Managing fungal diseases in warm-season turf
When was the last time you saw Pythium blight on bermudagrass or any other warm-season turfgrass? If you can't remember, don't worry--most warm-season turfgrass managers are in the same situation. Is it because you use lots of Pythium-specific fungicides? Probably not. Actually, Pythium blight is rare on warm-season grasses. I have seen it only three or four times in the last 10 years. And yet, this is the disease cool-season turfgrass managers fear the most! That is not to say we don't have our disease problems on warm-season turfgrasses. Obviously, we do. However, they can be quite different from diseases on cool-season turf--even when they involve the same pathogen.
Basic plant pathology, Part I--The disease triangle For a disease to occur, a susceptible turfgrass host, a virulent pathogen and an environment conducive for disease development must each be present. Pathologists refer to this as the disease triangle (see figure, page 19) because of the three components--host, pathogen and environment. By thinking about turfgrass diseases in this context, it's easier to understand why diseases affect turfgrasses in some situations and not others. The examples below illustrate differences in cool-season and warm-season turfgrass susceptibility to pathogens and differences in varieties or strains of the same pathogen.
A bermudagrass putting green located in the southeastern United States, overseeded with perennial ryegrass during the winter months. The fungal pathogen that causes Pythium blight always is present in the green, and the probability that environmental conditions conducive for disease development will occur at some point during the year is high. Such conditions include stressed turf, high humidity, persistent rainfall and night temperatures greater than 65†F. Experienced superintendents don't monitor the bermudagrass for Pythium blight, but they do monitor the ryegrass in the winter. Why? Because the ryegrass, not the bermudagrass, is susceptible to the Pythium species that cause Pythium blight.
Dollar spot on bentgrass or annual bluegrass vs. warm-season turfgrasses. While we do observe dollar spot on warm-season turfgrasses, it is not nearly as serious on these species as it is on the cool-season turfgrasses. On warm-season turfgrasses, a small amount of a quick-release nitrogen fertilizer normally eliminates the problem quickly and effectively. Again, the pathogen is present in the South and environmental conditions favorable for the disease exist, but the grasses just are not as susceptible. (An exception is zoysiagrass. However, even within this turf species, the fine-textured zoysiagrass cultivars generally are more susceptible than coarse-textured cultivars.)
Take-all patch on bentgrass vs. take-all root rot on St. Augustinegrass. This is a situation where the disease symptoms and the environmental conditions for disease development are the same, but the pathogens are different (though closely related). Gaeumannomyces graminis var. avenae causes the disease on bentgrass, whereas Gaeumannomyces graminis var. graminis is the pathogen that affects St. Augustinegrass. The difference between the two pathogens appears to be host specificity and temperature. The bentgrass pathogen G.g. avenae prefers cooler, temperate temperatures, whereas the St. Augustinegrass pathogen G. g. graminis prefers warmer, sub-tropical or tropical temperatures.
Rhizoctonia blight/brown patch on both cool- and warm-season turfgrasses. Both types of turfgrass are susceptible to the fungal pathogen Rhizoctonia solani, but the symptoms are different. On both types of turfgrass, the pathogen causes circular areas of turf (either patches or rings, small or large in size) to turn shades of brown. On cool-season turfgrasses, the leaves will have gray, tan or brown irregular-shaped lesions. However, we do not see these leaf lesions on warm-season turfgrasses. Instead, the fungus attacks the base of the leaf sheath where the leaf attaches to the stolon. The result is a basal leaf rot that allows you to easily pull affected leaves and shoots from the stolon. How can the same fungus produce such different symptoms? Because different strains of this fungus exist. The strain AG 1 causes the disease on cool-season turf, whereas the strain AG 2,2 appears to be the cause of the disease on warm-season turf. Only a laboratory with pure cultures of the pathogen can determine the strain.
"Helminthosporium"-type leaf spots. So many leaf spots, so little space to write about them! Helminthosporium is an old genus name for a group of fungi that scientists have broken down into four different genera--Bipolaris, Curvularia, Drechslera and Exserohilum. At least 21 species within these genera cause leaf spots on turfgrasses. Most Bipolaris spp. attack only warm-season turf. The exception is B. sorokiniana, which attacks just about all turfgrasses. This fungus infects warm-season turf during cool, wet periods from autumn through spring but attacks cool-season turf during warm, wet weather in midsummer. Most Drechslera spp. attack only cool-season turfgrasses. The exception in this genus is D. gigantea, which has a wide host range.
Despite the vast number of "Helminthosporium" species that cause leaf spots and the different environmental conditions that are conducive for disease development on cool- and warm-season turf, the control efforts are the same. Avoid excessive nitrogen-fertilizer applications and make sure turfgrass plants have adequate potassium. If it is necessary to irrigate, do so in the early morning hours to minimize leaf wetness. Make sure the amount of water is adequate to reach the root system. If possible, increase air circulation to keep the leaves dry. Also, it helps to raise the mowing height, whether on a putting green or a homeowner's yard.
Basic plant pathology, Part II--Infection vs. disease vs. disaster Just because a pathogen has infected (penetrated) a plant does not mean it will cause disease. Several steps must occur before you see a disease symptom such as a leaf spot or root lesion:
Inoculation. The pathogen must come in contact with the turfgrass plant. In turf, this is a relatively easy process because the pathogens are virtually always present in the turfgrass ecosystem. Inoculum is any part of the fungus that can cause infection--usually spores or mycelia.
Penetration. Just because the pathogen has come in contact with the turfgrass plant does not mean it necessarily will penetrate the plant. When the fungus does penetrate, it uses one of three methods. It enters through a natural opening such as a stomate, a wound (such as from mowing) or by directly penetrating the plant surface (either mechanically, like a drill bit, or with enzymes that degrade the cell wall).
Infection. This is the process by which the fungus invades many plant cells or tissues by growing and reproducing inside the plant. This negatively affects the plant and visible disease symptoms eventually begin to appear.
Again, infection does not automatically follow just because the pathogen has come into contact with the turfgrass plant and penetrated the plant surface. The plant must be susceptible to that particular pathogen (such as the examples I illustrated previously), the plant must be in a susceptible stage (for turf, this usually means it must be stressed), the pathogen must be a virulent strain capable of causing disease, and the environmental conditions must be favorable for the pathogen to grow and reproduce. Proper temperature and moisture usually are the two most important factors.
One leaf spot on one leaf in one yard is not a problem. Probably even multiple leaf spots on 1,000 leaves in one yard is not a problem. A disease is at epidemic proportions when it is widespread and severe. However, an epidemic can mean different things to different people. One person's minor problem is another person's disaster.
Disease monitoring and thresholds When we discuss monitoring and thresholds in reference to insects and weeds, it is fairly easy for everyone to understand the concept. However, discussions on disease monitoring and thresholds for turfgrass to determine if a disaster is about to strike cause even pathologists to throw up their arms in despair. Why? Because turf managers aren't producing a crop that you can measure quantitatively (for example, in bushels per acre). They are producing a putting green that looks and putts great, or an athletic field that prevents injuries or a landscape that is the envy of the neighborhood. More importantly, the desired look is different for each manager in each situation. That is why you don't see specific guidelines that tell the turf manager to spray a fungicide or initiate a cultural practice when you see, for example, a certain number of dollar-spot patches on the turf. You must develop your own tolerance or action threshold for diseases for each situation, and it will depend on environmental factors, primarily weather. On a golf course, you will probably even have different thresholds for greens and fairways.
Disease prevention and management It is all too common for turf managers to forget that preventive actions are an important part of an integrated-pest-management program, especially for diseases. This is unfortunate because the easiest way to control a disease is to prevent it from occurring or lessen its impact on the plant when it does occur. Prevention may include cultivar selection, cultural controls and fungicide applications. The problem is knowing what disease(s) you are trying to prevent. Unfortunately, experience is often the best predictor.
Whether you are just starting out as a turf manager or have been in the field for some time, it helps to develop a body of information on diseases that will become your individualized guide. If you don't know where to begin, I suggest two good references. A Guide to Integrated Control of Turfgrass Diseases, published by GCSAA Press (Lawrence, Kan.), consists of one volume for cool-season turfgrasses and another for warm-season turfgrasses. It includes all turfgrasses, not just those used on golf courses. The Compendium of Turfgrass Diseases, published by APS Press (St. Paul, Minn.), has excellent color photos that aid identification. These two references are good books with which to start.
Books, and even this article, are written to reach a wide audience. However, growing warm-season turfgrasses in tropical Southern Florida is different from growing the same turfgrasses in the Mediterranean and desert climates of southern California. Therefore, it is critical to develop a "profile" for each disease you encounter by adding information pertinent to your situation. This will help you develop the experience you need to effectively manage disease on your turf.
Dr. Monica L. Elliott is associate professor of plant pathology at the University of Florida's Fort Lauderdale Research and Education Center (Fort Lauderdale, Fla.).
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