L.P. Tredway, Extension Plant Pathology
There are somewhere in the neighborhood of 100 different diseases that affect turfgrasses. Just like human illnesses, each turf disease has a specific prescription for its cure and prevention. Some diseases can be suppressed by the application of nitrogen fertilizer, whereas others are encouraged by more nitrogen. Some diseases are suppressed by high soil pH, whereas others are encouraged by low pH. A long list of other turf management practices can be used to control disease, but the most effective prescription is specific for each disease.As the older, broad-spectrum fungicides are removed from the market due to environmental concerns, they are being replaced with a new generation of products with narrow control-spectrums. These new products are very effective, and safe to the environment, but they are effective against a small number of diseases. Because of this change in the turfgrass industry, accurate diagnosis of turfgrass diseases is becoming even more important.
Turfgrass diseases are very difficult to identify. Grass plants are very small, and most diseases are caused my microorganisms that can't be seen without a microscope. However, with some basic knowledge and a lot of practice, you can vastly improve your diagnostic skills. By learning how to diagnose just a few common diseases, you will be able to diagnose most of the disease problems that you encounter during the season. If you cannot diagnose a problem with reasonable certainty, then it may be worthwhile to submit a sample to a plant disease clinic.
The first thing to remember is that disease diagnosis is much like detective work. A detective rarely looks at a situation and solves the mystery on the spot. Rather, a detective develops a list of possibilities, gathers evidence by careful observation, and then arrives at a conclusion by a process of elimination. When you see a disease on your turf, first develop a list of possible diseases. This list can be developed based on the turfgrass species that is being affected and the time of year the disease developed. Then, closely observe and make note of the symptoms and signs that you see on the turf. Also, make note of recent weather conditions and major cultural practices that were performed recently.What are symptoms and signs?
Most diseases leave a unique and reliable pattern on the turf. These patterns are expressed as either symptoms or signs, and are very useful for disease diagnosis. Symptoms are the evidence of disease on the turf plants. Two types of symptoms are produced in turf: stand symptoms and plant symptoms.
Stand symptoms are most easily observed by standing and looking across the turf area. There are several different types of stand symptoms, which basically describe the pattern of the disease on the lawn or landscape. Diseases may produce spots, circles, patches, rings, or an irregular stand symptom. A spot is an area of diseased turf less than 4 inches in diameter (Figure 1). A circle is a perfectly circular area of diseased turf greater than 4 inches in diameter (Figure 2). A patch is an irregularly shaped area of diseased turf greater than 4 inches in diameter (Figure 3). As the name implies, a ring is circular area of diseased turf with healthy turf to the inside and outside, leaving a ring-like pattern on the turf stand (Figure 4). Rather than producing a regular pattern of symptoms on the turf, some diseases produce an irregular or non-patterned symptom across the turf stand (Figure 5). Most diseases produce a very specific stand symptom, but some diseases can produce more than one of the above symptoms. Nevertheless, the stand symptoms are very useful for narrowing the list of possible diseases.
Far too many turfgrass managers attempt to diagnose diseases just by looking at the problem from a distance. However, there is another important set of clues available on the individual plants, called the plant symptoms. Plant symptoms describe the location and pattern of symptoms on the individual plants. It is important, however, to look for plant symptoms in the right spot. Generally, the best plant symptoms can be observed along the border between healthy and diseased turf. The plants in severely affected areas that are already dead are not very useful for diagnosis of turfgrass diseases.
Symptoms observed on individual plants include leaf spots, foliar lesions, stem lesions, foliar blight, foliar dieback, crown rot, and root rot . A leaf spot is a round or oval area on the leaf with a distinct border, which is usually a different color than the center of the spot (Figure 6). A foliar lesion is irregular in shape and is typically larger than a leaf spot, but still has a distinct border that is usually a different color (Figure 7). A stem lesion is very similar to a foliar lesion, but is present on the stem or leaf sheath of the grass plant rather than on the leaves (Figure 8). Foliar blight and foliar dieback produce symptoms on whole leaves or entire plants (Figure 9); the two are distinguished in that a foliar blight produces a distinct border between healthy and diseased turf, whereas a foliar dieback does not. Crown rot is observed as a dark and rotten area at the base of the turfgrass plant (Figure 10). Root rots produce a visibly dark and rotten root system, and also a noticeable reduction in root depth in affected areas (Figure 11). Crown rots and root rots often occur together, and may also include rotting of stolons and rhizomes if present (Figure 12).
Signs are the visible evidence of the presence of a pathogen. Most turfgrass diseases are caused by fungi, and even though fungi are microscopic organisms, some produce larger structures at certain times in their life cycle that can be seen with the naked eye. Mycelium is a cottony or spider-web-like mass of fungal growth that certain fungi produce when the turf is wet or humidity is high (Figure 13). Spore masses are fuzzy or jelly-like growths produced on the diseased tissue by certain fungi, again usually when the turf is wet or humidity is high (Figure 14). Sporophores are enclosed structures that contain fungal spores. If present, sporophores are often seen as small, dark specks on the diseased tissue (Figure 15). Sclerotia are small, round, hard structures produced on the diseased turf or in the thatch layer by certain fungi (Figure 16). Sclerotia are actually survival structures that some fungi use to survive through periods of unfavorable weather conditions. Most people are familiar with mushrooms, which are the large spore-producing structures produced above-ground by Basidiomycete fungi. Some turfgrass pathogens, most notably the fairy ring fungi, produce mushrooms as a sign of their presence (Figure 17).What other clues can be used to diagnose diseases?Is the disease limited to or more severe in a particular microclimate on the turf stand? Some diseases are more severe in low-lying, wet areas, whereas others are worse in high and dry locations. Some diseases are encouraged by shade, whereas others are more severe in open areas. Take a few minutes to carefully look at the distribution of the disease and see if you can come up with any consistent relationships - the power of observation is very valuable when diagnosing a turfgrass disease.The development of disease is highly dependent on weather conditions, so recent weather conditions are important clues for disease diagnosis. In order to grow and cause disease, fungi need moisture, either in the form of rain, irrigation, or atmospheric humidity. Fungi are also very sensitive to changes in temperature; each fungus has a narrow range of temperatures in which it is able to grow and cause disease. Since most disease development occurs at night when the turf is wet from dew, nighttime low temperatures are the most important parameter influencing the development of foliar diseases. The development of root diseases is determined by soil temperature rather than air temperatures.
Consider any cultural practices that were (or were not) conducted on the turf in the last month. If a pound of nitrogen fertilizer per 1000 sq ft was applied two weeks ago, then your turf probably has a disease that is encouraged by high nitrogen levels. If you recently aerified and topdressed the turf, then it is possible that the turf is affected with a stress-related disease like anthracnose basal rot. Distinguishing diseases from other problems
There are many other problems that occur in turfgrasses that can be confused with disease. These include cultural problems, environmental problems, nutrient deficiency or toxicity, chemical spills, or a myriad of other problems. There are a few rules of thumb that can be used to distinguish these problems from disease. The first rule of thumb is that diseases progress. Disease symptoms are initially mild and localized in a small area, then the symptoms gradually become more severe and widespread over time. The rate at which a disease progresses depends on the disease, the host, weather conditions, and cultural factors, but this generally occurs over a period of several days to several weeks. In addition, there is usually not a sharp, well-defined line between healthy and affected turf, but this transition occurs very gradually. If large areas of turf die overnight or during the course of one day, or if there is a sharp line between healthy and diseased turf, then the damage was probably not caused by a disease (Figure 18). One simple way to determine if a problem is actively spreading is to mark the edge of the affected area with a surveying flag or stake. Monitor the area over a course of several days and determine if the problem is spreading in relation to the flag or stake.
Another rule of thumb is that diseases never occur in straight lines or other regular patterns. Instead, disease symptoms are randomly distributed across a turf stand. If the symptoms of a problem are in straight lines or some other recognizable pattern, then this damage was not likely caused by a disease. In most cases, management-related errors, such as misapplication of a fertilizer or chemical, are responsible for these types of symptoms (Figure 19).
Finally, diseases typically do not kill large areas of turf and leave adjacent areas of the same turfgrass species untouched. A disease may be more severe in a certain microclimate, such as a compacted area, but some symptoms of the disease will also be evident in adjacent, less compacted areas. Environmental, cultural, or management-related issues should be investigated when large areas of severe damage are surrounded by large areas of completely healthy turf.
Disease diagnosis is a process of elimination. First, start with a list of possible diseases based on time of year and the species of grass that is being affected. Second, look for stand symptoms, plant symptoms, signs, and other clues. Based on the evidence that you collect, narrow the list of possibilities. Repeat these two steps if necessary. If you can narrow the list down to one disease, then you have diagnosed the problem and can now develop a management program for its control. If you can only narrow the list down to two or three diseases, then you have two choices. First, you can design a management program that will control all of the possible diseases. This is a reasonable choice when the diseases can be controlled by similar cultural practices or fungicides. Second, you can send a sample of the affected turf to a plant disease clinic. This is recommended when different cultural practices or fungicides are needed to control the possible diseases. I still can't diagnose the disease, what do I do? If you can't narrow the list of possible diseases to a reasonable number, then it may be time to send a sample of the turf to a plant disease clinic. The Plant Disease and Insect Clinic at North Carolina State University is one of the few clinics in the United States that specializes in turfgrass disease diagnosis. For more information on how to submit a sample to the PDIC, please contact your county extension office or visit the following website: http://www.ces.ncsu.edu/depts/ent/clinic/. **April 2005