REVIEWED BY
Peng Tian
Assistant Professor, Plant Sciences

Manoj Chhetri
Department of Horticulture

ORIGINAL AUTHORS
Barb Corwin
Turfgrass Diagnostics LLC

Ned Tisserat
Colorado State University

Brad Fresenburg
Division of Plant Sciences

Unless otherwise identified, photos are by Barb Corwin.

Establishment and maintenance of turfgrasses is a multibillion dollar industry in the United States for good reason. Turfgrasses don't just add beauty to one's surroundings. Natural grasses provide safe and attractive playing surfaces for sports and other leisure activities. Vigorous, well-rooted turfgrass stands also protect local watersheds by filtering out harmful substances and reducing soil erosion and runoff. Turfgrasses cool the immediate environment in the heat of the summer and purify the air by removing carbon dioxide and releasing oxygen.

Millions of dollars are spent annually in the United States on fungicides to prevent or arrest turfgrass disease development on golf courses, high-maintenance lawns and other landscapes. Additional money is spent to repair and renovate severely damaged turf. Accurate diagnosis is the key to managing turfgrass disease in an economically and environmentally sound manner.

There are two types of pathogens that cause plant disease:

• Infectious or biotic
• Noninfectious or abiotic

Most of the common infectious turfgrass diseases are caused by plant pathogenic fungi. The symptoms of these fungal diseases and factors favoring pathogen development have been well described. With some background information, the identification of fungal diseases is a straightforward process.

Cultural practices such as mowing, aerification and irrigation or the misuse of herbicides, growth regulators, fungicides and fertilizers can cause turfgrass injury or decline. These abiotic plant diseases are often the most difficult to diagnose. Disease may also result from a complex of organisms in combination with environmental factors that reduce host plant vigor.

This publication is designed to be a useful reference for diagnosticians, turfgrass managers, industry representatives and others who want to learn how to diagnose and manage common turfgrass diseases caused by plant pathogenic fungi. Expert help in turfgrass disease diagnosis can be obtained from Plant Diagnostic Clinics at land grant universities or from the private diagnostic labs specializing in turfgrass disease diagnostics.

The plant disease triangle

The plant disease triangle, which illustrates how disease develops in a plant population, is a useful concept to help understand and manage turfgrass diseases. For turfgrass disease to occur, the following three components must all be present:

• The host
  A grass species susceptible to the pathogen
• The pathogen
  The disease-causing agent (biotic or abiotic)
• The environment
  Environmental conditions, such as temperature and moisture that favor pathogen development or reduce host vigor.

Identifying the grass species involved is an important clue for identifying the probable cause. Some diseases occur more commonly on one species than another. After you have identified the turfgrass species showing symptoms, refer to Table 1 for a list of the most common turfgrasss hosts for the diseases covered in this publication. Cross reference to Table 2 to check the prevailing temperature and time of year. The section on tips for diagnosing turfgrass disease can also help with the diagnosis.

Table 1. The most common turfgrass hosts of selected turfgrass diseases.

Table 2. The most likely temperature ranges for infection or symptom expression of selected turfgrass diseases.

Potential turfgrass pathogens are present wherever turfgrass is grown. But manipulating the other two components of the disease triangle can often provide effective disease management. Before applying fungicides, consider modifying the host, modifying the environment, or both. These considerations can be combined in a cultural disease management program that includes decisions related to selection of adapted and resistant turfgrass species and cultivars, fertility, mowing, water management, thatch removal and core cultivation.

Using cultural practices to manage disease

Modifying the host

Selecting turfgrass species and cultivars. Choose turfgrass species and cultivars that are adapted to site conditions and local climate. The National Turfgrass Evaluation Program (NTEP) was organized to evaluate cultivar performance of turfgrasses over wide geographic areas of the United States. Information in the NTEP reports is a good source for identifying regional adaptation and relative disease resistance. Visit ntep.org to review these reports. MU publication G6772, Cool-Season Grass Cultivars for Athletic Fields, presents ratings for Kentucky bluegrass, tall fescue and perennial ryegrass. Although the focus of the publication is athletic fields, the listings can be helpful in selecting cultivars for home lawns as well. It is often difficult for individuals to find seed of recommended cultivars. A good approach is to list what local retailers have available and then cross-reference to the cultivars recommended for your location.

Some diseases can be completely avoided by selecting or substituting grass species that are not susceptible to certain pathogens. For example, the severity of summer patch in Kentucky bluegrass lawns or golf green surrounds can be reduced by overseeding with tall fescue.

Fertilizing
Healthy, vigorous turfgrass often withstands infectious disease and adverse environmental conditions. A fertility program that promotes turfgrass vigor is an important part of a sound disease management program. Although the specifics of any program must be left to the turfgrass manager because of the on-site differences in use, expectation and budget, there are several generalizations that can be made. Decisions on amount and type of nutrients to apply should be based on the level of available nutrients in the soil and the plant's nutrient status as determined through soil testing and plant analysis.

For cool-season grasses, most of the nitrogen requirement should be met in the fall. Heavy spring applications of nitrogen result in lush, dense growth that is more disease prone. Growth of foliage at the expense of roots will also make the turfgrass more prone to decline situations during the heat and moisture stress periods frequently encountered in the summer.

For warm-season grasses, nitrogen requirements should be spaced out over the summer months (May to August) when these grasses are actively growing. Late-summer to early-fall applications of nitrogen will make these grasses more susceptible to cold/cool weather diseases.

Mowing
Turfgrass plants mowed shorter than their optimal height of cut are, in general, more susceptible to disease. Optimal cutting heights for cool-season grasses range from 2.5 to 4.0 inches, depending on the species. Height of cut for warm-season grasses can range between 1 and 2 inches.

Frequency of cut should be determined by the "one-third rule" of mowing. Do not remove more than one-third of the leaf growth 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 blade inflicts more and bigger wounds that not only increase the potential for infection by turfgrass pathogens but also increase the potential for moisture stress. Observe leaf tips or grass clippings collected on the mower deck immediately after a mowing to determine the quality of cut. Use this as an indicator of when to sharpen blades.

Modifying the environment

Although ambient temperature, relative humidity and amount of precipitation cannot be controlled, there are ways to modify the local environment to reduce the negative impact of turfgrass disease. Development of many turfgrass diseases is favored by extended periods of leaf wetness. Extended periods of leaf wetness in the turfgrass environment are caused by dew, guttation (water droplets that form at the tips of grass leaves) and frequent irrigation or precipitation. Remove dew and guttation from grass leaves by dragging a hose across the surface, using a whipping pole or briefly irrigating it with large droplets, only long enough to remove the dew from the leaf surface. The goal of all three methods is to spread the concentrated dew or guttation droplets over a larger surface area so the turf canopy will dry faster in the morning. Recent research results suggest that dew removal can improve the performance of certain contact fungicides.

Improper irrigation alone can create a disease problem that normally would not exist. Ideally, irrigation should be applied between midnight and early morning when the turfgrass canopy is normally wet. This avoids extending the duration of leaf wetness. Avoid frequent irrigation that also extends the duration of leaf wetness, especially when environmental conditions are favorable for Pythium foliar blight, brown patch and dollar spot. Generally, deep and infrequent irrigation is recommended so that some soil and leaf drying can occur, permitting a gaseous exchange between soil and atmospheric air.

Irrigation water should be tested regularly (every two to three years) for pH, soluble salt concentration, level of bicarbonates and other factors that influence irrigation water quality.

Adequate drainage is necessary for vigorous turfgrass growth. Turf areas that stay constantly wet because of poor drainage are prime targets for decline. Surface contouring and subsurface drainage are costly, but permanent solutions. Core cultivation and slicing are turf management practices that can be repeated during the year to temporarily increase infiltration rates and promote soil drying.

Increase light penetration and air movement by selectively pruning trees and shrubs. This will speed the drying of poorly drained areas and also reduce the humidity in localized turfgrass areas.

The negative impact of turfgrass disease can be 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 an excellent habitat for both active and dormant stages of many plant pathogenic fungi. When environmental conditions are optimum, fungi can rapidly grow and infect living turfgrass tissue.

Remove excess thatch when it accumulates to 1/2 inch or more in taller-mowed turf (1.5 to 4 inches) and 1/4 inch in lower-mowed turf (below 1.5 inches). Dethatch when the turfgrass is actively growing, so it will recover quickly. Generally dethatching should occur in the spring and fall for cool-season grasses and midsummer for warm-season grasses.

Core cultivation is another tool for thatch control. On sand-based or modified sand-based putting greens, cores are generally removed and the holes are backfilled with sand. On other turf areas, the cores can be broken up and filtered back into the turf canopy to promote the activity of soil microbes that break down organic matter in the thatch layer.

Using fungicides to manage disease

Fungicides are a tool for managing turfgrass disease but are most effective when combined with cultural practices that reduce plant stress. Fungicide applications can be made before the onset of disease (preventive) or after disease development (curative). On high-visibility, high-profile sites such as golf course putting greens and surrounds, a preventive approach is usually desired. For diseases such as brown patch or dollar spot, experienced turfgrass managers can adopt a modified preventive approach by using the concept of sentinel plots to monitor disease potential in a local geographic area. For the professional lawn care operator, the sentinel plot could be a lawn on which brown patch tends to break first. For the golf course superintendent, the sentinel plots could be those greens that historically exhibit the first symptoms of dollar spot or brown patch. Once disease is observed in the sentinel plots, the turfgrass manager can then treat other lawns or other greens on the course preventively. Some diseases have to be managed before the onset of symptoms. Summer patch and large patch are just two examples. The decision to use a preventive or curative approach or a combination of the two ultimately depends on expectations, budget and the diseases in question.

Don't expect immediate improvement to the turfgrass following a curative fungicide application. This is because disease often occurs under conditions that are not favorable for optimum turfgrass recovery. Although suppression of the fungus will occur, some time may pass before the prevailing environmental conditions become favorable for growth and recovery of the turfgrass. Grasses can naturally recover from some diseases once environmental conditions favor growth of the turfgrass. Large patch of zoysiagrass and brown patch of tall fescue are two examples. Owners or managers of high-visibility turfgrass areas are often unwilling to wait for recovery. Also, the thinned areas may be invaded by undesirable grasses, broadleaf weeds or algae.

Classifying fungicides

Table 3. Basic fungicides labeled for management of turfgrass diseases.

Fungicides are classified in several different ways. In Table 3, fungicides are classified in ascending order of FRAC code. The FRAC code was assigned by the Fungicide Resistance Action Committee, an international committee of scientists representing chemical manufacturers. Fungicides with the same FRAC code have the same or very similar biochemical mode of action, i.e., they affect their target fungi in the same way. Most registrants are voluntarily printing the FRAC code on product labels. This will make it easier for turfgrass managers to avoid repeated use of products with the same mode of action, which could result in the selection of resistant fungal populations.

Fungicides can also be classified on the basis of topical mode of action. Topical mode of action describes what happens to a fungicide when it contacts a plant. Fungicides have two basic topical modes of action — contact and systemic. Systemic fungicides are subdivided based on the direction and extent of movement inside the plant. There are three possible fates of systemic fungicides once they have entered a plant: xylem-mobile (translocated upward), phloem-mobile (translocated upward and downward) or limited translocation from the entry site (locally systemic).

Another category of fungicides includes those that have been registered as reduced risk by the Environmental Protection Agency (EPA). Several fungicides labeled for use on turfgrass have been registered as reduced risk because of low impact on human health and low toxicity to nontarget organisms, including birds, fish and other plants (Table 4). Other criteria that are considered under the reduced risk registration protocol include low potential for groundwater contamination, low use rates, low pest resistance and compatibility with integrated pest management (IPM) practices.

Table 4. Reduced-risk fungicides labeled for use on turfgrass, as of 2007.

Choosing the right fungicide

In today's turfgrass fungicide market, there is a high level of competition for market share of fungicide dollars. Although there are numerous trade names to choose from, many of these are competing products with the same active ingredient or with the same or similar mode of action. Although active ingredients may be the same, product formulations can be quite different, leading to the possibility of varying levels of efficacy. In addition, the level of company support for products varies widely from one company to the next.

Developing a fungicide plan of action can help with fungicide choice. To help develop the plans, information on the basic fungicides labeled for management of turfgrass diseases is presented in Table 5 PDF. A fungicide plan of action is a season-long program outlining the targeted diseases, application dates and products. The fungicide plan of action is based on the disease history, fungicide budget and expectations of the facility. The environment and the grass species and cultivars you are managing will also influence your plan. Ideally the fungicide plan of action should be developed side-by-side with the plans for other chemical inputs such as fertilizers, herbicides and growth regulators and scheduled core cultivation and other maintenance practices. A well thought out fungicide plan of action makes the most of fungicide dollars.

 

Figure 1
Daconil Ultrex applied at 1.8 ounces per 1,000 in 41 gallons per acre. Top: coarse droplets; bottom: fine droplets. D. Shepard, Syngenta Professional Products photos.

Applying fungicides correctly

Once you have developed a fungicide plan of action, there are other factors that need to be considered to ensure optimum product performance. Thorough coverage of the targeted part of the plant (e.g., foliage crowns or roots) must be achieved and the correct amount of product according to label recommendations must be applied. Two of the common causes of poor product performance are spray droplets that are too large to provide the complete coverage needed to control certain diseases and spray volumes that are too low.

Droplet size is highly dependent on nozzle type. Droplet size at a given pressure influences the amount of product applied per unit area, the uniformity of coverage in that area and the off-target drift potential. Reducing droplet size by half will result in about eight times more droplets per unit area. For example, applying a fungicide in a water volume of 44 gallons per acre through a nozzle that delivers 800-micron droplets distributes about 88 droplets per square inch, whereas a nozzle that delivers a 400-micron droplet will result in about 704 droplets per square inch. The smaller the droplet, the more even the distribution of the product in the turf canopy (Figure 1), but the greater the drift potential. Most nozzle manufacturers provide color-coded tables that list droplet sizes for different nozzles at various pressures.

There are two basic types of nozzles: flat fan and hollow cone. A flat fan nozzle is the most widely used in the turfgrass industry and under varying ranges of pressure is a good choice for applying both contact and systemic fungicides. Flat fans (specifically designed for low pressures) offer good control of spray drift when operated at low pressures. Nozzles can operate at different pressure ranges, but 30 to 60 psi is typical for turfgrass applications.

There are two types of hollow cone nozzles, one producing very fine droplets and one producing very coarse droplets. The fine-droplet hollow cone distributes droplets finer than flat fan nozzles but has no overlap of the spray pattern. The finer droplets are prone to drift and there is lack of uniform coverage if the nozzle is used on sites other than level ground. The raindrop nozzle is a course-droplet hollow cone that is less prone to drift; however, this nozzle results in less uniform coverage.

When fungicides are mixed with water, the solution becomes heavier than water alone (8.34 pounds per gallon). Heavy liquids will form narrower spray bands and larger droplets, resulting in poorer coverage. Liquids less dense, or lighter, than water can form wider spray angles and increase flow rates. Nozzle manufacturers have conversion charts for solutions heavier or lighter than water.

Coverage is also determined by spray volume. Nozzle manufacturers provide tables that list spray volumes per unit area for each nozzle at different pressures and speeds. Recent research has shown that a foliar disease such as dollar spot is suppressed effectively by fungicides (both contact and systemic) applied in a spray volume as low as 1 gallon per 1,000 square feet (44 gallons per acre). In contrast, fungicides (contact and systemic) targeting brown patch were most effective when applied at 2 to 4 gallons per 1,000 square feet (87 to 175 gallons per acre). Fungicides targeting root- and crown-infecting pathogens should be applied at rates of at least 2 gallons per 1,000 square feet; some product labels suggest 2 to 4 gallons per 1,000 square feet. Because most fungicide plans of action are designed to cover more than one disease at a time, turfgrass managers should, as a general rule, use a flat fan nozzle at a pressure sufficient to apply at least 2 gallons per 1,000 square feet.

Finally, make sure that the sprayer is properly calibrated. For step-by-step instructions, refer to MU publication G6751, Calibrating Sprayers and Spreaders for Athletic Fields and Golf Courses.

Preventing or delaying fungicide resistance

Repeated use of fungicides with similar modes of action can result in the selection of fungal populations with resistance to the fungicide. Strains of the dollar spot fungus resistant to cadmium-based, MBC and DMI fungicides and iprodione have been reported. Pythium species resistant to metalaxyl and anthracnose fungi resistant to MBC and QoI fungicides have also been reported. The likelihood of selecting for resistant fungal strains depends on the fungicide mode of action and the biology of the fungus. Table 5 presents the relative risk of resistance for basic fungicides labeled for control of turfgrass diseases.

Plant pathologists generally agree that certain practices can delay the selection of resistant fungal strains. Here are some examples:

• Do not rely on fungicides alone. Combine fungicide use with cultural practices that reduce disease severity.
• Use contact fungicides alone or in combination with systemics as part of the management program.
• Use fungicides on a preventive, rather than curative, basis.
• Rotate or mix systemic fungicides with different modes of action. Repeated use of the same or a similar fungicide selects resistant members of the population.

Using other approaches to manage disease

A number of other strategies may play a role in a turfgrass disease management program, especially when used in combination with other, more traditional methods.

Biological control

Research efforts have looked at two ways to achieve effective biological control practices in a turfgrass disease management program. One approach is to introduce microbial inoculants that have a demonstrated negative effect on target plant pathogenic fungi. The second is to stimulate resident populations of competitive or antagonistic microorganisms through the application of organic amendments and other microbial food sources.

Research trials with various microbial inoculants, including fungi and bacteria, have been ongoing and well supported by the United States Golf Association, Golf Course Superintendents Association of America, industry, state turfgrass organizations and the federal government for more than 25 years. Although many of the microbes have given promising results in greenhouse or small-scale field trials, few have been successfully developed as commercial products. Of the 33 microbe-disease combinations studied as biological control agents in a turfgrass system (summarized by Eric Nelson, Cornell University, in a 2003 review), only six have been registered as microbial pesticides by the EPA. The six organisms include two fungi (Trichoderma harzianum strain 1295-22 and Gliocladium catenulatum strain J1446), three bacteria (Bacillus licheniformis strain SB3086, Bacillus subtilis strain GB03 and Pseudomonas aureofaciens strain TX-1) and one actinomycete (Streptomyces lydicus strain WYEC 108). Since the first product was registered in 1996, there have been numerous changes in trade names and manufacturers, and the availability of these six products is limited in the United States. Visit epa.gov/pesticides/biopesticidesfor the most up-to-date registration information. One of the major limiting factors to successful use of microbial inoculants in a turfgrass system is the difficulty of delivering and sustaining adequate populations of the microbial inoculant in the turfgrass environment.

The second method of biological control — promoting resident microorganisms — gives promise for a more sustainable approach. This approach ranges in scope from composting with different organic amendments to applying food sources such as molasses, with the ultimate goal of "feeding" or promoting a diversity of naturally occurring soil microorganisms that suppress disease. Again, the level of success with these methods has varied considerably and is hampered on a commercial scale by difficulties in achieving high-quality, consistent product from one batch to the other.

Biopesticides

Biopesticides include the microbial pesticides mentioned above as well as biochemical pesticides and plant-incorporated protectants. Biochemical pesticides are naturally occurring substances in contrast to conventional products that are generally synthetic. One such product registered for use on turfgrass by Arvesta Coorporation and licensed to Cleary Chemical is Endorse. Endorse is polyoxin D zinc salt, a naturally occurring product produced by a soilborne bacterium. The bacterium is grown commercially and the polyoxin D zinc salt is extracted and formulated as the fungicide Endorse. Another example is hydrogen dioxide (registered as Zerotol by Biosafe Systems). Essential oils and other plant extracts are included as biochemical pesticides. A more complete listing of registered biopesticides by active ingredient can be found on the EPA website cited above. Unfortunately the list is not searchable by crop, so finding all the products labeled for use on turfgrass is difficult.

Plant-incorporated protectants (PIP) are pesticides produced within a plant as the result of the addition of foreign DNA. There are currently no PIP products registered for use on turfgrass.

Plant defense activators

Plant defense activators are naturally occurring or synthetic products that have been shown to induce systemic acquired resistance in various plants. Rather than impact the plant pathogen directly, these products enhance or stimulate a plant's own natural defense mechanisms. The efficacy of several such products against brown patch and dollar spot has been evaluated in recent years. Although some disease suppression has been shown, the level of disease control with these products alone has not been acceptable. However, some plant defense activators show promise when used in combination with fungicides or cultural practices, such as the application of foliar nitrogen.