Many of you feel very comfortable trying to diagnose a plant problem, readily separating the noninfectious from the infectious causes or quickly distinguishing insect injury or site stress. Still, I know from telephone inquiries that others are uncomfortable making such diagnoses and have requested some help. Here are some step-by-step suggestions on how to go about diagnosing a plant problem.
Above all else, do not let anyone rush you into a diagnosis. “On-the spot” diagnoses and partial information are the most direct route to a misdiagnosis! This may cause unnecessary expense, concern, and delay in getting to the actual cause of the problem.
Step 1. Identify the plant species. In fact, knowing the cultivar or hybrid may also come in handy. If you know the host, you can eliminate many problems that won’t occur on that host, and you can start looking in reference books, on Web sites, or in files for problems that might occur on that host. For instance, sudden stem tip necrosis and death on crabapples could be the result of fire blight infection, but that disease will not occur on redbud.
Step 2. Determine what is wrong. This may seem obvious, but sometimes drooping foliage, variegated leaves, or bark splitting is normal for a species. Sometimes the yellowing of leaves is a problem, while other times it is a cultivar quality.
Step 3. Dig out the facts. This is probably the most important part of the diagnostic process. Details on symptom expression are much more useful that generalizations such as “my plant is dying.” The presence of fruiting bodies, mushrooms, conks, bacterial ooze, insects, insect parts, insect trails, odd growth, or information on anything abnormal can help the diagnosis. Critical information includes a description of the pattern of the problem over the entire planting and a description of the pattern of the problem on one affected plant. For instance, the problem might occur only along one side of the property and only on one side of all plant species. Such a description would strongly suggest a wind or weather stress or possibly a chemical drift. As another example, only a few of the pines are affected, and they have suddenly turned brown over the entire tree. This might involve a root problem or possibly pine wilt. Finally, it is helpful to know how things have changed on the site just before the problem was manifested. Often this information leads us to investigate cultural problems, construction injury, weather stress, or the like.
Investigate conditions around the plants, weather, chemicals, and any other facts that might have a bearing on symptom development.
Step 4. Consult references. There are textbooks that list problems by host; books on insect problems, diseases, chemical injury, nutritional problems, and the like. There are university fact sheets, Web sites, and advice from specialists. Start with a general plant problem reference that lists many problems for each host. Extension offices have many useful references to get you started.
Step 5. Determine probable cause(s). At this point, you might want to ask for diagnostic help if you are uncertain. There is always the University of Illinois Plant Clinic! Check out the Plant Clinic services at http://plantclinic.cropsci.uiuc.edu/. Most Illinois Extension unit offices also have Master Gardeners who have been trained to help in plant problem diagnosis. The Master Gardener Web site is http://www.extension.uiuc.edu/mg/.