The return of cool weather in fall means good news and bad news. The good news is that the number of days spent working in the hot, blazing sun decreases. The bad news is that along with more hospitable working conditions comes a whole bevy of fungal diseases that can cause problems for sports turf managers. The best place to start when dealing with fall turf diseases is following a step-by-step approach to identification.
Start the process by first evaluating the entire landscape to identify how the problem spots are distributed. The next step is getting down on your hands and knees, using a hand lens to get a closer look at any unusual blotches or markings on leaf blades, roots and crowns. In addition to leaf symptoms, because most turf diseases follow a specific pattern, make notes about general appearances such as round or oval patches of browned turf.
Recall the history of the field, both in the past month and this time last year. History can play a big role in disease management and give you further clues to aid your diagnosis. In addition to actual pathogenic foliar diseases that can lead to damage, “mimics” can cause similar symptoms. If you can identify such causes as heat/drought stress, compaction injury and various insects, success will be more likely. These maladies often appear to be foliar diseases at first glance. Correct identification of a problem will help avoid unnecessary spray applications, saving money and time. Following are the specifics of common fall diseases of turf to help distinguish between them and counterfeits.
Stem and leaf rust
The severity of rust outbreaks varies from year to year, usually occurring from late summer to midfall following hot, dry periods when the growth rate of the turf has slowed. Turf injury becomes severe when poor growing conditions reduce turf vigor. Warm days and cool to moderate nighttime temperatures along with long morning dew periods or nighttime irrigation create optimal growing conditions for the fungi. In addition, low-fertility turf will typically have more rust.
Heavily rusted turf appears yellow or orange when viewed from a distance. In severe infections, small clouds of rust spores can discolor shoes, mowing equipment and pant legs. Close examination of rusted leaf blades reveals the presence of the orange to brick red pustules, which rub off easily when touched. Each rust pustule produces a vast number of spores, each one capable of infecting a grass blade. New infections can occur about every seven to 10 days. Infected turf becomes weak and thin, predisposing it to winter injury and other abiotic stressors.
Controlling rust begins with the use of improved, rust-resistant turf cultivars. Maintaining turf in a vigorous, but not lush, condition through balanced fertilization, aeration and early morning irrigation will limit the severity of injury from rust. For most rust-infected turf, a light application of nitrogen made at the onset of symptoms will result in a marked improvement. On turf with a history of stem rust, judicious applications of preventative fungicides containing azoxystrobin, chlorothalonil, mancozeb, propiconazole, pyraclostrobin or triadimefon are an important part of an overall rust management program.
Leaf spot of tall fescue.
Leaf spot/melting out
The leaf spot pathogens, Bipolaris and Drechslera spp., are favored by cool, wet weather. Under these conditions, large numbers of spores are produced and spread through the turf. The spread to new growth is enhanced by wind, mowers, foot traffic, dragging hoses, splashing water and infected grass clippings. Once dispersed, the spores germinate within a few hours, symptoms appear, and a whole new crop of spores can be produced in seven to 10 days.
Early symptoms of leaf spot are small purple to black spots on the leaf blade. As the disease progresses, round to oval spots with buff colored centers develop. In some cases, they are outlined or surrounded by a dark brown margin. After several weeks of infection, the disease progresses to a “melting out” stage, where infected leaf sheaths turn a uniform dark chocolate brown, causing leaves to yellow and drop from the plant. From a distance, the stand appears thin and will have a yellowish cast.
Incorporation of improved cultivars and usage of other good management practices will go a long way toward control of leaf spot. Thatch management, watering in the morning hours and avoiding fertilization procedures that encourage lush turf growth are some of the most important ones.
When necessary, apply a preventative fungicide. Products containing azoxystrobin, chloroneb, chlorothalonil, fludioxonil, fluoxastrobin, iprodione, mancozeb, maneb, myclobutanil, PCNB, polyoxin D, propiconazole, pyraclostrobin, thiophanate-methyl, trifloxystrobin, vinclozolin or combination products containing one or more of these active ingredients should be applied beginning in early spring. Two or three additional applications, spaced three weeks apart, will improve the chance of success.
In shady sections of a home landscape, powdery mildew can become a problem on both turf and ornamentals. In most cases, it’s a matter of a violation of the tried and true landscape theme of “right plant, right place.” Fortunately, most sports fields are not located near large shade trees; however, this is not true in all cases.
City parks and community ball fields are locations where powdery mildew can be problematic due to the surrounding landscape. Of course, shade is a double-edged sword. Turf managers may encounter an increased incidence of certain diseases due to heavy shade, while spectators are grateful for its presence, especially on a hot summer day as they watch their youngster play baseball.
The disease is quite aptly named, in that a white powdery substance becomes coated on leaf surfaces as if a baker had dusted them while coating a pan with flour. Obviously, this becomes aesthetically unappealing, but also a concern in that the growth of the fungi blocks sunlight reception by the chloroplasts and, as a result, photosynthesis is reduced.
Powdery mildew leaves appear as if they have been dusted with flour.
PHOTOS COURTESY OF UNL UNLESS OTHERWISE NOTED.
Fungicides can be applied to suppress powdery mildew, particularly in areas that are prone to infection. One or two applications are usually sufficient if applied before the onset of symptoms. Products containing fenarimol, myclobutanil, propiconazole, pyraclostrobin, thiophanate-methyl, triadimefon or combination products containing one or more of these active ingredients are effective.
Outbreaks of red thread are caused by cool, moist conditions, especially on turf that is under stress or slow in growth due to low fertility or low temperatures. It can be a serious problem on low-budget fields due to a trend toward reduced nitrogen rates. Drizzly days and temperatures in the 50s or 60s are conducive to the development of the disease. As with many other diseases, it can be spread by equipment contaminated with the causal fungus.
Symptom patterns are circular to irregular patches of turf with a pinkish to tan cast. Often, a ragged appearance develops due to the intermingling of infected turf plants with others. Infected leaves die from the tip downward. Damage is primarily confined to the leaves and leaf sheaths. During continuous wet weather, pink gelatinous mycelium that binds the leaves together can develop.
Controlling red thread involves maintaining a balanced fertility program, collection of grass clippings and application of fungicides. On turf with a history of previous infection, begin treatment prior to the onset of symptoms. Products containing azoxystrobin, chlorothalonil, fenarimol, fluoxastrobin, flutolanil, iprodione, mancozeb, maneb, polyoxin D, propiconazole, pyraclostrobin, triadimefon, trifloxystrobin, vinclozolin or combination products containing one or more of these active ingredients have shown good to excellent suppression of red thread.
Stripe smut is a cool-weather disease that is prevalent in spring and fall. Temperatures in the 50 to 65-degree range favor its development. The spores that perpetuate the fungus reside in the thatch and can be spread by wind, rain and traffic. Infection causes the turf plants to take on a general stunted and yellowed appearance. The key indicator of the disease is the presence of stripes of color that run vertically along the turf blades. At first, the stripes are yellow, then turn gray, and then turn to black. In time, the infected leaves shred and curl. When this occurs, masses of spores are released and can fall to the thatch or to nearby plants. Death of infected plants is inevitable.
Unfortunately, adherence to good cultural practices has little suppressive effect on the development of stripe smut. Selection of resistant turfgrass cultivars and application of appropriate fungicides will go a long way toward the control of stripe smut. Products containing fenarimol, myclobutanil, propiconazole, thiophanate-methyl, triadimefon, or combination products containing one or more of these active ingredients are critical to successful strip smut suppression. Stripe smut can be increased in severity with applications of chlorothalonil or thiram.
John Fech is an extension educator specializing in turf and ornamentals at the University of Nebraska-Lincoln. Dr. Loren Giesler is an extension plant pathologist at the University of Nebraska-Lincoln. He works with the green industry to deliver research-based information to assist with their management decisions.