While the discovery that a disease has invaded your garden is bothersome, attempting to diagnose the problem can be downright confounding. Using a step-by-step process is your best bet in working toward a solution. By the time you diagnose a plant disease, it may be too late to help that particular plant, but you will likely gain sufficient information to prevent future problems.
Before catapulting yourself into the study of various plant diseases, it is wise to identify not only the common name of the plant, but also the scientific name. Accurate diagnosis cannot take place without knowing which specimen you are working with.
Define the Problem
Avoid snap judgments when diagnosing plant diseases. Take time to completely examine the plant. Inspect all parts, even the roots, if possible. Look for pests, as well. Note what is not thriving on the plant. Determine if more than one plant is involved and if symptoms are on more than one species. Keeping an open mind will allow you to see warning signs. Using a tool such as a magnifying glass can be immensely helpful.
Next look at the location of the plant. Be aware of soil and site conditions. Determine the approximate age of the plant. Try to recall past and current weather conditions as well as management practices, such as type and consistency of irrigation and frequency and type of fertilization. A first step toward diagnosis is to look at very obvious or common causes first. Continue to look beyond immediate symptoms for less apparent clues.
Look for Patterns
Observe the surrounding areas in which affected plants are located, as well as areas that are near but not damaged. Generally a pattern is present, though that pattern may not be immediately obvious. Compare all plant parts, including old and new growth, leaf margins and whole leaves. Don't assume that the afflicted plants have only one problem. Often, by the time a disorder crops up visually, more than one problem may be acting on the plant. Take into consideration the plant's requirements, its current location and plant disorders that thrive in similar conditions.
Formulate a Tentative Diagnosis
After identifying the type of plant, processing information about the site and determining the degree of plant distress and disease signs and symptoms, it is time to make a tentative diagnoses. A complete diagnosis does not necessitate the narrowing down to one exact disease. It is best to err on the side of caution and leave your options open to two or three possibilities. At this point you are ready to consult references or an expert in the field. Keep observing, however, because you never know what may turn up next. The more attuned you become to garden diseases, the more able you will be to handle them effectively.