IPM Identification and Mangement Of Turfgrass Diseases

Author(s):

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

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.

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 as a pullout section. 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.

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.

Figure 1. Daconil Ultrex applied at 1.8 oz/1,000 in 41 gal/acre. Left: coarse droplets; right: fine droplets. Photo by D. Shephard, Syngenta Professional Products

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/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 lb/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/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 (87–175 gallons/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.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.