spot diseases in turfgrass.
How it forms. A wet-dry weather cycle produces ripe conditions for the fungi to actually fruit.
"If you have a nice, generally summer weather pattern where you get periodic showers, those are the periods when you're more likely to see leaf spot develop," Hagan says. "If you have continuous rain, you might not see as much disease as you might if you just get that rain cycle, and a few dry days and then back to rainy weather. At the same token, if it's really dry, almost nothing will happen, it will suppress the disease."
The fungi usually survive year to year in thatch or leaf debris, and under wet-dry conditions will produce spores or conidia, Hagan says. When there is a change in relative humidity, the spores move up the leaf canopy and fall on the leaf through air current.
"If you get at least 12 hours of leaf wetness, then the spores germinate and then the fungus moves into the leaf and begins to attack the leaf tissue," he says. "After about 10 or 12 days, you'll see a distinct leaf spot form in that area."
The cycle will continue to repeat itself as long as the conditions are right.
Identification. Leaf spot symptoms will vary depending on the type of grass in which the disease is found.
Generally, a dark brown margin with a tan center forms and the remainder of the leaf may have yellowing, Hagan says.
"Then, if you get enough spots on the leaves, those diseased leaves will wither and die," Hagan says. "From a distance, what you tend to see is the grass will thin out – that's a good way to describe it – or fade out."
Prevention. Over the years, leaf spot resistant turfgrass varieties – from Kentucky bluegrass to bent grass to tall fescue to Bermuda grass – have been developed and gone a long way to suppress the diseases.
"If someone was looking to establish a new turf or lawn, the ideal method of control would be, in the first place, simply to establish one of these varieties that are considered to be resistant to leaf spotting and melting out," Hagan says.
Of course, there are general turfgrass management practices that can decrease the odds of the disease forming. Hagan says those include: mowing at the proper height, ensuring optimal fertility, making multiple light applications instead of one single high-rate application, irrigating every five to seven days as long as the turf growth is maintaining sufficient water and collecting clippings.
"Normally I'm not that worried about collecting clippings, but if they do have an active disease issue, it's probably a good idea to go out there and collect the clippings," he says.
Control. There are numerous fungicides available to control leaf spot diseases in different turfgrasses. Because of the variation in grasses and geography, it's best to consult a local extension agent on what products are most effective in that area.
"The best type of program is basically a preventative program where you're out in front of the disease and you simply prevent it from developing," Hagan says.
The author is an associate editor at Lawn & Landscape. She can be reached at firstname.lastname@example.org.