The word spring has many connotations for turfgrass managers: spring training, spring leagues and even spring ahead. Unfortunately, spring may also mean the occurrence of some not-so-pleasant turfgrass diseases. Some are worse than others in terms of damage, but all are worthy of your time and attention.
Weather sets the stage for disease
For pathogenic infection to occur, three factors must be satisfied: a favorable host (susceptible turf species and cultivars), a virulent pathogen and weather conditions that support the growth of the organisms. In most scenarios, it’s reasonable to assume that disease spores are like the common cold, present everywhere all the time. The second factor, the turf plant, can be variable depending on its inherent genetic resistance to the pathogen.
Each season brings its own distinct set of weather conditions that are foundational to the establishment of pathogenic diseases. Generally, we think of spring as cool and wet, but each time period can be unique and difficult to predict. Be sure to keep an eye on the weather forecasts for up-to-date information on the third part of the turf disease equation.
Inspection and identification
The first step in managing diseases is to regularly inspect and identify maladies that might be damaging. Routine scouting is important in early detection of symptoms. Scouting is not difficult, but it does take time and practice. Take a page out of the training manual for spotting counterfeit money: One of the most effective ways for spotting fake money is to know the real stuff so well that a fake is easy to spot. The key to knowing which causal agent is responsible for possible turf disease is to know the way that the turf is supposed to look, as well as the way it would look if infected, so well that it’s easy to tell when it’s starting.
In addition to actual pathogenic foliar diseases that can lead to damage, “mimics” can cause similar symptoms. If you can accurately identify such causes as winter desiccation, water and ice damage, frost injury and various insects, success will be more likely. These maladies often appear to be pathogenic 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 spring diseases of turf to help with distinguishing between them and counterfeits.
Pink snow mold is caused by prolonged periods of cool, wet weather, especially during conditions where temperatures are between 32 and 45 with light drizzle or fog. Heavy, wet snows that fall on unfrozen turf can cause severe infection. On closely cut turf, roughly circular, rusty brown patches ranging in size between 6 inches and 2 feet appear. On higher cut turf, more or less circular 4 to 12-inch spots occur, with the grass being matted and bleached. Under moist conditions, a white to salmon-pink (hence the name, pink snow mold) moldy growth is visible at the edges of the patches. These patches are often scattered in a random pattern, where the contrast in color between the dormant or greening turf and diseased spots helps to detect the disease.
Gray snow mold infects turf in the late fall but, unlike pink snow mold, is more likely to become damaging if extended periods of snow cover and freezing temperatures characterize the winter period. Symptoms are usually more pronounced when snow has drifted or piled and is slow to melt in spring. Gray snow mold expresses itself as rough looking, bleached tan areas up to 1 foot in diameter. When wet, a whitish-gray moldy growth may cover the turf. Upon close inspection, small orange to black sclerotia or fungal storage organs may be visible.
Control of snow mold injury can be prevented through good management practices and judicious use of fungicides. Fertilization with slow-release sources is preferred to using quick-release forms, such as ammonium nitrate and urea. Installation of snow fences can prevent drifting onto high-value turf areas or ones with a history of previous injury.
Preventative fungicide application for pink snow mold include products containing azoxystrobin, chlorothalonil, fenarimol, copper hydroxide + mancozeb, iprodione, PCNB, propiconazole, pyraclostrobin, thiophanate-methyl, trifloxystrobin or combinations will assist with control. Iprodione and chlorothalonil combinations or PCNB offer good control for gray snow mold. An initial application can be made in the fall, soon after the last fertilization of the year, and repeated as necessary during winter thaws. Rescue treatments in the spring may partially arrest further development, but are often ineffective.
Leaf spot/melting out
The leaf spot pathogens Bipolaris sorokiniana and Drechslera poae are favored by cool, wet weather. Under these conditions, large numbers of spores are produced and spread through the turf. The movement to new growth is enhanced by wind, mowers, foot traffic, dragging hoses, splashing water and infected grass clippings. Once transmitted, 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 takes on a thin yellowish cast.
Incorporation of improved cultivars and usage of other good management practices will go a long way towards 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 fungicide containing azoxystrobin, chlorothalonil, fludioxonil, iprodione, mancozeb, trifloxystrobin or vinclozolin beginning in early spring. Two or three additional applications, spaced three weeks apart will improve chances of success.
Powdery mildew develops in areas of moderate to dense shade. Poor air movement across the turf enhances the development of the disease. Unlike most foliar diseases, moisture on the turf blades is not required for infection to occur. Moderate temperatures, high humidity and cloudy weather are the conditions that favor its development. Turf growing under shade trees and along the north and east side of buildings are especially likely to become infected.
The surface of the grass blades that are infected with powdery mildew appear as if they’ve been dusted with flour or lime. The white leaves later turn yellow and finally tan to brown as they die from the disease. Susceptible cultivars can be severely thinned as a result of a heavy infection.
Powdery mildew can be controlled by selecting and utilizing cultivars that have a high degree of resistance. In some situations, tree or building removal may be an option. Tree pruning is not a feasible treatment option. In order to achieve enough light and air penetration through a tree to the turf to discourage powdery mildew, severe pruning is required. This is usually not desirable for at least two reasons. First, this severe pruning is harmful to a tree, depleting carbohydrate reserves and creating an undesirable tree architecture that is more susceptible to solar and wind damage. Second, the effects are short term, in that within a few years the tree is likely to regain the same density as before.
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 propiconazole or triadimefon work well on powdery mildew. Powdery mildew may also occur in the late summer and fall if conditions are favorable.
Stripe smut is a cool-weather disease that commonly infects fields in spring and sometimes in autumn. 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 grey, and then to black. In time, the infected leaves shred and curl. When this occurs, masses of spores are released that 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 towards the control of stripe smut. The fungicides benomyl, thiophanate-methyl, fenarimol, propiconazole and triadimefon are registered for stripe smut.
Red thread/pink patch
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 the use of reduced nitrogen rates to save maintenance dollars. 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 can develop that bind the leaves together.
Controlling red thread involves maintaining a balanced fertility program, collection of grass clippings and application of fungicides. On turf with a history of previous damage, begin treatment prior to the onset of symptoms. Azoxystrobin, chlorothalonil, flutolanil, iprodione, polyoxin D, propiconazole, pyraclostrobin, and triadimefon containing products are efficacious on red thread.
John Fech is an extension educator specializing in turf and ornamentals at the University of Nebraska-Lincoln. Roch Gaussoin is a professor of horticulture and extension turf specialist at the University of Nebraska-Lincoln.