Diagnosing diseases is common to plant pathology, human medicine and veterinary medicine. Before a problem can be treated, doctors need to find out what ails the patient, whether the patient is a plant, human or animal.
Because animals and plants aren’t verbal, vets and plant doctors rely on symptoms for clues. In plants, symptoms are the changes in appearance of a sick plant compared to a healthy plant. To accurately diagnose a disease, it is helpful to be familiar with a disease-free plant of the same species.
It is also valuable to remember disease is a process that develops over time. If a plant can be observed where it is growing, a home gardener can see if a zinnia, for example, wilts only during the middle of the day, or it if wilts persistently. In the first case, the problem is likely water stress, which can be remedied easily because it is not a disease.
Persistent wilting, however, is a common symptom of root rot on many plants. To confirm this diagnosis, the plant should be dug up carefully, so some of the soft, rotting roots come along with the plant. Based on the symptoms of brown, soft roots, or a lack of roots, root rot can be diagnosed.
Plant diagnosticians often go another step, especially if the plant is a farmer’s crop. To determine if applying a fungicide can help manage the disease, the pathogen that caused the disease must be identified. Different fungicides will be recommended if the cause is a fungus or a bacterium.
If the pathogen grows on the outside of a diseased plant, rather than inside, the pathogen may be visible to a trained eye. One of the characteristics most useful for identification is the type of spores a fungus or water mold makes.
If the pathogen is inside a diseased plant, it must be coaxed out. Pieces of diseased plant tissue are soaked in diluted chlorine bleach for one minute, rinsed in sterile water, blotted dry on sterile paper towels, and placed on culture medium in a petri dish.
If the culprit is a water mold, it will grow from the diseased piece onto and into the culture medium within 48 hours. If the culprit is a fungus, it may take three to five days for enough growth to occur to identify it.
The traditional method to identify fungi and water molds is to look at them under a microscope. This step in diagnosis is very similar to the previous step of identifying pathogens on diseased plants.
A newer, and more precise, way to identify any pathogen (fungus, water mold, bacterium, nematode or virus) is to use various DNA fingerprinting techniques, as crime labs do. Just like merchandise in a store, many pathogens now have “bar codes,” characteristic pieces of DNA that are unique to a species. DNA techniques require specialized supplies and equipment, so many plant disease diagnostic labs don’t provide this service, unless the sender requests and pays an extra fee for it.
Plant disease diagnosis often involves detective work because the diagnostician wasn’t present while the disease developed. The plant variety name, planting date, and how much sunlight and water the plant receives can be informative.
Another important observation is whether a single plant or multiple plants are affected. If many different types of plants are symptomatic, the problem is likely something related to the growing conditions, or a misapplication of an herbicide, rather than a disease. As this example illustrates, plant diagnosticians, most of whom are plant pathologists, identify many kinds of plant problems in addition to diseases.
Modern communication methods aid diagnosticians and help solve plant problems quicker. Mailing diseased plants or parts via overnight delivery helps the sample arrive at the diagnostic clinic in (relatively) good shape. Answers and control recommendations are e-mailed back to submitters.
Digital photos have revolutionized plant disease diagnosis. Diseases with characteristic symptoms, like powdery mildew or downy mildew, can be diagnosed based on a clear photo emailed to a clinic. Diagnosticians e-mail photos to each other or to specialists when they are stumped.
The Clemson Plant and Pest Diagnostic Clinic provides diagnosis of plant problems and identification of pest insects and weeds for South Carolina residents. The current fee is $20 per sample.