Taking an environmentally sensitive approach to pest management
Turfgrass disease is one of the serious and costly reasons for injury and death to grasses used in home lawns, golf courses, sport fields, and other areas where grasses are desired. Plant pathogenic fungi are the main cause of lawn diseases. Other organisms, such as nematodes, and several non-parasitic problems are also sources for diseases. An accurate diagnosis of the problem is essential to any successful control program. Diagnosis of lawn diseases can be performed at diagnostic clinics, such as the Plant Diagnostic Clinic on the MU campus at 23 Mumford Hall, Columbia, Mo. 65211.
Disease identification and control involve more than just waiting for diseases to appear, then trying to make a rapid diagnosis and applying a fungicide. Most disease identification guides show only the symptoms of developed diseases. This is helpful, but it is more important to know the conditions that can lead to a disease, and to follow basic cultural practices that can reduce your potential for a disease. Knowing when and under what conditions to anticipate various turfgrass diseases, an individual can prepare for what to do about them, saving time and achieving better results in disease control. See MU Extension Guide Sheet G6756 - Turfgrass Disease Control.
Environmental conditions strongly influence disease occurrence. Although many of the causal agents are always present in turf, diseases do not occur until conditions are favorable for pathogen development. For example, brown patch disease requires wet, humid conditions during warm to hot weather. Being aware of the conditions that increase disease potential is important in taking preventive measures such as applying fungicides before symptoms appear. But before fungicides are considered, there are several turfgrass management practices that need discussion in hopes of reducing the potential for disease.
Several diseases can be avoided by selecting grass species that are not susceptible to certain pathogens. For example, summer patch is a severe problem on Kentucky bluegrass but has little effect on tall fescue. An area historically prone to summer patch disease can be planted to tall fescue to reduce that potential. Likewise, within species of grasses, selected cultivars can offer more disease resistance than others. A cultivar of Kentucky bluegrass may show a higher level of tolerance to rust disease and perhaps be selected as part of a blend or mixture. Even though these grasses are termed “disease resistant,” it does not mean that they are 100 percent disease free. Selecting cultivars with higher disease resistance will reduce your potential for turfgrass diseases and becomes the first step in a line of cultural practices to manage turfgrass diseases.
The National Turfgrass Evaluation Program was organized to test species and cultivar performance in several locations of the United States. Most of the data and information generated by this program can be accessed through their web site at http://www.ntep.org. You can also contact your local MU Extension center for grasses that have been recommended for Missouri.
Soil fertility is an important factor in disease development. High nitrogen levels increase the susceptibility of cool-season grasses (Kentucky bluegrass, tall fescue, perennial ryegrass, fine fescue and bentgrass) to leaf spot, Rhizoctonia brown patch and Pythium blight. Low nitrogen levels increase turfgrass susceptibility to dollar spot and red thread. Low potassium levels in the soil reduce turfgrass tolerance to high temperatures and drought stress, which can increase the potential of diseases such as summer patch. Low pH is often associated with diseases such as brown patch as well.
Knowledge of soil fertility as it relates to turfgrass diseases can help guide an individual in deciding how to manage a lawn. A tall fescue lawn can receive two or three fertilizer applications throughout the fall and perhaps receive no additional fertilizer in the spring to reduce the potential for brown patch. Like tall fescue, Kentucky bluegrass can receive fall fertilization but can also receive fertilizer in the spring to help keep dollar spot from infecting the bluegrass.
To minimize the potential for disease, supply enough nitrogen that proper mowing is required on a weekly basis. Sometimes a light application of nitrogen will produce enough active leaf growth that disease symptoms are no longer visible.
Turfgrass plants mowed shorter than their optimal height of cut are, in general, more susceptible to diseases. Optimal cutting heights for cool-season grasses range from 2.5 to 4.0 inches, depending on the species. Warm-season grasses can range between 1 and 2 inches.
Seasonal variation in mowing height was once thought to be highly beneficial and is still considered beneficial by some. We know that mowing cool-season grasses a little taller in the summer months can have benefits through summer stress periods (deeper roots, better cooling effect). We also know that cool-season grasses mowed a little taller in the spring and fall compete more successfully against weeds. Therefore, select the tallest, acceptable mowing height for your species of grass and maintain that height during the entire season. This provides benefits throughout the season -- competition against weeds as well as reduced summer stress.
Frequency of cut should be determined by the “one-third rule” of mowing. You should make sure that no more than one-third of the leaf growth is removed during a single mowing.
Mowing creates wounds through which fungi can enter the plant and infect it. Leaf cuts made by a sharp mower blade are cleaner and heal faster than the tearing and shredding caused by a dull mower blade. A dull mower blade inflicts more and bigger wounds that increase potential for infection by turfgrass diseases. Observe leaf tips or grass clippings collected on your mower deck immediately after a mowing to determine the quality of cut. Use this as an indicator of when to sharpen blades.
Nearly, all turfgrass diseases require water for their development. Some disease problems such as Pythium blight, brown patch, and dollar spot are accentuated by extended periods of free moisture. Extended periods of free moisture in turfgrasses can be caused by dew, guttation and frequent irrigation or rainfall. Guttation is the formation of water droplets at the tips of grass leaves. These droplets contain exudates of sugars and proteins and serve as an excellent food source for pathogens. Remove dew and guttation from grass leaves by dragging a hose across the surface of the lawn, using a whipping pole or briefly irrigating only long enough to wash the dew from the surface of the leaves. Following these methods will spread the concentrated dew or guttation over a larger surface area, causing the turf canopy to dry faster.
Improper irrigation alone can create a disease problem. Avoid frequent irrigation that results in extended periods of free moisture. Avoid late evening watering that extends the free moisture period throughout the night. Coolseason grasses can be allowed to have drying periods (near wilting) to disrupt the growth cycle of fungi favored by free moisture.
Irrigation in the early morning not only limits extended periods of dew and guttation but also applies water at a time of the day when temperatures are low (reduced evaporation) and winds are calm (better distribution). A rule of thumb is to avoid puddles and runoff during irrigation, put the water where it is needed, and irrigate only as much as your particular soil type can absorb in one cycle.
Essentially, all turfgrass diseases are reduced by good thatch control.
Thatch is a layer of dead and living plant material located between the soil surface and green turf canopy. It is excellent habitat for active and dormant stages of disease-causing organisms. When environmental conditions are optimum, fungi can rapidly grow and infect living turf tissue.
Remove excess thatch when turf is actively growing to promote quicker recovery from power-raking or verticutting. Remove thatch in the spring before application of crabgrass preventer, or in the fall for cool-season grasses and midsummer for warm-season grasses. Core aerification (removing soil plugs) is a slower process of thatch control but will cause less direct stress on the turf. Breaking up soil plugs and filtering soil into the turf canopy allows soil microbes to breakdown dead organic matter in the thatch layer.
Remove excess thatch when it accumulates to a half inch or more in taller-mowed turf (1.5 to 4 inches) and one-quarter inch in lower-mowed turf (less than 1.5 inches).
Good exchange of air between the soil and atmosphere is necessary for vigorous turfgrass growth. Turf areas that stay constantly wet because of poor soil conditions are prime targets for water-favoring, soil-borne diseases such as Pythium blight and brown patch. Surface contouring and subsurface drainage can be costly but permanent solutions to wet soils.
Core aerification and slicing are turf management practices that can be repeated during the season to temporarily increase air exchange and soil drying. You can also increase light penetration and air movement by selectively pruning your trees and shrubs. This will speed the drying of poorly drained areas and also reduce the humidity in localized turf areas. By implementing some of the cultural practices outlined above, turfgrass managers can reduce their risk of turfgrass diseases. However, in extended conditions favorable to the development of particular diseases, cultural practices alone are usually not enough to maintain disease free turf. If you can tolerate a few patches of disease without the use of fungicides, then so be it.
See MU Guide Sheet 6756 for commercial fungicide controls. Be sure to properly identify the problem before selecting a control product. Turf managers can help to narrow down possible diseases by first, determining the species of grass infected, the time of year, levels of fertility and water, and finally present environmental conditions. Always read the entire label of the product chosen. Remember, difficult problems can be diagnosed at the Plant Diagnostics Lab at the University of Missouri – Columbia. Go to http://soilplantlab.missouri.edu/soil or call 573 882-0623.
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REVISED: October 23, 2012