Part 1: Turfgrass pathogens
It is hard to say what a new disease is when we don't know what happened 100 or 1,000 years ago. What we do know is that new, emerging and re-emerging diseases result from a multitude of influences, including cultural practices, pesticides and plant genetics. These diseases keep turfgrass managers on their toes and make management challenging. Following is a discussion of three such diseases that researchers have discovered or extensively studied over the past several years. Control tips are also included if you encounter them in your turfgrass.
Gray leaf spot
Gray leaf spot (GLS) on perennial ryegrass is one of the newest diseases in the cool-season regions of the United States. So far, no one has found it anywhere else in the world. Since its discovery, researchers have intensively studied this disease, but we still don't completely understand it.
In the past, we thought gray leaf spot occurred only during hot, humid weather. However, the Midwest experienced relatively mild weather last year, and the disease appeared during cooler, wet periods that followed a few hot, dry days.
Golf course superintendents often see this disease following aerification and overseeding, even during cooler fall temperatures. Researchers have seen this disease infect untreated immediate roughs and spread directly into treated perennial ryegrass fairways.
In addition to the confounding factors of environment and pathogen, turfgrasses show little or no resistance to this disease. Fortunately, even if GLS eventually moves in, perennial ryegrass can be re-established quickly, and it performs well throughout most of the growing season.
So far, I have not seen extensive damage on low-maintenance sites. Sparsely irrigated, low-fertility sites such as most public parks, home lawns and private establishments are not targets for this pathogen. We have seen it more in highly maintained golf course settings.
Look for GLS from mid-summer through fall in cool-season growing regions. You will first notice a smoky appearance similar to brown patch. In the early stages of development, look for gray to black masses of spores on the leaf surface. Also, look for twisted, straw-colored grass tips.
In later developmental stages, look for yellowing, water-soaked leaf blades that resemble symptoms of Pythium. However, look on the outer fringe of the leaf-spot patches for black spores. These spores indicate that it is gray leaf spot. Use a microscope to definitively diagnose GLS. Look for flame-shaped conidia (asexual spores) — they are easy to spot.
The disease can be spread by wind, splashing rain, mowing and traffic. Plant death can occur within 48 to 72 hours. Seedlings are especially susceptible when planted in infected areas. Take necessary precautions to limit the spread of this disease and to successfully reseed damaged areas.
It isn't clear whether cultural methods can help control this disease. In general, we expect to see less damage under low-intensity management, but this has not been scientifically proven. Apply fungicides to achieve the most effective control. Strobilurin chemistries and thiophanate-methyl are effective for gray leaf spot control. (The Fungicide Update on page 38 lists more controls.) Apply fungicides with enough water to penetrate the leaf canopy unless label directions dictate otherwise. Breeders are developing resistant varieties. They will likely be the best way to control this disease in the future.
Localized dry spot
Localized dry spot is not a new problem, but researchers have extensively studied it in recent years. Turfgrass managers are commonly plagued by turf that won't recover when given modest amounts of water. Because many factors may be involved, the problem is difficult to pinpoint.
Often, obvious, physical, non-pathogenic factors account for this problem. Tree roots can out-compete turf in certain locations, and mounds and ridges are prone to dry spots, particularly on southern and southwestern exposures. Also, in low areas, algal crusting, poor soil structure, shallow root zones and excess thatch create conditions that inhibit efficient use of water.
Basidiomycete fungi that cause fairy rings are also associated with localized dry spot. These fungi decompose organic matter in the soil and are beneficial to the ecology of the turfgrass rhizosphere. However, they can also create hydrophobic (water-repelling) conditions in the soil. The fungi release organic substances that coat soil particles, causing them to bind together. Water does not adhere to the soil particles, and it rapidly passes through the soil.
In the past, managers completely excavated these sites. Now, we feel it's best to use the cultural methods mentioned above combined with fungicides. You can also apply wetting agents. They act like mild detergents to dissolve hydrophobic substances. Use a 5-gallon bucket with small holes drilled through the bottom to apply wetting agents overnight. By using this method you will not disrupt daytime activities.
Ophiosphaerella dead spot
In the fall of 1998, researchers at the University of Maryland identified this new disease on creeping bentgrass. It appeared in Maryland, Virginia and Ohio with symptoms similar to dollar spot. Researchers subsequently called it “ball-mark disease.” The causal organism is in the genus Ophiosphaerella.
You will first see this disease appear as small spots, 1- to 2-centimeters in diameter. As the symptoms progress, the spots may eventually reach 8 centimeters in diameter. You will recognize symptoms similar to spring dead spot. However, you will not see the characteristic dark strands of spring dead spot on the root surface. When diagnosing for Ophiosphaerella, look for reddish-brown spots around the periphery of the infected turf patch. You may also see pinkish mycelia on newly infected leaves. At this point, few reports on the epidemiology, symptomatology and control of this disease are available. However, the next time you see symptoms of dollar spot, don't automatically assume that it is. It may be ball-mark disease.
Dr. Andy Hamblin is an assistant professor at the University of Illinois, (Urbana-Champaign, Ill.).
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