What Is Turfgrass Disease
Disease of sickness in turfgrasses, as in other plants, develops from an interaction between a susceptible plant, a disease-producing organism (usually a fungus), and an environment favorable for the disease-causing organism to attack. The sick turfgrass plant then gives visible evidence that something is wrong. This evidence of disease we call symptoms. This is the same situation that occurs in human disease. For example, the disease-causing streptococcal bacterium produces the symptoms (fever, sore throat, and rash) that physicians diagnose as the disease scarlet fever. Scientists who work with turfgrass diseases sometimes use a disease triangle to illustrate the concept of disease. The three sides of the disease triangle represent three factors that interact to produce turfgrass disease: the disease-causer, the susceptible grass, and a favorable environment (Figure 1). Three factors interact to cause turfgrass disease; therefore,
- we must observe all three factors to gather information for diagnosis of the problem, and
- we can change any or all of these three factors to combat the disease.
Figure 1. Disease triangle.
The Importance of Correct Diagnosis
The first step in turfgrass disease management is the identification of the problem. Diseases are only one cause of turf loss, and disease control measures will do nothing to alleviate damage from other causes such as insect attack or drought stress. It is essential to determine whether the problem is disease and, if so, which disease it is. Disease management strategies that are effective against one disease may have no effect on or may even worsen another disease. This is particularly true when, because of incorrect diagnosis, the wrong fungicide is selected. The best result that can be hoped for in this situation is that nothing is lost except time, effort, and money. A worse possibility is that the disease problem may be increased by application of an inappropriate chemical.
The three disease factors (grass, disease-causer, and environment) provide the sources of information for diagnosis (Figure 2). The environment during the onset of the disease is one source of diagnostic information. For example, what were the temperature, the light intensity, and the moisture conditions just prior to and during disease development? The nature of the disease site is also important. Air and water drainage, soil conditions, sun/shade, slope, and nearness of other plantings or buildings all may be important in development of turfgrass diseases. Prior chemical applications to the site, including pesticides and fertilizers, may be contributive. Heavy thatch accumulation and poor mowing practices that stress the turf may trigger or amplify certain disease problems in turf areas.
Figure 2. Diagnosis triangle.
The nature of the symptoms on the grass is a very important source of diagnostic information. Two kinds of symptoms should be looked for in diseased turgrass areas--symptoms on the stand and symptoms on individual plants. A home lawn, an athletic field, and a golf green or fairway are all examples of turf stands. Symptoms on the stand are the appearance and the visible patterns of the disease on the planting. These are extremely important in turfgrass disease diagnosis because different diseases appear differently on turf stands. The visible differences in pattern are often critical factors in identifying particular diseases. Diseases can appear on the turf stand as spots, patches, rings, circles, or may be unpatterned. Certain diseases never appear as rings, while others always appear as rings. Symptoms to look for on individual plants include leaf spots, leaf blight, wilt, stunt, yellowing, and root discoloration or rot. Leaf spots can be very good diagnostic clues because the leaf spots of different diseases are usually unique in shape, color, and size. Leaf blighting can be any size or shape and may involve the entire leaf.
Certain life stages of turfgrass disease-causers can be seen without magnification. The fungi that cause most turfgrass diseases are microscopic. But in stripe smut, powdery mildew, and rust diseases, the spores of the causal fungi pile up in such numbers that they become visible as black, white, or orange powder on grass leaves. In red thread disease, the fungus sticks together and forms the pink or red antlerlike threads that typify the disease. When the causal fungus can be seen, its appearance is often the most important clue for diagnosis.
Disease Management of Turfgrass
Because the three components of disease development combine to influence the onset on turfgrass disease, the task of disease management on turfgrasses involves manipulation of these three--the environment, the grass, and/or the disease-causing organism--to favor the grass and inhibit the causal fungus (Figure 3).
Figure 3. Management triangle.
The environment can be altered in many ways. The ones chosen depend on the disease to be managed. Water manipulation can be a valuable tool in disease management. For example, some diseases require free water on leaves for development. Effective strategies to reduce free water include morning irrigation, removal of dew, and reduction in amount and/or frequency of irrigation. Other management strategies may involve some forms of improved air and water drainage, improved soil conditions by aeration, thatch reduction, manipulation of light conditions, regulation of fertilization levels, and implementation of proper mowing practices. Environmental modifications may be appropriate methods for reducing damage from particular diseases and ensuring a vigorous turf that recovers from disease injury.
When establishing new turf areas or when renovating disease-damaged turf, it is important to select grasses that are resistant to diseases known to be common in the area or that have damaged the existing stand. Disease-resistant grasses can be seeded to minimize turf loss from disease. For example, certain varieties of Kentucky bluegrass are resistant to spring leaf spot, a disease that is devastating on many Kentucky bluegrass turfs. Varietal resistance to other common diseases of bluegrass is available. For diseases to which varietal resistance is weak or not available, it may be possible to seed a grass species that is resistant to a prevalent disease problem. For example, ryegrass may replace bluegrass in an area damaged by necrotic ring spot, or bluegrass might replace ryegrass in an area when Pythium blight is a problem. Disease severity can often be reduced by appropriate changes in the grass that is being grown. It is a bad practice to replant the same grass that has been killed by the same disease year after year, if there is another option.
In selecting grasses for turf establishment or renvation, it is preferable, where possible, to use mixtures of different grasses or blends of different varieties rather than seeding a single kind of grass. The seeding of mixtures or blends produces a diverse population of grass plants. Such turf is usually more likely to survive stress caused by disease. Diversity in plantings almost always increases odds of survival.
The causal organism may be attacked by applying chemicals that will either kill the organism or keep it from growing. Most fungicides do not actually kill fungi; they work by preventing growth. Again, it is important to have identified the causal organism correctly so that an appropriate fungicide can be selected. Arbitrary selection and application of fungicides without knowledge of the disease cause can do as much harm as good. Using the wrong fungicide wastes money and may increase the amount of disease or produce other undesirable side effects.
With rising prices, declining budgets, and increasing environmental awareness, those who manage turfgrasses are faced with the necessity of making reasonable and defensible choices. Planning an effective disease management program, therefore, involves not only "spraying something," but selecting cost-effective and environmentally sound disease control strategies. The financial, environmental, and aesthetic costs of disease management strategies must be considered. Good, common-sense approaches to disease management should employ all available disease management strategies (Figure 4).
Figure 4. Use all three management approaches to reduce the amount of turfgrass disease.
Prepared by Patricia L. Sanders, former professor of plant pathology.