Above: drought-stressed turf may appear to mimic disease or insect damage
As an Extension Educator, I have been called upon from time to time to help homeowners understand what is causing their lawns to look sick. Sometimes, we can get to the root of the problem right away, and other times the factors causing unthrifty grass are not easily defined.
First of all, understand that the grasses most of us grow in our lawns are cool-season grasses, such as Kentucky bluegrass, tall fescue, or perennial ryegrass. This means that they grow very well in spring and fall, but can be stressed by summer heat, lack of adequate rainfall, and other factors.
So, a lawn can begin to look sick as a reaction to stressful environmental conditions, and not necessarily a disease or an insect.
With that said, there are several turfgrass diseases of concern. Each has its typical season of injury, and most prefer a certain species of turfgrass.
Lee Miller, Purdue assistant professor of botany & plant pathology specializing in turfgrass diseases, recently wrote, “The two most common diseases on cool season lawns are dollar spot and brown patch,” he said. “Dollar spot occurs most frequently on Kentucky bluegrass lawns, while brown patch is one of the few diseases that affect tall fescue.” Both diseases form localized dead spots in turf.
Miller said that knowing if your lawn is predominantly Kentucky bluegrass or tall fescue provides good evidence as to what disease may be affecting it.
“Kentucky bluegrass spreads by rhizomes and the leaf is smoother with a single translucent mid-vein that runs down the leaf center,” he said. “Held up to the sunlight, the translucent mid-vein on Kentucky bluegrass is conspicuous.” He adds that Kentucky bluegrass also has an infamous boat-shaped leaf tip, whereas tall fescue comes to an abrupt point.
“Tall fescue is a bunch type grass, meaning a single plant does not spread and make daughter plants from ‘runners’ (a.k.a. stems called rhizomes [underground] or stolons [above ground]),” he said. “Tall fescue is characterized by relatively thicker leaf blades with prominent, evenly spaced, rough feeling veins running down the leaves.”
He said that leaf symptoms provide additional confirmation of the identification and activity of these two diseases. Along the edges of a patch or spot, drive by sweep a handful of leaves. Using a hand lens, look closely at the lesions on the leaf blade. “Brown patch is characterized by a dark, irregularly shaped margin with a beige to straw interior,” he said. “Dollar spot will have a slightly less dark margin, and the lesions often characteristically drive straight across the leaf blade causing an hour-glass shaped lesion.”
Miller said that for both diseases, leaf wetness is a key driver of the disease epidemic, so reducing early morning shade and limiting irrigation to the early morning hours rather than at dusk can help decrease disease severity. “Dollar spot is a low nitrogen disease, so when occurring on Kentucky bluegrass it is an indicator that nitrogen is deficient in the plant and fertilizer should be applied,” he said. “Brown patch occurrence on tall fescue doesn’t necessarily indicate a nitrogen excessive or deficient state.”
“If a fungicide is warranted, which is rare in most cases, the active ingredients azoxystrobin or pyraclostrobin are most effective against brown patch but have little to no effect on dollar spot,” he said. “Along with fertility, dollar spot can be suppressed by several other different fungicide chemistries.” He concluded his comments on management by saying that fall overseeding can often be used to simply fill in the gaps left by these diseases, perhaps with better new improved cultivars.
Some typical insect damage we see with turf can stem from below-ground insects and above-ground insects. The primary sub-terranean insect of concern is white grubs. They eat grass roots and form patches of dead turf. Above-ground insects include armyworms, cutworms, sod webworms, billbugs, and chinch bugs. Find out more in the Purdue Extension publication, “Integrated Management of Turfgrass Insects at www.edustore.purdue.edu.
Find Miller’s original article entitled, “Know thy Host, Know thy Disease,” at the Purdue Landscape Report website: https://www.purduelandscapereport.org/.