Diagnosing turfgrass disease
In August and early September 1998, many golf courses in Nebraska, Iowa and Kansas experienced a sudden and devastating outbreak of gray leaf spot on perennial-ryegrass fairways. Initially, superintendents brought this to my attention because they were noticing thinning turf in ryegrass fairways that they had not treated with a fungicide. I took a cursory look at the turf on a couple of courses and passed the problem off as brown patch. Why did I, and others, misdiagnose the problem? Two reasons: First, we had our minds locked on brown patch, our region's most serious disease of perennial ryegrass. Second, we weren't familiar with the symptoms of gray leaf spot because it wasn't supposed to occur this far west, or so we thought. With that attitude, we violated an important principle of plant disease diagnosis-keeping an open mind.
When you diagnose turfgrass problems, whether they are caused by pathogens or environmental factors, you must follow a prescribed sequence of steps that requires your knowledge of the host (turfgrass) species, your understanding of infectious and non-infectious disease agents and your ability to observe. Thus, to become proficient in diagnosis, you must educate yourself about prevalent diseases and gain experience through repetition and training.
Field diagnosis Often, you can identify the cause of a disease with reasonable certainty with field diagnosis. First, approach the problem without a preconceived diagnosis. If you expect dollar spot, that's probably what you'll find even if dollar spot is not the primary cause of the turf's problem. Its presence could be due to stress from nitrogen deficiency or soil compaction. If you limit your diagnosis only to dollar spot and overlook the stress factor, then you will not detect the underlying problem. In other words, you have addressed the symptoms but not the cause.
Having the appropriate tools available and following some key steps in field-diagnostic procedures allows you to improve your chances of accurately diagnosing both the symptoms and the cause. * Have the right tools on hand, including a 20x hand lens, hand trowel, soil probe and a good plant-disease reference (see box, "Turfgrass disease diagnostic references," page G 11). * Note the time of the year when symptoms first occur. For example, patch-disease symptoms occurring in May and June on Kentucky bluegrass could be necrotic ring spot. Identical symptoms occurring in July and August are probably summer patch. * Identify the grass species and, if possible, the variety. Note whether only certain grass species are affected in mixed stands. * Note the overall pattern (patches, thinning, streaks, etc.) and distribution (such as in low or wet areas or next to a street or sidewalk) of symptoms in the affected area. * Observe whether the symptoms are specific or general. Generally poor plant growth could indicate a root problem. Look for specific symptoms on individual leaf blades, such as spots, orange pustules or black stripes. Your hand lens is useful for looking for signs of the pathogen. * Use a trowel or cup cutter to remove plants and examine the roots. Remove the soil and clean the roots in a pail of water. Diseased roots are usually brown and may have discolored lesions. Healthy roots are white to pale green. Compare the roots of diseased plants to those from healthy plants. * Use a soil probe to check the affected area for soil moisture, compaction and thatch thickness. * Obtain background information on cultural practices, fertilizer and pesticide applications and any past occurrences of the problem. * Determine when the symptoms began and how this might relate to specific environmental conditions and cultural practices that took place before symptoms became apparent. Note if there is a consistent correlation between disease development and weather conditions or cultural practices. * Usually more than one disease is active in a given turf area. Determine if both diseases are causing injury or if one is merely cosmetic. Be prepared for multiple diseases on one site.
Consult a good turfgrass-disease reference with color photos. Comparing symptoms of your field sample with pictures and descriptions in your reference will make diagnosis easier. The reference also should provide a discussion of conditions and practices that affect the occurrence of particular diseases. This allows you to use your knowledge of the site's history and conditions to aid diagnosis. Immunoassay diagnostic kits are available for certain turf diseases, such as dollar spot, brown patch and Pythium blight. You can use these kits in the field, office or laboratory to confirm your diagnosis based on symptoms.
Collecting a sample for clinical diagnosis If you are having trouble pinpointing the cause of symptoms or you would like a greater level of certainty before deciding on a treatment, you should use the services of a plant-diagnostic clinic. Clinical diagnosticians must base their diagnosis only on what they find in the sample you send. Thus, to help their diagnostic accurracy, you must: * Collect a good sample representative of the symptoms. * Only collect the sample from an area that you have not recently treated with a fungicide.
Also, remember that diagnosticians cannot see the overall pattern of symptoms on the affected area. Therefore, the accuracy of their diagnosis heavily depends on your sample and your thorough description of the site.
Here's an example. Let's say you find symptoms with a pockmarked pattern of roughly circular patches that resemble small doughnuts. You suspect one of the patch diseases, such as summer patch. Because you're not totally comfortable with your field diagnosis, you decide to send a sample to a plant-disease diagnostic clinic.
First, collect the sample at the margin of one of the patches so that it includes both live and dead grass. The larger the sample the better. With turf, a 6- to 8-inch-diameter section of sod usually is sufficient. Be sure to include the root system with the sod. Place the sample in a plastic bag, but do not add water. If you can't send the sample immediately, keep it in a cool location until you can. Package the sample in a container so that it will arrive intact. Then ship or mail it to the clinic by express-delivery service before Friday. Samples you send late in the week will spend the weekend in the post office and could reach the diagnostic clinic in a deteriorated condition.
Include with the sample essential background information and a color photo of the affected turf area. At the University of Nebraska, our Plant & Pest Diagnostic Clinic uses a standardized Specimen Identification Form. Most other plant-disease diagnostic clinics use a similar form, or at least provide a detailed list of information they need you to submit with your sample.
Remote diagnostics using digital images Many university plant-disease diagnostic clinics are exploring remote diagnostics via digital images sent through a computer. For the last 2 years, the Plant & Pest Diagnostic Clinic at the University of Nebraska has routinely received images of plant disease samples sent via e-mail. Advances in computer-imaging technology and digital cameras have made this method of plant-disease diagnostics possible.
This process starts with capturing images of the diseased plant sample with a digital camera. You then download the image onto a computer and send it via e-mail to the plant-diagnostic clinic. At the University of Nebraska, we are taking remote diagnosis a step beyond the e-mail approach. Our diagnostician is developing a web page for the clinic that will include the specimen-identification form. When this is operating, clients can take pictures of the diseased plant sample and affected turf area with their digital cameras and download these images to their computers. The next steps will be to enter our diagnostic clinic's web site, go to the specimen-identification form, complete the necessary information on the form, attach the digital images and electronically send it all to the clinic. Thus, the clinic receives the diseased plant images as well as the accompanying background information. If we can diagnose the disease from the images and information provided, we place the diagnosis and management recommendations on the original specimen-identification form and electronically send it back to the client. We hope to have this system up and running by this summer.
When you use remote diagnostics, remember the following tips: * Take a close-up photograph of symptoms on individual leaves or roots. * While standing, take a photograph of the symptoms. Provide an indication of the size of the patch or spot using a pencil or pocketknife for scale. * Take a photograph that illustrates the overall symptom pattern of the affected area.
Remote diagnostics has its limitations and will not totally replace traditional live-plant diagnosis in the near future. One limitation is the resolution of the digital images. For example, last fall, I received via e-mail a series of digital images of wheat plants. From the images, I could see that the leaves were chlorotic with spots, but the resolution was not clear enough to allow me to determine if the spots were rust pustules, aphid-feeding sites or a fungal leaf spot. Only after I received a live wheat-plant sample could I identify the cause of the spotting. With the improvement in digital-camera resolution, the capability of remote plant-disease diagnostics will improve. I'm sure that remote diagnostics will eventually become a routine tool in plant-disease diagnostics. Check with your local university or extension service. Several other universities have similar programs in operation or in development.
Accurate diagnosis of a turfgrass disease is the key to successful control of the problem. You play a critical role in the diagnostic process by collecting a good sample and providing pertinent background information. Although diagnosing diseases can be a baffling process, you will pinpoint the problem more often than not if you always follow proper procedures.
Dr. John E. Watkins is an extension plant pathologist with the University of Nebraska (Lincoln, Neb.).
At a diagnostic clinic, procedures range from diagnosing the problem based on the visual symptoms to DNA-based technology for the rapid detection of pathogenic microorganisms. If the diagnostician cannot make the diagnosis based on easily visible symptoms or signs on the sample, the next step is to directly identify the pathogen (usually a fungus).
The technician can do this is in two ways. After surface-disinfecting the leaf or root in running water, the diagnostician places it on a microscope slide and crushes or teases it apart in an attempt to observe the spores of the pathogenic fungus. Sometimes this works and sometimes it doesn't. If it doesn't, then petri-plate isolation is the next step. The objective here is to isolate the pathogen in a pure culture and identify it based on its mycelial-growth pattern or spore morphology. These procedures can take anywhere from 30 minutes to a week or longer, depending on the growth habits of the pathogen. With some of the patch-disease-causing pathogens, such as Leptosphaeria korrae (the cause of necrotic ring spot), isolation in pure culture can take weeks.
Now, however, rapid detection is possible using a specific nucleic-acid (DNA) probe or immunoassay. This rapid-assay diagnostic technology is one of the greatest benefits biotechnology has provided to the turfgrass industry.
Occasionally, a disease develops in a geographic area where it hasn't previously occurred. When this happens, plant pathologists go through a procedure known as Koch's postulates to establish the cause of a disease. This procedure includes four basic steps: * Consistently associating the organism suspected of causing the disease with the symptoms. * Isolating the suspect organism in pure culture. * Producing the same symptoms in a disease-free host when you inoculate that host with the suspect organism. * Re-isolating the same suspect organism in pure culture from the host in Step 3.
When these four criteria are satsified, you have established that a particular organism is the cause of a certain disease.
Turfgrass-Disease Diagnostic References Color Atlas of Turfgrass Diseases, J.B. Beard and S. Tani, 1997. Ann Arbor Press, Chelsea, Mich.
Compendium of Turfgrass Diseases, R.W. Smiley, 1983. The American Phytopathological Society, St. Paul, Minn.
Integrated Turfgrass Management for the Northern Great Plains, 1997. F.P. Baxendale and R.E. Gaussoin, eds. University of Nebraska Cooperative Extension (Lincoln, Neb.), publication EC97-1557.
Management of Turfgrass Diseases, J.M. Vargas, 1994. Lewis Publishers, Boca Raton, Fla.
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