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Guide to Turfgrass Fungicides

Alfredo Martinez and Lee Burpee
Department of Plant Pathology — Griffin Campus

Tom Allen
Texas Agricultural Experiment Station, Bushland, Texas

Common Turfgrass Diseases and Chemical Controls
Turfgrass Fungicide Formulations
Classification of Fungicides
Biofungicides
Methods to Maximize Efficacy of Turfgrass Fungicides
Turfgrass Fungicide Resistance Management
Turfgrass Fungicide Costs
Turfgrass Fungicide Internet Resources
Turfgrass Fungicide Manufacturers and Distributors
Bibliography

Common Turfgrass Diseases and Chemical Controls

Rhizoctonia complex (Brown Patch and Large Patch). Caused by Rhizoctonia spp. (mostly R. solani in Georgia). The disease can affect all species of warm- and cool-season turfgrasses, including bentgrass, ryegrass, tall fescue (Brown Patch) and St. Augustinegrass, zoysiagrass, centipede and bermudagrass (Large Patch). The most favorable conditions for disease development usually occur from late April through October. Brown patch is favored by high relative humidity during the day and a minimum temperature of 60 degrees F at night. Excess soil moisture, extended leaf wetness and/or high levels of nitrogen increase severity of the disease. Symptoms: Leaf blades or sheaths are blighted in circular patches measuring several inches to feet across. In bermudagrass, centipede and ryegrass, patches are brown. In St. Augustinegrass, patches appear straw-colored. Smoke rings may be present in bentgrass. Mycelium may be evident early in the morning.

Dollar Spot. Caused by Sclerotinia homoeocarpa. All species of warm- and cool-season turfgrass are susceptible. Tall fescue, Kentucky bluegrass, perennial ryegrass, bentgrass, centipede, zoysiagrass and bermudagrass hybrids are particularly susceptible. The disease is promoted by dry soil conditions, extended leaf wetness and low nitrogen levels in soil. Symptoms: Circular straw-colored infection centers are only a few inches across, although spots may merge, resembling brown patch. Live blades may have straw-colored lesions along one edge. Lesion expands across blade, causing tip to dieback. Abundant white fungus growth evident early in the morning when dew is present may be seen during periods of severe disease development.

Pythium Blight. Caused by Pythium spp. Annual bluegrass, tall fescue, perennial ryegrass and varieties of bentgrass, bermudagrass, centipede, zoysiagrass and St. Augustinegrass are susceptible. The disease is promoted by excessive soil moisture caused by over irrigation or heavy rain showers. Some Pythium species favor temperatures between 32 degrees F and 50 degrees F while others thrive in temperatures between 70 degrees F and 90 degrees F. Symptoms: Small, irregular spots may enlarge and appear dark and water-soaked in early stages. White, cottony mycelia may be evident. Turfgrass in affected spots dies rapidly, collapses, and appears oily and matted. Gray to white fungal growth may be evident early in the morning on leaves of some turfgrass species.

Helminthosporium Leaf Spot. Caused by Drechslera spp; Bipolaris spp; (formerly Helminthosporium spp). Perennial ryegrass, tall fescue and some varieties of bentgrass, bermudagrass, zoysiagrass and centipede are susceptible. The conditions that promote the disease are low potassium levels, extended leaf wetness and/or high nitrogen levels in soil. Drechslera is favored by cool wet weather whereas Bipolaris is active during warm weather of midsummer. Leaf spot occurs in areas that experience more than 10 hours a day of foliar wetness for several consecutive days. Disease is also favored by high amounts of nitrogen and a low mowing height, and drought stress. Symptoms: Small, dark spots or streaks on grass blades and sheaths. Leaf spots are more numerous near the collar area of the leaf blade. Severely affected turfgrass may become brown and thin.

Curvularia Diseases. Caused by Curvularia spp. Perennial ryegrass, tall fescue, bentgrass, bermuda-grass, zoysiagrass and centipede are susceptible to the disease. The conditions that promote the disease are similar to Helminthosporium leaf spot. Usually weakly pathogenic unless turfgrass is predisposed to high temperature stress. Symptoms: Vary with type of grass. Fescue — indefinite yellow/green dappled pattern extending down from leaf tip. Affected area turns brown, then gray, and then dies. Reddish brown margin may or may not be present. In Bentgrass, affected parts of blades are tan instead of brown or gray.

Gray Leaf Spot. Caused by Pyricularia grisea. Gray leaf spot affects a wide variety of hosts. However it is of primary importance on St. Augustinegrass and perennial ryegrass. Bermuda, centipede, bentgrass and various species of fescue may also be affected, albeit to a lesser degree. Conditions promoting disease include: high humidity, warm temperatures, and high nitrogen levels. Symptoms: On St. Augustinegrass, gray leaf spot first appears as small, brown spots on the leaves and stems. The spots quickly enlarge to approximately ¼ inch in length and become bluish-gray in color and oval or elongated in shape. The mature lesions are tan to gray in color and have depressed centers with irregular margins that are purple to brown in color. A yellow border on the lesions can also occur. Severely affected blades wither and turn brown.

Fairy Ring. Caused by few species of mushroom and puff ball-producing fungi (Basidiomycetes). The disease is particularly damaging on centipede and St. Augustinegrass in south and coastal Georgia. The conditions promoting disease are: Presence of fungus and moist, warm weather. Symptoms: Circular or semi-circular band of darker-than-usual green grass. Grass inside ring usually is not as vigorous and may be declining. Grass may be dead inside young rings; a band of dead grass with greener grass in the middle may be seen in older rings. During rainy, moist conditions, a ring of mushrooms may appear.

Take-All Patch, Take-All Root Rot, Bermudagrass Decline. Caused by Gaumannomyces graminis. Bentgrass, St. Augustinegrass, bermudagrass and centipedegrass are susceptible. Bluegrass and fescues are rarely affected in Georgia. Take-all patch typically occurs in wet soils and in areas with soil pH 6.5 or above. This disease is more severe on less fertile and sandy soils. Symptoms: Wilted circular patches that are brown or bronze-colored and measure up to several feet in diameter. Infected plants have dark-brown roots.

Rust. Caused by Puccinia spp., Uromyces spp. Susceptible turfgrass include Kentucky bluegrass, ryegrass, bentgrass, zoysiagrass, bermudagrass and fescue. Rusts occur in early spring through mid-summer. Rusts favor moist, low-light areas. Depending on the species, rusts favor temperatures between 65 degrees F and 85 degrees F. Severe rust infections occur on slow-growing turfgrass maintained under conditions of low nitrogen and/or drought stress. Symptoms: Light-yellow flecks on the leaf blades and sheaths. The flecks enlarge, elongate, and turn yellow in color. The infected areas expend above the epidermis and then rupture, releasing spores that are yellowish-orange to reddish-brown in color.

Turfgrass Fungicide Formulations

Fungicides are available in many different formulations. Below are the common formulations and their standard abbreviations.

1. Wettable Powders (WP)

Most fungicide are formulated as wettable powders. The active ingredient is normally combined with a solid material (clay or talc) and is suspended in water with the aid of an emulsifier. Many of these suspensions settle out quickly, so an agitator is needed in the spray tank to keep the particles in suspension. Even though plant injury is rarely a problem with wettable powders, there can be excessive wear on spray nozzles with prolonged use of these formulations.

Examples: Bayleton, Prostar, Tersan, Alliette etc.

2. Water Soluble Packets (SP or WSP)

Some WP fungicides are now available in water soluble pouches. These pouches contain enough product for a single application. Pouches are placed in the mixing tank to release the fungicide; this reduces the exposure of mixer and loader personnel to dust from the fungicide. Fungicides available in this form do not require continuous agitation.

Example: Eagle

3. Granules (G)

Small pellets of inert ingredients (normally clay) are sprayed with the desired amount of active ingredient. Generally, granules are large in size (ranging from 40 to 80 mesh). Granules can be applied to turf with a fertilizer spreader or incorporated directly into the soil.

Examples: Compass G; Andersons Bayleton etc.

4. Emulsifiable Concentrates (EC)

A liquid fungicide, insoluble in water, dissolved in an organic solvent. An emulsifying agent is incorporated in the formulation, so when the product is mixed with water, an emulsion is formed. When the product is added to water it forms a “milky” mixture consisting of the active ingredient and the emulsifier. An emulsion is a suspension of very tiny droplets of the solvent/fungicide in water. Once mixed with water in a spray tank, an EC can require some agitation to maintain a suspension.

5. Flowables (F or L)

Flowables are insoluble fungicides ground into very fine particles, usually by a wet grinding process, and impregnated on a carrier such as clay or talc. The active ingredient is mixed with a liquid and an inert ingredient to form a suspension. The liquid is thick and requires dilution in water before application. Flowables remain suspended in water for relatively long periods of time, but they should be agitated before use. Flowables are dust-free, easy to mix, remain in suspension longer than wettable powders, and may also resist washing off the plant better than wettable powders. They need to be protected from freezing, however.

Example: Lesco Twosome

6. Water Dispersible Granules (WDG) or Dry Flowable (DF)

The two names are interchangeable with one another. These are similar to wettable powders. The active ingredient is prepared as granule-sized particles and must be mixed with water. The small granules can be poured from a container like a liquid. They are virtually dust-free, more readily measured than wettable powders, and disperse easily in water to form a suspension. They require constant agitation and cause some nozzle wear on spray equipment.

Examples: Heritage, Spectro, Systar, Buscalid

7. Fumigants

Fumigants are chemicals that turn into a poisonous gas after application. They are generally used for soil fumigation. Some are formulated only as gases, while others are liquids that turn to gas when injected into the soil. Advantages of fumigants include toxicity to a wide range of pests and effectiveness with a single application. The area treated with fumigants must be enclosed or covered, and special safety and application equipment are needed.

Example: Curfew

Classification of Turfgrass Fungicides

Fungicides may be classified by similarities in their chemical structure, topical activity and mode of action.

Chemical Structure

There are 13 chemically related groups or “chemical families” of fungicides (Table 1).

Fungicides in a common chemical family are similar in their topical activity and mode of action. If a fungus develops resistance to one fungicide in a chemical family, then it is usually resistant to other fungicides in the same family.

Topical Activity

Topical activity refers to the fungicide’s activity. According to their site of interaction, fungicides can be divided into two main categories:

Contact: A contact fungicide kills or inhibits the fungus that it comes in contact with. These fungicides can also be applied to leaves, but can only inhibit the growth of the fungus if it is present on the leaf surface. Contact fungicides do not enter the plant.

Systemic: Fungicides that have systemic qualities are absorbed by leaves or roots and transported within the plant to provide fungicidal activity. Some systemics move short distances from the site of application, such as across a leaf blade (local systemic or translaminar). Some fungicides are weakly systemic and can move farther from the application site than local systemics.

Most systemics move more extensively because they are mobile in xylem tissue. When applied to the root zone, these are absorbed by roots and then move upward through the plant with the transpiration stream (xylem-mobile systemic).

Xylem-mobile systemics applied to leaves move throughout the leaf where deposited, but cannot be redistributed out of that leaf; however, any material deposited on stems can move upward into leaves. Phloem-mobile systemics (also known as “true” or amphimobile systemics) have bi-directional mobility; some material moves in phloem out of the leaf where it is deposited upward to other leaves and downward to roots. Systemics cannot move again after translocation.

Mode of Action

Mode of action refers to physiological processes of the fungus that are affected by the fungicide. Fungal processes affected are normally at the cellular level. Fungicidal action can be expressed in one of two visible ways: (1) inhibition of fungal growth and (2) inhibition of spore germination.

Becoming familiar with a fungicide’s mode of action is important for resistance management, and for incorporating fungicides with different modes of action into a disease management program.

There are four categories of fungicide mode of action. Each of the four categories are discussed in detail below with examples of chemical families and specific fungicides that fit each category.

Specific examples of each of the fungicides within these categories are included in Table 1.

Electron Transport Chain Inhibition

The specific members of this group inhibit mitochondrial respiration, which blocks the production of energy. Some examples include:

Strobilurins — Members of this family of fungicides are derived from naturally occurring fungicides in edible mushrooms and wood-decaying mushrooms. They provide good control of downy and powdery mildews, leaf spots, rusts and many other turfgrass diseases. Specifically, members in this group contain the active ingredients azoxystrobin, pyraclostrobin, or trifloxystrobin. Specific fungicides that contain strobilurins include Heritage, Compass and Insignia.

Enzyme Inhibition

These fungicides inhibit the action of important fungal enzymes.

Carbamates and Dithiocarbamates — These fungicides are contacts that cause a direct interference with the respiratory processes of the cell and inactivate Sulfyhydroxyl (SH) groups in amino acids, proteins and enzymes. There are several fungicides in this group that are commonly used on turfgrasses to control a broad spectrum of diseases. Specific fungicides in this group include: Dithane, Mancozeb and Manzate.

Carboxamides — The fungicides in this group interfere with respiration of the fungal cells by blocking the activity of certain respiratory enzymes. Flutolanil (Prostar) is a carboximide that is commonly used in turfgrass disease control.

Copper — Fungicides containing copper are the oldest group of fungicides used for disease control. Their use dates back to the 1700s. The majority of these products are either blue, green, red or yellow powders that are insoluble in water. Copper fungicides are contacts that cause a non-specific breakdown of proteins and enzymes and are most effective at controlling leaf spot fungi. Specific examples of fungicides that contain copper include Junction and Kocide.

Aromatic hydrocarbons (substituted aromatics) — These chemicals are diverse in mode of action, but basically they reduce growth rates and sporulation of fungi. Specific fungicides in this family include Terraneb and PCNB.

Phenylpyrrole — The fungicides in this family are systemic in nature, and act by disrupting amino acid metabolism. The majority of the members in this family are insecticides, but some are fungicides. Specific fungicides that include this chemistry are Maxim and Medallion.

Nitriles — There is only one member of this group that is labeled for disease control on turfgrass, chlorothalonil. This fungicide disrupts normal cell metabolism and the regulation of cell functions. Examples include Daconil.

Inhibition of Nucleic Acid Metabolism and Protein Synthesis

Benzimidazoles — The fungicides in this family cause morphological distortion of germinating spores by inhibiting DNA synthesis (nuclear division). These chemistries are not toxic to fungi in their initial form but are broken down into metabolites that are toxic to fungi. Active ingredients with this specific chemistry used for turfgrass control are limited to thiophanate-methyl. Specific fungicides that contain this active ingredient are Cleary’s 3336, Topsin and Cavalier.

Phenylamides — These materials depress nucleic acid synthesis by decreasing the levels of RNA synthesis. The fungicides in this family provide good control of soil-borne diseases caused by omycetes, which include Phytophthora and Pythium. Active ingredients in the family include mefenoxam, metalaxyl, and oxadixyl. Specific fungicides containing these active ingredients include Subdue MAXX.

Dicarboximides — Inhibit DNA and RNA synthesis, cell division and cellular metabolism. Chemistries in this family are effective at controlling a wide range of pathogenic fungi including Fusarium, Rhizoctonia and Sclerotinia. Active ingredients in this family include iprodione and vinclozolin. Specific products available include Touché, Chipco 26019 and Vorlan.

Sterol Synthesis Inhibition

Ergosterol is the major sterol in most fungi and is essential for membrane structure and function. Ergosterol is the target of fungicides that inhibit sterol synthesis. Examples include:

Demethylation inhibitors (DMI) — This group of fungicides is also referred to as the sterol biosynthesis inhibitors (SBIs). The fungicides in this group work specifically by inhibiting the syn-thesis of ergosterol, which is necessary for the formation of the cell membrane. One of the vital functions of the cell membrane is to protect cell contents from outside chemicals by screening what flows into the cells. There are subgroupings within the DMIs that include the pyrimidines and the triazoles. Active ingredients in the DMI group include fenarimol, mycobutanil, triadimefon and propiconazole. Specific fungicides that are demethylation inhibitors include Rubigan, Eagle, Banner Maxx and Bayleton.

Antiobiotics. They specifically interfere with fungal cell wall synthesis by inhibiting chitin synthase. Examples of fungicides that are chitin synthase inhibitors include Endorse.

Multi-Site Activity

Phosphonates — The mode of action of the members of this group is complex and includes both direct and indirect mechanisms. One example is fosetyl-Al, which is registered for control of Pythium blight on turfgrasses. Once fosetyl-Al enters the plant, it degrades to phosphonate (phosphonic acid), which functions as a fungicide. In general the members of this group have direct fungitoxic activity against fungi and enhance the plant’s natural defenses. Specific fungicides in this group include Aliette and Prodigy.

Biofungicides

Biofungicides are naturally based microbial or biochemical products. There are two types of biopesticides: (1) Microbial biopesticides with an active ingredient that is a biological control agent (organism capable of attacking or competing with a pathogen or pest), and (2) plant biopesticides or plant-incorporated protectants are “pesticidal substances that plants produce from genetic material that has been added to the plant.”

Efficacy of Turfgrass Fungicides