Pathogens that are responsible for causing diseases in turf tend to be thought of as agents of infection for only one part of the plant. Foliar diseases like ascochyta leaf blight and dollar spot are maladies that can cause significant aboveground injury, but rarely cause the demise of the entire turf plant. However, for other diseases, the initial infection is root system oriented, but because of the importance of their function, all plant parts may eventually succumb.
Many well-documented research studies indicate there is a direct relationship between the height of cut and the depth of the root system: the higher the height of cut, the deeper the root system will grow. The inverse is also true. Because of the low height of cut on most fields, shallow roots are the norm.
Roots function by providing anchorage and extracting soil moisture and nutrients. If damaged or limited, in concert with the short depth of rooting, turf plants usually respond to infection of the root system by going into severe stress. Sure, the shoots are important for making sugars and carbohydrates, but deterioration of the root system is more impactful.
How do you know you are dealing with a root-damaging disease? Stem rust is another leaf disease that produces symptoms on leaf blades: orange to red pustules covering leaf surfaces. Diagnosing root system diseases is usually more difficult, as the symptoms are less distinct.
While each disease or causal agent produces its own set of unique symptoms, in general, the aboveground symptoms are that of off-color plant tissues, sometimes in characteristic patterns and sometimes not. Most appear softened, flattened or lackluster. When you observe this from above, you’ll know it’s time to dig.
In order to gain more information, soil excavation is necessary to obtain a full picture of the problem at hand. In other words, there’s no other way to know for sure than to dig and look closely. Fortunately, the tools needed for such inspection are inexpensive and usually already in your shop. For the most part, a fishing knife, sod spade and a shallow bucket will facilitate the gathering of information about root issues.
Once the roots are unearthed, general belowground symptoms can be characterized as well. Most problems produce softened, flaccid roots that are brown to black in color. Healthy roots should be white and relatively firm, with abundant root hairs visible. While inspecting the roots, take note of color, size and the extent of the root system.
Summer patch and necrotic ring spot
Summer patch and necrotic ring spot are two of the most damaging of all diseases. Generally, necrotic ring spot occurs during the spring and fall, while summer patch symptoms appear in summer. Symptoms of these diseases are quite difficult to distinguish between, yet can be identified in the laboratory.
Affected turf shows 6 to 12-inch circular, semicircular or serpentine patches of light tan grass blades. A “pockmarked” or doughnut-shaped appearance often develops, with brown, matted turf plants, containing a tuft of apparently healthy turf in the center. This is sometimes referred to as the frogeye effect. Nearby plants on the edges of the patches are commonly unthrifty due to slowed function of the roots, caused by either disease.
The most effective control measures for these fungal diseases are to avoid stressors such as compaction, heavy nitrogen fertilization, excessive irrigation and ultra-close mowing. These factors limit growth of the root system, a part of the plant that becomes limited after infection. Products containing azoxystrobin, fenarimol, iprodione, myclobutanil, propiconazole and thiophanate-methyl have been shown to be effective on summer patch and necrotic ring spot if timed correctly.
Spring dead spot
Injury from spring dead spot produces roundish patches of dead looking turf and can range from 6 inches to 2 feet in diameter. Spring dead spot is commonly only found on bermudagrass. In severe cases, the spots will merge, creating larger, irregular patches in the turf. The affected turf appears brown to black in color; in most cases the center of the patches will be slightly sunken. Spring dead spot is aptly named, as the infected turf is actually dead, requiring either replacement or encouragement to fill in from the edges. Because of its high degree of tolerance to the spring dead spot pathogen Poa annua commonly is the first plant to reestablish in the patches of dead turf.
Various research studies have revealed that a low to moderate degree of control may be achieved with fungicides alone, but that an acceptable level of control is only achieved with a combination of cultural practices and well-timed fungicide product applications in an integrated approach. The important factors of consideration include mowing height, fertility, aerification, thatch management, pH and fungicide use. Other factors, such as compaction and cultivar, can also influence the virulence of the fungus.
Leaf spot/melting out
Even though it starts out as a leaf disease, the advanced stage of this malady is characterized by a necrosis of the crown and root system. 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 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 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 the chance of success.
During cool, wet conditions in spring and fall, take-all patch may infect turf and produce depressed circular patches of blighted turf. The patches range from 6 inches to 2 feet in size and usually appear bronze to reddish-bronze in color. In some scenarios, the frogeye pattern may develop, which can make it quite difficult to distinguish take-all patch from necrotic ring spot. Shortly after the infection period, roots begin to soften and lose function.
High pH soils have been linked to the development of take-all patch. Maintaining pH levels below 6.5 and using a balanced fertility program will lessen the effects. Applications of sulfur or ammonium sulfate may be necessary as dictated by soil test results. Products containing azoxystrobin, fenarimol, fluoxastrobin, hydrogen dioxide, propiconazole, pyraclostrobin, triadimefon and triticonazole have a good to very good effect on take-all patch.
Caused by several species of the Pythium fungi species, Pythium blight is caused by wet, saturated soil conditions. The responsible fungi live in the thatch and upper soil layers. A wet turfgrass canopy also leads to the development of damage, as do conditions of high relative humidity and warm nighttime temperatures. Early symptoms are small, roughly circular reddish-brown spots that appear suddenly in the turf. The onset of symptoms is rapid during hot, humid weather. When dew is present, infected leaves are water-soaked, slimy to the touch and may contain a fungal mycelium that resembles a fluffy cotton ball. There is sometimes a fishy odor associated with the pathogen. Pythium blight is most easily diagnosed in the early morning.
Management techniques that promote good surface drainage are helpful in control. Fertilizing in a manner to avoid flushes of growth and thinning nearby ornamental plantings (especially tall, dense canopy trees when feasible) to increase air movement over the turf and redirecting traffic from wet turf are other helpful practices. There is a laundry list of active ingredients labeled for Pythium blight and/or Pythium root dysfunction. Products or combination products containing azoxystrobin, chloroneb, copper hydroxide + mancozeb, cyazofamid, ethazole, fluoxastrobin, fluopicolide +propamocarb, fosetyl-Al, mancozeb, mefenoxam, metalaxyl, phosphite (salts of phosphorous acid), propamocarb and pyraclostrobin have good to excellent activity depending on application timing and formulation.
While not a fungal, viral or bacterial disease, nematodes are microscopic organisms that feed on roots, producing symptoms similar to other root maladies. Plant pathogenic nematodes that attack turfgrasses live in thin water films on soil particles. Aboveground symptoms caused by root-feeding nematodes are a nondescript yellowing and thinning of the turf, which can commonly be confused with damage from other causes. Belowground symptoms are more telling; roots appear stunted, with many developing galls, knots or lesions. Others develop an excessively branched root system or combination of these symptoms.
Control of nematodes is complicated. Positive preventative effects from following sound cultural practices have not been well documented. The decision to apply a nematicide should be based on a comparison between the baseline number of nematodes, the number actually present and accepted threshold levels-the minimum population required to cause economic and aesthetic damage. Not all nematode species are equally damaging, nor are all species of turf equally tolerant of damage. When sampling for nematode damage, it’s a good idea to collect samples from unaffected areas as well as ones where nematodes are suspected as the causal agent. Contact the testing laboratory prior to sampling to obtain their preferred sampling procedures.
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.