Integrated Management of Turfgrass Diseases Lane Tredway, Ph.D.

An Integrated Pest Management (IPM) plan aims to reduce pest development to acceptable levels in the most economically efficient and environmentally sound means possible. It is a common misconception that IPM seeks to eliminate the use of pesticides. Nothing could be further from the truth! In fact, pesticides are an integral part of any IPM plan. However, a well-designed IPM plan will ensure that pesticides are used in the most effective manner, which often results in a reduction of pesticide usage. One of the first steps in development of an IPM plan is the establishment of damage tolerances. How much damage from disease can your customers or clients withstand? This varies widely depending on the situation and turf use. For example, the threshold for damage on golf course putting greens is very low and, usually, zero. In contrast, most homeowners will tolerate 5% to 10% damage in their lawns from disease. Some will tolerate more, others will tolerate less. Identifying the tolerance threshold is a critical first step because it determines the intensity of the IPM plan that is developed. What are the key components of an IPM plan for turfgrass diseases? Once again, let’s consider the disease triangle. Since there are three factors that are required for disease to develop, altering any of these three factors will help to reduce the development of disease. The Management Triangle (Figure 31) is used to depict this relationship. For example, planting a disease-resistant turfgrass species or variety will shorten the ‘host’ side of the triangle and reduce the amount of disease that develops. Using cultural practices to adjust the environment so that conditions are less favorable for disease development will reduce the ‘environment’ side of the triangle and further reduce disease development. Using a resistant grass and proper cultural practices will often control diseases to acceptable levels. However, in cases where disease pressure is high or tolerance thresholds are low, fungicide applications may be needed to suppress pathogen growth and protect the turf from attack. Fungicide applications suppress pathogen growth and shorten the ‘pathogen’ side of the triangle to further reduce disease development.

Figure 1. The diagnosis triangle depicts the clues that are useful for diagnosing turfgrass diseases. Species and Variety Selection: The First Line of Defense Disease management begins long before construction with the selection of a grass species and variety. Unfortunately, disease is often not considered in this selection phase, and as a result, the turfgrass manager is left with a major disease problem that persists for many years. In general, grass species that are well-adapted to the climate, microclimate, and intended use will be more resistant to disease than less well-adapted species. An excellent example of the importance of variety selection in disease management is provided by ‘Crenshaw’ creeping bentgrass. This variety was planted widely in the Southeast United States in the early 1990’s due to its excellent tolerance to heat and drought stress. Unfortunately, it was later discovered in University trials that Crenshaw is extremely susceptible to dollar spot. Most golf course superintendents who manage Crenshaw greens now have a chronic dollar spot problem that is very difficult to control, costing the golf course thousands of dollars more per year in fungicides for dollar spot control. There are several sources for information on the performance of turfgrass varieties in various regions of the United States. The National Turfgrass Evaluation

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Program (NTEP) conducts variety trials on all major turfgrass species across the United States. The results from these trials are available in summarized form on the NTEP website (www.ntep.org). Results from trials conducted in North Carolina are available on the Turffiles website (www.turffiles.ncsu.edu). No matter how good a new variety sounds, do not plant it in large areas until it has been tested in cultivar trials in your area! If necessary, you can plant an area of the new variety in a nursery to ensure that the grass is well-adapted to your situation and is not highly susceptible to disease. Cultural Management of Turfgrass Diseases Now that you have selected a well-adapted turfgrass species and a disease-resistant variety, it is time to establish and maintain the turf. Nearly every cultural practice that is performed on turf influences the development of one or more diseases. Some practices may help to reduce the development of certain diseases while making other diseases more severe at the same time. For example, diseases such as brown patch and Pythium blight are encouraged by application of high rates of water soluble nitrogen, however, anthracnose basal rot and dollar spot can be controlled by this same practice. Disease management can be equated to walking a tightrope – too much of a nutrient or management practice can be just as bad as too little. Remember our goal – to encourage the healthy growth of the turf while minimizing the growth of the pathogens that may cause disease. When establishing turfgrasses from seed, it is very important to apply the seed at recommended rates. Seeding rates that are higher than recommended can increase disease development during establishment and also for several years into the future. Seedling plants are immature and have less resistance to disease than mature plants. High seeding rates increase competition among seedlings for light, water, and nutrients and slow the maturation process. As a result, the turfgrass seedlings remain in a susceptible state for a longer period of time. In addition, seedling diseases such as Pythium damping-off and Rhizoctonia damping-off are encouraged by high humidity and extended periods of leaf wetness. Dense stands of turfgrass seedlings retain moisture and humidity longer and therefore encourage the development of these diseases. Research conducted in New York demonstrated that dollar spot and pink snow mold were more severe in creeping bentgrass plots seeded at 2 lbs per 1000 ft2 compared to plots seeded with the recommended 0.75 lb per 1000 ft2 up to two years after seeding.

Mowing is the management practice that is conducted most frequently and has a major impact on turfgrass health. Turfgrasses evolved under grazing pressure and have been further selected by breeding programs for performance under regular mowing. Because mowing has such a major impact on turfgrass health, it is not surprising that this practice has a significant impact on disease development. Mowing height is probably the most important consideration with respect to managing turfgrass diseases. Each turfgrass species has a range of mowing heights under which it performs best (Table 1). Mowing heights that are too low remove an excessive amount of leaf tissue and reduce the amount of energy that the plant is able to produce through photosynthesis. As a result, low mowing heights reduce root growth and also reduce the amount of energy that the plant can expend on protection from disease. Several diseases, such as algae, anthracnose, and dollar spot are encouraged by low mowing heights. Mowing heights that are too high can also encourage the development of certain diseases. High mowing heights produce a turfgrass canopy that is dense, matted, and retain moisture and humidity for extended periods of time. As we will see, most foliar diseases require extended periods of leaf wetness in order to develop. Gray leaf spot is an example of a disease that is encouraged by mowing heights that are too high (Figure 32). Table 1. Optimal mowing heights for common turfgrass species. Turf Species Mowing Heights (in.) Fine fescues 1.5 – 2.0 Tall fescue ≥ 1.5 Kentucky bluegrass 0.75 – 2.5 Annual bluegrass ≤ 1.0 Perennial ryegrass 1.5 – 2.0 Creeping bentgrass 0.125 – 0.5 Bermudagrass 0.125 – 1.0 Zoysiagrass 0.5 – 1.0 Centipedegrass 1.0 – 2.0 St. Augustinegrass 2.5 – 3.5 The optimal mowing frequency depends on the mowing height and should be determined by using the ‘1/3 rule’. This rule states that no more than 1/3 of the leaf tissue on a turfgrass plant should be removed by any one mowing. For example, for turf that is being maintained at a height of 2 inches, mowing should be performed before the turf reaches a height greater than 3 inches. Mowing less frequently can result in scalping and severe

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injury to the turf, which can encourage the development of several diseases. Infrequent mowing also allows the turf canopy to become dense and matted, which holds moisture and encourages the development of foliar diseases. Another disadvantage of infrequent mowing is that foliar pathogens have an extended period of time to damage the turfgrass plant and produce spores before they are removed by mowing.

Figure 2. High mowing heights can increase leaf wetness and encourage the development of foliar diseases. Gray leaf spot is strongly encouraged by high mowing heights, as seen here. The disease is very severe in the perennial ryegrass rough, which is mown at 2 inches, but the perennial ryegrass fairway is completely healthy. Over the years, there has been considerable debate over the benefits or disadvantages of collecting clippings when mowing. Contrary to popular belief, leaf clippings do not contribute to thatch accumulation. Leaf tissue breaks down rapidly after mowing because it is easily degraded by soil microorganisms. Instead, thatch is primarily composed of stems, rhizomes, stolons, and roots. Therefore, there is no real advantage to collecting clippings in most circumstances. However, there are certain situations in which collecting clippings can help to control diseases. Collecting clippings can help to reduce the development of gray leaf spot, rust, red thread, and Helminthosporium leaf spots. These are all diseases which produce spores or other propagules on the turfgrass leaves. Collecting clippings removes the infected leaves away from the turf site and therefore slows the spread of the disease. It is important to note that clipping collection is only recommended when these diseases are actively developing. On the other hand, clippings are an excellent source of nitrogen and other nutrients, which are released as the leaves decompose. Not collecting clippings is therefore advantageous because it reduces the amount of fertilizer that must be applied to the turf. Removing clippings can

encourage the development of diseases that are encouraged by low fertility, such as dollar spot or anthracnose. If clippings must be collected due to customer demands, which is common for golf course or athletic field turf, it is important to increase fertility levels to account for the nutrients that are being removed in clippings. When mowing, it is very important that the mower blades are sharp and well-adjusted. Dull rotary mower blades or poorly adjusted reel mowers fray the tips of leaf blades rather than making a clean cut. These frayed wounds take longer to heal and therefore the turf plant is susceptible to infection by foliar pathogens for an extended period of time. Also, the turf plant must expend more energy to repair these wounds, which may cause an overall reduction in the plant’s health. Red thread is one disease that is commonly encouraged by the use of dull mower blades in landscape turfgrasses (Figure 33).

Figure 3. Red thread is greatly encouraged by dull mower blades. The fungus infects the turf through the leaf tip, and frayed leaf tips are more prone to infection than cleanly cut tips. Some pathogens can be spread by mowing, especially early in the morning when the leaves are wet and the pathogens are actively producing spores or mycelium (Figure 34). Therefore, when there is a disease actively developing, it is best to allow the turf to dry completely before mowing so as to reduce the spread of the pathogen. Irrigation is also an important cultural practice in managing turfgrass diseases. Some diseases, such as dollar spot or anthracnose basal rot, are encouraged by drought stress. Drought-stressed turf is not able to function properly physiologically and is less able to protect itself from attack by these diseases. However, there are also many other diseases that are made worse by over-irrigation. Wet soils prevent the development of a deep, dense root system, maintain high humidity in the

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turf canopy, and also encourages the growth of many turf pathogens. Again, the goal here is to provide the turf with a sufficient amount of soil moisture without keeping the soil too wet.

Figure 4. Some foliar diseases, such as Pythium blight, can be spread by mowers or other equipment when the disease is actively developing. In addition to the amount of irrigation, the frequency and timing of irrigation is also important, especially with respect to foliar diseases. Most foliar diseases require at least 10 hours of continuous leaf wetness or high relative humidity (>95%) in order to develop. The longer the leaves remain wet, the more severe these diseases become because the pathogen has more time to grow, infect, and cause disease. So what causes turfgrass leaves to become wet? There are three important sources of leaf wetness: dew (condensed atmospheric water vapor), guttation (vascular fluid exuded from the leaf tip), and irrigation water. There is little that can be done to prevent the formation of dew or guttation water. Turfgrass leaves are typically wet for at least 8 hours each night from dew and guttation. We do, however, have control over the timing of irrigation and it is critical that irrigation is applied when it will not extend this period of natural leaf wetness. Irrigation water is best applied between midnight and 6 am. This removes the large droplets of dew and guttation water and replaces it with smaller droplets of irrigation water, which dry more quickly once the sun rises. Irrigation water should not be applied before sunset or after sunrise; this will increase leaf wetness duration and may trigger an outbreak of a foliar disease. The frequency of irrigation is also important. In general, irrigation should be applied as infrequently as possible to encourage the development of a deep, dense root system and to avoid increases in leaf wetness. This can be accomplished by applying irrigation on a deep and infrequent schedule. This is done by applying a sufficient

amount of water to thoroughly wet the entire root zone and then repeating when the root zone has become dry or when the turf begins to show signs of mild wilt. The frequency and amount of irrigation will be dictated by root depth, soil type, sunlight intensity, relative humidity, and temperature. Following this type of program requires the regular use of a soil probe to assess the depth of the root system and the amount of moisture in the soil profile. In addition to the timing of irrigation, several other practices can be implemented to reduce leaf wetness duration. Mowing is an effective way to remove dew and guttation water and can be used to reduce leaf wetness duration. If mowing is not being performed, many turfgrass managers reduce leaf wetness by dragging a pole, hose, or chain across the turf in the morning to knock the large water droplets from the turf. Shade or restricted air movement are a common cause of increased leaf wetness. Trees, landscape plants, buildings, fences, or other obstructions that block sunlight or wind movement will increase disease pressure by increasing the duration of leaf wetness. This effect can be reduced by pruning or removal or trees, installation of mulch beds in place of turf, or establishment of shade-tolerant turfgrass species such as the fine fescues. When tree removal is not an option, many golf course superintendents install high-powered fans surrounding putting greens (Figure 35). These fans do help to increase air movement over the turf surface, but obviously do not increase sunlight penetration.

Figure 5. High-powered fans can be used to reduce leaf wetness in areas where trees or other obstructions reduce air circulation. Fans, however, do not solve the shade problems that often exist in such locations.

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Fertility levels have a huge impact on turfgrass health and disease development. Nitrogen is the nutrient that has the greatest effect on turfgrass growth, and this nutrient also has the greatest impact on disease development. Some diseases are enhanced by excessive nitrogen levels, such as brown patch, Pythium blight, gray leaf spot, and summer patch. High nitrogen levels cause the turf to put all of its energy into foliar growth at the expense of root development and storage of carbohydrates. As a result, the plant develops a shallow root system and becomes stressed during times of heat and drought. In addition, a turf plant growing under excessive N levels has less energy available for defense from disease. Deficient nitrogen levels also encourage several diseases, such as dollar spot, anthracnose basal rot, and red thread. Turf that is deficient in nitrogen grows slowly, cannot produce an adequate amount of chlorophyll, and does not have sufficient energy available for protection from pathogen attack. The use of controlled-release fertilizers or light-frequent applications of soluble fertilizers (‘spoon-feeding’) are effective ways to provide the turf plant with adequate nitrogen while preventing surges of foliar growth. Virtually all other essential plant nutrients have an effect on one or more turfgrass diseases. Deficient levels of other nutrients seem to be more problematic than excessive levels from a disease perspective. For example, red thread development in perennial ryegrass can be encouraged by calcium or magnesium deficiencies. Manganese deficiencies increase the severity of take-all patch in creeping bentgrass. Low levels of zinc and manganese have also been associated with increases in dollar spot activity on creeping bentgrass. Soil pH has a significant effect on a group of root diseases, called the ‘patch diseases’ after the most common symptom that they induce (Figure 36). Most of the major turfgrass species are attacked by one or more of these patch diseases. Summer patch occurs on bluegrasses, fine fescues, and creeping bentgrass. Take-all patch attacks all of the bentgrass species grown for turf. Spring dead spot is a persistent problem in bermudagrass and has recently been identified in zoysiagrass. All of these diseases are most severe when soil pH is above 6.5, so reduction of soil pH below this level is a very effective management tool. Soil pH can be lowered through the application of elemental sulfur or by using ammonium forms of nitrogen, such as ammonium sulfate. Elemental sulfur must be used with extreme caution because it can cause direct burn to turfgrass foliage, can contribute to the

development of black layer, and can also drive the soil pH too low too quickly. For this reason, elemental sulfur is only recommended when soil pH is greater than 7. When it is used, it should be applied at a maximum rate of 0.5 lb per 1000 ft2 to putting green turf or 5 lb per 1000 ft2 to other turf areas. Apply when soil temperatures are between 50°F and 70°F and wait at least 8 weeks before making a follow-up applications. Soil tests should also be conducted to determine if a follow-up application is necessary. If soil pH is below 7, then ammonium forms of nitrogen, such as ammonium sulfate, can be used to reduce soil pH gradually. When a root absorbs an ammonium ion (NH4

+), a hydrogen ion (H+) is exuded into the soil to keep the charges balanced. The hydrogen ion is what causes the soil to become more acidic. These nitrogen forms can be used as part of a normal management program and do not have the potential for negative side effects as does elemental sulfur. Turfgrass species vary in their sensitivity to acidic soils, so this must be considered when attempting to reduce soil pH for patch disease management. Bermudagrass grows optimally when pH is as low as 5.0 and bentgrasses suffer no adverse effects at a pH as low as 5.5. Annual and Kentucky bluegrasses are not as tolerant of low pH, so pH should not be lowered below 6.0 for these turfgrass species.

Figure 6. Summer patch is a patch disease that occurs on Kentucky bluegrass. All of the patch diseases are most severe when soil pH is above 6.5. Reducing soil pH below this level through application of sulfur or ammonium nitrogen is an effective management technique.

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Soil drainage also has a major impact on disease development. Many diseases are more severe in soils that remain wet for extended periods of time due to compaction, lack of surface drainage, or slow subsurface drainage. Wet soil conditions prevent the development of a deep, dense root system and also encourage the growth of many turfgrass pathogens. Maintaining adequate soil drainage is a very effective disease management practice. Some diseases may be encouraged by soils that drain too quickly and do not hold an adequate amount of moisture and nutrients. Dead spot, take-all patch, and nematodes are examples of diseases that are most severe in dry, sandy soils. Turfgrasses growing in these types of soils are constantly stressed due to a lack of adequate water and nutrients. Increasing the frequency of irrigation and fertilization, or amending the soil with organic matter, are effective methods for management of these diseases. Excessive thatch is a common contributor to disease problems in turfgrasses, especially in grass species that accumulate thatch rapidly, such as bentgrasses, bermudagrasses, zoysiagrasses, and bluegrasses. Thatch encourages disease development through several mechanisms. First, many fungal pathogens survive in the thatch layer while they are not actively causing disease. The more thatch that is present, the higher pathogen populations will be, and the greater the potential for disease development (Figure 37). Also, excessive thatch accumulations often lead to scalping injury as mowing equipment sinks into the spongy thatch layer, imposing an additional stress on the turf.

Figure 7. Several turfgrass diseases are encouraged by excessive thatch accumulations. When not causing disease, fungal pathogens survive in the thatch layer. As thatch accumulates, the pathogen population becomes higher and disease pressure increases. Fairy ring, shown above, is one disease that is encouraged by excessive thatch. You can see a large amount of fungal mycelium growing in the thatch layer, which is well over 2 inches deep in this bermudagrass sample.

Fungicides for Turfgrass Disease Control Lane Tredway, Ph.D.

Fungicides are an important component of many IPM plans for turfgrass diseases. Fungicides, however, are not always necessary. Because of the expense and potential for adverse side-effects on the turf or environment, fungicide applications should always be viewed as a last resort, only after all other possible means of disease management have been implemented. In many cases, diseases can be managed to acceptable levels by selecting resistant grasses and implementing proper cultural practices. This is especially true in landscape turfgrasses, where damage thresholds are higher, management practices are less intense, and more flexibility exists with respect to grass selection. In contrast, fungicides are a critical component of disease management programs in golf course turfgrasses as discussed in Chapter 1. The term ‘fungicide’ literally means ‘fungus-killing substance’. Based on the use of this term, it is a common misconception that fungicide applications actually kill fungal pathogens and reduce their populations in the turf. This was true with many of the heavy metal fungicides such as mercury and cadmium that have long been outlawed in the United States. Unfortunately, most of today’s fungicides do not kill fungi, but instead only temporarily inhibit their growth. These products can be more accurately described as ‘fungistats’, meaning a substance that inhibits fungal growth. We continue to use the term ‘fungicides’, but just keep in mind that these materials are only temporarily inhibiting fungal growth and are not necessarily reducing their populations. When fungicide applications are needed to control a disease, a turfgrass manager must strive to maximize his or her fungicide use efficiency, or the amount of disease control provided by each dollar spent on fungicide products. There are four primary means by which a turfgrass manager can maximize fungicide use efficiency:

1. Select the best fungicide for each situation. 2. Time applications based on weather conditions

that are favorable for disease development. 3. Apply fungicides properly. 4. Manage the development of fungicide

resistance. The information provided in this chapter will provide a foundation of knowledge related to each of these points. The most successful turfgrass managers are those that follow these three steps, as they are able to maintain

disease-free, high-quality turfgrass on a reasonable budget. Remember that learning about fungicides is a continuous process that will continue throughout your career in the turfgrass industry. Fungicide Terminology Fungicides are a confusing topic for students studying turfgrass science. But don’t feel bad, there are many turfgrass managers who have been in the field for decades and still struggle with this topic. Why is this such a difficult topic? The primary reason is that there is a lot of knew terminology associated with this subject. Take the time to learn this terminology, and fungicides will become much less mysterious to you. Definitions of the key terms related to fungicides are provided in Appendix A. There are two different ways in which fungicides can be applied to control turfgrass diseases. Preventative applications are made before symptoms of the disease appear with the goal of protecting the healthy plants from infection. On the other hand, curative applications are made after symptoms of the disease have appeared. Curative applications have two goals: (1) to cure plants that have already been infected by the pathogen; and (2) to protect still healthy plants that have not yet been infected. Preventative control is recommended for most turfgrass diseases because curative applications require increased rates and shorter application intervals and are therefore more expensive. There are somewhere in the neighborhood of 100 different fungicides that are applied to turfgrasses. To make matters worse, each fungicide has three different names (Table 2). The chemical name is the complete chemical formula for the active ingredient. This name means something to chemists but is too long and complicated to be used by practitioners in the field, including myself. We use the common name, which is a simplified name for the active ingredient. This name is used much more readily in conversation, books, or magazine articles. The trade name is the commercial name that is used for the formulated product. Note that the same active ingredient may be sold under several different trade names. However, since they contain the same active ingredient, these products are essentially the same even though they are sold under different trade names.

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Table 2. Chemical name, common name, and trade names for a common turfgrass fungicide. Chemical name tetrachloroisothalonitrile Common name Chlorothalonil Trade names Daconil, Chlorstar,

Concorde, Echo, Manicure The fact that fungicides containing the same active ingredient are nearly identical simplifies things for us quite a bit, as there are only 24 different active ingredients that are applied to turfgrasses for disease control. This means that you only have 24 different fungicides to learn instead of 100! But wait, we can simplify this even more. Fungicides can be placed into chemical classes, which are groups of fungicides with similar chemical properties. Because they are chemically similar, fungicides within a class inhibit fungal growth in a similar manner and also move within the plant in a similar fashion. For practical purposes, fungicides within a class are nearly identical. There are 13 different classes of fungicides that are applied to turfgrasses (Table 3), further reducing the number of different fungicides that you need to learn! Considerations in Selecting a Fungicide The first step toward maximizing fungicide use efficiency is selecting the best fungicide for each situation. What are the factors that must be considered when choosing a fungicide? Of course, a fungicide must be effective for control of the disease you are trying to control. How do you determine if a fungicide provides good or excellent control of a certain disease? The logical first step would be to read the fungicide label to see if the disease is listed. Unfortunately, this means very little because many fungicides are labeled for diseases that they do not control adequately (Figure 36). Maybe you could just ask the next salesperson that walks through the door? If you take this strategy, it may be no small coincidence that the most effective fungicide happens to be the one that he or she has loaded on the truck! The best way to select an effective fungicide is to refer to an unbiased source of information, such as your local Cooperative Extension Service or other turfgrass managers in your area. Ratings of currently available fungicides for control of common turfgrass diseases are provided in Appendix B. Another important consideration when selecting a fungicide is residual control, or the number of days of disease control provided after application. Some

fungicides are very short-lived and only provide 5 to 7 days of disease control, whereas others are long-lasting and provide several weeks of disease suppression. Because fungicides that provide long residual control are generally more expensive than short-residual products, it is often advantageous to make more frequent applications of a short-residual products if this is logistically feasible. However, in a landscape situation, applications can only be made every 4 to 6 weeks, so a long-residual product is essential.

Figure 8. Buyer beware! Many fungicides are labeled for diseases that they do not control very well. For example, Eagle 40WP (myclobutanil) is labeled for control of brown patch but does not provide adequate control of this disease in tall fescue. In most cases, there will be more than one disease, and sometimes many, that have the potential to develop at a given time. This is why control spectrum is an important consideration when selecting a fungicide. Control spectrum simply refers to the number of different diseases that are controlled by a fungicide: a broad spectrum fungicide controls a large number of diseases, whereas a narrow spectrum fungicide controls a small number of diseases. There is no official line between what constitutes a broad spectrum vs. a narrow spectrum fungicide; it is more of a subjective term. Chlorothalonil is a good example of a broad spectrum fungicide and is labeled for control of 14 different diseases. Boscalid, on the other hand, is only labeled for control of two diseases and is considered a narrow spectrum fungicide. The topical mode of action is also an important consideration when selecting a fungicide. Topical mode of action describes how the active ingredient moves on and in the plant after application. A contact fungicide stays on the leaf surface after application. A localized penetrant is absorbed into the leaf but is not translocated, and a systemic fungicide is absorbed into the leaf and translocated in the vascular tissue of the plant. There are two types of systemic fungicides that

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Table 3. Chemical class, common name, and other characteristics of fungicides used turfgrass diseases control.

FRAC #1

Chemical Class Common Name Topical MOA

Resistance

Risk Trade Name(s)

14 Aromatic chloroneb contact low Terraneb, Terremec

Hydrocarbon ethazole contact low Koban, Terrazole

quintozene contact low Terrachlor, Turfcide, PCNB

1 Benzimidazole thiophanate-methyl acro. penetrant high 3336, Systec, Fungo

28 Carbamate propamocarb local. penetrant low Banol

7 Carboxamide boscalid acro. penetrant high Emerald

flutolanil acro. penetrant moderate ProStar

2 Dicarboximide iprodione local. penetrant moderate Chipco 26GT

vinclozolin local. penetrant moderate Curalan, Touche, Vorlan

M2 Dithiocarbamate mancozeb contact low Fore, Dithane, Mancozeb

thiram contact low Spotrete, Thiram

3 DMI fenarimol acro. penetrant moderate Rubigan

myclobutanil acro. penetrant moderate Eagle

propiconazole acro. penetrant moderate Banner MAXX

triadimefon acro. penetrant moderate Bayleton

M4 Nitrile chlorothalonil contact low Daconil, Chlorostar, Manicure

4 Phenylamides mefanoxam acro. penetrant high Subdue Maxx

12 Phenylpyrolle fludioxonil contact low Medallion

H4 Polyoxins polyoxin D translaminar moderate Endorse

33 Phosphonate fosetyl-Al systemic low Aliette Signature

phosphorous acid systemic low Alude, Resyst, Vital

11 QoI azoxystrobin acro. penetrant high Heritage

pyraclostrobin mesostemic high Insignia

triflozystrobin mesostemic high Compass1Designation assigned to each chemical class be the Fungicide Resistance Action Committee. According to

new EPA guidelines, these numbers are to be printed prominantly on the front page of fungicide labels.

are used in turfgrasses. An acropetal penetrant is absorbed and translocated only in the xylem of the plant. These fungicides, therefore, are only translocated upward in the plant. On the other hand, a true systemic is absorbed and translocated in both the xylem and phloem, meaning that these fungicides are moved both upward and downward in the plant after application. Topical mode of action is an important consideration when selecting a fungicide for several reasons. First, the topical mode of action determines the length of residual control provided by a fungicide. Because contact fungicides remain on the leaf surface, they are quickly washed off of the leaf by rain or irrigation and broken

down by UV light. For this reason, contact fungicides in general only provide 7 to 14 days of residual control. Localized penetrants are absorbed into the leaf and therefore are not washed off of the leaf surface and last longer than contacts, up to 14 to 21 days. Because they are absorbed and translocated throughout the plant, systemic fungicides last for at least 21 days after application, and in certain cases, up to 6 weeks. Topical mode of action also determines how effective a fungicide will be for curative applications. Contact fungicides are not effective for curative control of turfgrass diseases. Contacts will help to prevent further spread of the disease by protecting plants that are not

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yet infected, but they will not help to cure plants that are already infected because they remain on the surface of the plant. Systemic fungicides are most effective for

curative applications because they are translocated throughout the plant and can help to cure plant tissues that are already infected by the fungal pathogen. For

Table 4. When selecting a fungicide, purchasing decisions should be made based on cost per unit area rather than cost per pound. When application intervals vary among products, the price per unit area per day should be considered.

Product Rate (oz/1000 ft2) Interval (days) Price ($/lb or gal) $/1000 ft

2$/1000 ft

2/day

Heritage TL 1 28 $576 (gal) $4.50 $0.16

Heritage 50WG 0.2 28 $356 $4.56 $0.16

Insignia 20WG 0.7 28 $111 $4.88 $0.17

SysStar 80WDG 2.25 28 $44 $6.19 $0.22

Compass 50WG 0.25 21 $300 $4.69 $0.22

Prostar 70WP 2.25 28 $52 $7.17 $0.26

Banner Maxx 2 14 $289 (gal) $4.52 $0.32

Bayleton 50DF 1 14 $81 $5.06 $0.36

preventative applications, all fungicides are effective because the pathogen has not yet gained entry into the plant. Topical mode of action is also an important consideration when combining more than one fungicide, a practice called tank-mixing, which is common on golf course putting greens. When tank-mixing fungicides, the mixture components should have different topical modes of action. For example, a tank-mixture could include a contact + localized penetrant, contact + acropetal penetrant, or a localized penetrant + acropetal penetrant. Combining different topical modes of action creates multiple layers of protection in and on the turfgrass plant and greatly increases the efficacy of disease control. The biochemical mode of action of a fungicide describes the mechanism by which fungal growth is inhibited. For example, chlorothalonil inhibits fungal growth by stopping thiol-dependent reactions in the fungal cell. Azoxystrobin binds to the QoI protein on the inner mitochondrial membrane and by doing so inhibits energy production in the fungal cell. For the sake of simplicity, fungicides can be divided into two groups based on their biochemical mode of action: multi-site inhibitors interfere with more than one biochemical reactions in the fungal cell, whereas single-site inhibitors interfere with a single biochemical reaction. Chlorothalonil is an example of a multi-site inhibitor because there are several hundreds or even thousands of thiol-dependent reactions that occur in the fungal cell. Azoxystrobin, on the other hand, is a single-site inhibitor because it only binds to one single protein. Biochemical mode of action is important because it determines the fungicide’s risk for the development of fungicide resistance. Fungicide resistance occurs when naturally-occurring strains of a pathogen, which are not

inhibited by a particular fungicide, become dominant in the pathogen population following repeated applications of the fungicide. Because they only inhibit a single biochemical reaction, single-site inhibitors are at high risk for fungicide resistance. Multi-site inhibitors have a moderate or low resistance risk depending on the number of reactions that are inhibited in the fungal cell. Some fungicides may have adverse side effects on the turf that warrant consideration. The DMI fungicides are well known for their growth regulating side-effects on turfgrasses. This growth regulation results in significant thinning of close-cut turf, especially if applied repeatedly, at high rates, or during hot weather. To prevent turf thinning, the DMI fungicides should never be applied to close-cut turf when high temperatures are above 90°F. Other fungicides have been shown to cause adverse side effects. For example, repeated applications of benzimidazole fungicides have been shown to increase thatch accumulations and also encourage the development of Pythium blight. Finally, cost is an important consideration when selecting a fungicide. The cost of a fungicide application not only includes the purchase price of the fungicide, but also labor and equipment operation costs. Far too many turfgrass managers select fungicides based on price per pound or price per gallon. Because application rates and intervals vary considerably, cost per pound or gallon means absolutely nothing! In order to compare fungicides, you must calculate the cost of an application on a per acre or per 1000 ft

2 basis. If application

intervals vary, one can also calculate the price per unit area per day as well (Table 4).

Fungicides for Turfgrass Disease Control Page 11

Differences within a Chemical Class Earlier, we learned that fungicides within a chemical class are identical. While this is a good rule of thumb, there are exceptions. Fungicides within a chemical class may differ in several characteristics, including cost, control efficacy, control spectrum, potential for side-effects, or even topical mode of action. The QoI class is one in which there are some important differences among fungicides. The QoIs differ in topical mode of action, residual control, and control spectrum. Azoxystrobin (Heritage), the first QoI fungicide released in 1996, is an acropetal penetrant. Pyraclostrobin (Insignia) and trifloxystrobin (Compass) have a mesostemic mode of action, which is similar to a localized penetrant. As a result of their different topical modes of action, these fungicides also differ in residual control, with azoxystrobin typically lasting the longest (28 days or more) and the others lasting for shorter periods of time (21 days or less). The QoIs also differ in their control spectrum. One of the major drawbacks of azoxystrobin is that it does not control dollar spot, one of the most common turfgrass diseases. As a result, dollar spot control was a top priority for other companies as they were developing QoIs to compete with azoxystrobin. Both pyraclostrobin and trifloxystrobin have some dollar spot activity and are labeled for ‘suppression’ of this disease, but only provide a 25% to 50% reduction in dollar spot development. Because of their mesostemic mode of action, pyraclostrobin and trifloxystrobin also tend to be less effective for control of root diseases than azoxystrobin, which can be watered into the soil, absorbed by the roots, and then translocated throughout the root system. Some important differences also exist among the DMI fungicides, particularly with respect to their side-effects. The DMI fungicides are closely related to a group of herbicides and growth regulators called the triazoles. This group includes paclobutrazole, which is commonly applied to turfgrasses for suppression of annual bluegrass. All of the DMI fungicides also have herbicidal or growth regulating effects on turfgrasses and can cause significant thinning of putting green turf when applied at high rates. Among the DMIs, fenarimol (Rubigan) has the most severe herbicidal properties and is rarely applied to putting green turf for this reason. Propiconazole (Banner Maxx) generally has the least growth regulating effects, followed by myclobutanil (Eagle) and triadimefon (Bayleton).

Differences within a Common Name We also learned earlier that different trade names which contain the same active ingredient are identical. There are some exceptions to this rule as well. In reviewing Table 3, you will notice that some products are sold under multiple trade names (example: chlorothalonil is sold as Daconil, Chlorstar, Manicure, Echo, and others), whereas other products are sold under only one trade name (example: flutolanil is only sold as ProStar). Why is this? When a new fungicide is discovered, the company that discovered it applies for patent protection through the U.S. government, which provides them the sole right to produce and market that fungicide for 17 years. After the 17 year period has expired, then any company or individual may produce and market that fungicide. The fungicides that are sold under multiple trade names are no longer under patent and can be marketed by anyone. So what is the difference between products containing the same active ingredient? The companies who market these products usually purchase the active ingredient from the same producer, so there are no differences in the active ingredient. What does differ is the ingredients and methods that are used to formulate the active ingredient. These differences can impact control efficacy, residual control, and potential side-effects. Research has shown that formulations of chlorothalonil differ significantly in the level of dollar spot control that they provide (Figure 37). The differences are most pronounced among the dry flowable (DF) formulations of chlorothalonil. Certain formulations of propiconazole and mefanoxam, such as Spectator and Mefanoxam, use oil-based EC formulations that may cause foliar burn on turfgrasses during hot weather. Fungicide Application Timing The timing of application has a tremendous impact on the efficacy of fungicides. Because today’s fungicides only temporarily inhibit fungal growth, they are most effective when applied on a preventative basis. In general, turfgrass diseases should be controlled preventatively, before the pathogen has infected the plant. How do you know when infection is going to occur? Since disease development is triggered by the environment, you can predict disease development with a fair degree of accuracy just based on weather conditions. For example, the dollar spot pathogen begins to grow and infect the turf in the spring when nighttime temperatures consistently exceed 50°F. A preventative dollar spot program should therefore be initiated at this time. Preventative applications for control

Chapter 4: Fungicides for Turfgrass Disease Control Page 24

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Infe

cti

on

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rs p

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Daconil

Ultrex

Concorde

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Wstik

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Echo 720 Control

Figure 39. Comparison of chlorothalonil formulations for dollar spot control in creeping bentgrass. Products were applied once at

2.7 oz a.i. per 1000 ft2, and data was collected 14 days after

application (Data provided by B.B. Clarke, Rutgers University).

of root diseases should be timed based on soil temperature since this parameter is most influential to these diseases. For example, the fungus that causes take-all patch begins to grow and infect creeping bentgrass roots in the fall when soil temperatures decline to 60°F. A preventative program for take-all patch control should be initiated at this time. It is very important that fungicide applications should not be timed based on the calendar, as the correct timing can vary from year to year by as much as several weeks due to differences in weather conditions. Applying Fungicides Properly The manner in which a fungicide is applied has a major impact on how effective it is. The primary considerations when applying a fungicide are:

1. Application Rate 2. Nozzle Type 3. Nozzle Pressure 4. Dilution Rate or Delivery Rate 5. Post-Application Irrigation

On fungicide labels, application rates are typically listed as ‘preventative’ or ‘curative’, with the preventative rates being lower than the curative rates. It is important to note, however, that these are only guidelines and do not have to be followed precisely. As long as you stay within the rate range listed on the label, one can apply any rate that he or she chooses regardless of whether it is a preventative or curative application. In general, low rates should be used when disease pressure is low or when repeat applications are to be made on short intervals.

High rates should be used when disease pressure is very high, when long periods of residual control are desired, or for curative applications. The type of nozzle that is used to apply a fungicide has a major impact on its effectiveness. Many turfgrass managers have switched to using ‘rain-drop’ style nozzles that are operated at low pressures. These nozzles produce large droplets that minimize the potential for drift of the applied material to neighboring properties or environmentally sensitive areas. Unfortunately, the coverage provided by raindrop nozzles is not as uniform as that provided by standard flat-fan nozzles. Consequently, fungicides applications made through rain-drop nozzles are less effective than those made through flat-fan nozzles. This is particularly true for foliar and stem diseases, as uniform coverage is not as critical for control of root diseases. Nozzle pressure impacts disease control in a similar fashion as nozzle type. Low nozzle pressures of 20 to 30 psi are often desirable because of the reduced potential for drift, but they do not cover the turf as well as nozzles operated at 40 psi. Nozzle pressures above 40 psi dramatically increase the amount of drift that will occur and do not significantly increase the quality of coverage. Again, nozzle pressure is primarily a concern with foliar diseases and is not as important for control of root diseases. In order to maximize fungicide use efficiency, all fungicides applied for control of foliar or stem diseases should be applied through flat-fan nozzles operated at 40 psi. Dilution rate, also known as delivery rate, is the amount of water per unit area in which a fungicide is applied. The industry-standard dilution rate for application of fungicides for control of foliar diseases is 2 gallons per 1000 ft

2 or 87 gallons per acre. Dilution rates below 2

gallons per 1000 ft2 do not provide adequate control of

foliar diseases because coverage becomes less uniform. Please note that dilution rate has absolutely nothing to do with the fungicide application rate. The same amount of fungicide is being applied regardless of the dilution rate. Increased dilution rates should be used when applying fungicides for control of stem, crown, or root diseases. Applications in high water volumes are also known as drench treatments. Remember that most systemic fungicides only move upward in the plant after application, so these fungicides must be moved downward in the turf canopy so that they can protect the stems, crowns, or roots from attack. For control of stem or crown diseases, dilution rates of 3 gallons per 1000 ft

2 is recommended, whereas applications in 5 gallons

Fungicides for Turfgrass Disease Control Page 13

per 1000 ft2 are most effective for control of root diseases. Post-application irrigation, or ‘watering-in’, is another option for delivering fungicides deeper into the turf canopy for control of stem, crown, or root diseases. Generally, application of 1/8” of water is recommended for putting green turf or 1/4” for fairway or athletic field turf. This method is faster and more convenient than increased dilution rates because the sprayer does not have to be recalibrated and fewer spray tanks need to be mixed. This method is only an option if the irrigation system provides uniform coverage. If this method is used, it is critical that the irrigation be applied to the turf immediately after application, before the spray begins to dry on the turf foliage. Depending on the capacity of the irrigation system, the spray rig operator often winds up waiting for the irrigation system to catch up. In this case, it may be more efficient to use a higher dilution rate rather than post-application irrigation. Fungicide Resistance: A Developing Crisis As discussed in Chapter 2, the types of fungicides that are available to turfgrass managers are changing. The old contact, broad spectrum fungicides such as chlorothalonil and mancozeb are slowly but surely being removed from the market due to environmental and human health concerns. These products are being replaced with a new generation of fungicides that are safe, environmentally friendly, and very effective. However, these new fungicides all have site-specific modes of action, meaning that they are prone to the development of fungicide resistance. For this reason, fungicide resistance is the most important issue that turfgrass managers will deal with over the next decade. A fungal pathogen is actually a population of individuals, and they, like humans, differ in many characteristics. Even before a given fungicide is applied to a turfgrass area, there are naturally-occurring strains of the pathogen that are not controlled by that particular fungicide. Initially, these strains are present at such low levels that they are never seen and the fungicide provides excellent control. However, if the same fungicide is applied to the turf repeatedly, these resistant strains are able to grow and reproduce without competition. When the resistant strains become dominant in the population, a control failure occurs, often with devastating results (Figure 37). Fungicide resistance occurs when a fungicide becomes ineffective due to a shift in the pathogen population toward strains that are naturally resistant to the fungicide.

Once a pathogen develops resistance to a given fungicide, the use of that fungicide for control of that disease is usually lost forever. With few exceptions, fungicide-resistant strains are able to compete and persist in the population for many years even if the fungicide is not applied. The fungicide still can be used to control other diseases, and the disease in question can still be controlled with other fungicides. However, if care is not taken to prevent the development of fungicide resistance, a turfgrass manager will soon run out of fungicide choices, and will probably be out of a job as well.

Figure 38. If you are a golf course superintendent, this is not how you want to make the front page of your local newspaper! The consequences of fungicide resistance are often devastating. This is the result of resistance to the QoI fungicides in the gray leaf spot pathogen, Pyricularia grisea, in Lexington, KY. Because fungicides within a chemical class have identical biochemical modes of action, a pathogen that is resistant to one fungicide will also be resistant to all of the other fungicides in that group. This is called cross-resistance. For example, a golf course superintendent repeatedly applies Banner Maxx (propiconazole) for control of dollar spot. After 3 years, the pathogen population has become resistant to this fungicide and is no longer controlled. The dollar spot population at this golf course will also be resistant to the other DMI fungicides – fenarimol, myclobutanil, and triadimefon. For this reason, we often refer to fungicide resistance according to the chemical class that is affected (i.e. benzimidazole resistance, dicarboxamide resistance, DMI resistance, QoI resistance, etc). Pathogens are also able to develop resistance to more than one chemical class, a condition called multiple resistance. Let’s consider the previous example of the golf course superintendent with DMI-resistant dollar spot. If the superintendent switched to using thiophanate-methyl repeatedly, then benzimidazole

Fungicides for Turfgrass Disease Control Page 14

resistance would eventually develop. Since the DMI resistance did not go away, now the population is resistant to both the DMIs and the benzimidazoles. You can see that this poor superintendent is quickly running out of fungicides for dollar spot control. Not all fungicides are at risk for resistance development. The potential for resistance is determined by the fungicide’s biochemical mode of action. Single-site inhibitors are at high risk for resistance because they only inhibit one biochemical reaction in the fungal cell. There is a high likelihood that there will be a strain that is resistant to a single-site inhibitor in a given pathogen population. On the other hand, multi-site inhibitors have a low or moderate risk for fungicide resistance, depending on the number of biochemical reactions that are inhibited. Fungicides that inhibit several hundred or even thousands of reactions will have a low resistance risk; it is almost impossible for a fungus to develop resistance to these kinds of fungicides. Fungicides that inhibit 3 or 4 biochemical reactions will have a moderate resistance risk. It is possible for a pathogen to develop resistance to these fungicides, but it will take much longer than with a single-site inhibitor. Fungal pathogens vary in their ability to develop fungicide resistance also. Some fungi develop resistance to fungicides very quickly, others do not. The ability of a pathogen to develop resistance depends on many factors, such as growth rate, number of generations per year, mode of reproduction (asexual vs. sexual), means of survival, and other unknown factors. Pathogens such as Sclerotinia homoeocarpa (causes dollar spot), Pyricularia grisea (causes gray leaf spot), and Colletotrichum graminicola (causes anthracnose basal rot) have the ability to develop resistance very quickly. Most cases of fungicide resistance in turfgrasses have been associated with these three diseases. Other pathogens, such as Rhizoctonia solani (causes brown patch and large patch) are at low risk for fungicide resistance. In fact, there has never been a case of fungicide resistance in this pathogen, even though fungicides have been applied for its control for well over 60 years! Strategies for Preventing Fungicide Resistance There is much controversy among turf pathologists in how to prevent the development of fungicide resistance. Some say that tank-mixing products from different chemical classes is the best way to prevent resistance. The theory is that there is small chance that a strain will have resistance to both fungicide classes, especially when one of the mixture components is a low- or medium-risk product. Other turf pathologists claim that

rotating among different fungicide classes is the best way to prevent resistance. One notable turf pathologist even recommends that turf managers should just use a fungicide until it stops working, then switch to another one until it stops working, and so on. This would be a good strategy if there were an endless supply of fungicide classes, but unfortunately there is not! One fact that is not debatable is that the best way to prevent fungicide resistance is to reduce fungicide use. This can be done by implementing an integrated disease management plan: planting disease-resistant cultivars, using proper cultural practices, and maximizing fungicide use efficiency. Even with an integrated management plan in place, fungicides will still be necessary to control certain diseases, such as dollar spot, anthracnose basal rot, gray leaf spot, and others. Many of these are the same diseases that are at high risk for fungicide resistance! It is recommended that these high-risk diseases be controlled on a preventative basis. This keeps the pathogen populations low and reduces the chance that a resistant strain will be present. What about tank-mixing and rotating? Which strategy is best? The answer to this question depends on the disease you are controlling and the fungicide you are applying. Since both fungicides and pathogens vary in their risk for fungicide resistance, one needs to take both into account when developing a resistance management strategy. In Table 5, each fungicide has been assigned a risk value of 1 (low risk), 2 (moderate risk), or 3 (high risk) based on its biochemical mode of action and history of resistance problems. Each pathogen has also been assigned a risk value based on its ability to develop resistance to fungicides. When using a given fungicide to control a certain disease, you can determine the overall resistance risk by taking the fungicide risk value and multiplying it by the pathogen risk value. Then, based on this overall risk value, follow the recommended resistance management strategy. For example, lets say that you are developing a fungicide program for dollar spot control in creeping bentgrass. Boscalid (Emerald) is very effective for dollar spot control, so you want to use it in your program. The risk value for boscalid is 3, and the risk value for dollar spot is 3 also. Multiplying these numbers gives an overall resistance risk of 9. This is a very high-risk situation since both the fungicide and pathogen are prone to fungicide resistance. In this situation, you must rotate to a different chemical class after every

Chapter 4: Fungicides for Turfgrass Disease Control Page 27

application and tank-mix this product with a low- or moderate-risk fungicide, such as chlorothalonil. Based on this recommendation, you select iprodione for your next application for dollar spot control. This fungicide has a risk value of 2, yielding an overall risk value of 6. The resistance risk is lower in this situation

because iprodione is a medium-risk fungicide. Based on this risk value, you should rotate to a different chemical class after 1 or 2 applications. Tank-mixing with a low- or moderate-risk fungicide is strongly recommended but is not absolutely necessary.

Table 5. Risk values for fungicides and turfgrass diseases. To determine the overall resistance risk, take the fungicide risk value and multiply

it by the pathogen risk value. Based on this overall risk value, follow the recommended resistance management strategy in the table below.

Fungicide Class Risk Value Disease Pathogen Risk Value

azoxystrobin QoI 3 Algae Cyanobacteria 1

boscalid carboxamide 3 Anthracnose Colletotrichum graminicola 3

chloroneb aromatic hyrdocarbon 1 Brown Patch Rhizoctonia solani 2

chlorothalonil nitrile 1 Copper Spot Gleocercospora sorghi 2

ethazole aromatic hyrdocarbon 1 Dead Spot Ophiosphaerella agrostis 2

fenarimol DMI 2 Dollar Spot Sclerotinia homoeocarpa 3

fludioxonil phenylpyrolle 1 Fairy Ring Basidiomycetes 1

flutolanil carboxamide 2 Gray Leaf Spot Pyricularia grisea 3

fosetyl-Al phosphonate 1 Large Patch Rhizoctonia solani 2

iprodione dicarboxamide 2 Leaf Spot/Melting Out Helminthosporium spp. 2

mancozeb dithiocarbamate 1 Pink Patch Limonomyces roseipellis 1

mefanoxam phenylamide 3 Pink Snow Mold Microdochium nivale 2

myclobutanil DMI 2 Powdery Mildew Blumeria graminis 3

polyoxin D polyoxins 2 Pythium Blight Pythium aphanidermatum 3

propamocarb carbamate 1 Pythium Root Rot Pythium spp. 2

propiconazole DMI 2 Red Thread Laetisaria fuciformis 1

pyraclostrobin QoI 3 Rust Puccinia spp. 3

quintozene aromatic hyrdocarbon 1 Southern Blight Sclerotium rolfsii 2

thiophanate-methyl benzimidazole 3 Spring Dead Spot Ophiosphaerella spp. 1

thiram dithiocarbamate 1 Stripe Smut Ustilago striiformis 2

triadimefon DMI 2 Summer Patch Magnaporthe poae 1

trifloxystrobin QoI 3 Take-all Patch Gaeumannomyces graminis 1

vinclozolin dicarboxamide 2 Yellow Patch Rhizoctonia cereals 1

Based on the overall risk value, the following strategy is recommended to delay resistance:

Risk Value Recommended Action

9 Rotate to different chemical class after EVERY application; Tank-mix with low or

moderate risk product for EVERY application

6 Rotate to different chemical class after 1-2 applications; Tank-mixing with low or

moderate risk product recommended

4 Rotate to different chemical class after 1-2 applications; Tank-mixing not necessary

3 Rotate to different chemical class after 2-3 applications; Tank-mixing not necessary

2 Rotate to different chemical class after 3-4 applications; Tank-mixing not necessary

1 Rotating and tank-mixing not necessary, but recommended to avoid potential side

affects from continuous use of same chemical class