Problems of ornamentals usually do not arise from an isolated cause. There may be a primary cause such as spider mites on spruce or rust disease on hawthorn. However, associated environmental conditions may be present that need to be diagnosed. Drought (for instance) often precedes mite problems. Formulating questions to ask about the ornamental plant with poor health symptoms is a key to successful diagnosis.
One of the most difficult diagnostic areas in ornamental plant care is determining the causes of problems that result in nonspecific symptoms. Such nonspecific symptoms as leaf yellowing, leaf drop, browning of the tips of leaves and branch diebacks may be serious and easily detected. However, they can result from a variety of environmental imbalances, alone or in combination, as well as from some kinds of pathogens.
Such nonspecific symptoms commonly, but not always, result from poor root health in the urban landscape. Hundreds of factors can cause root health problems. Some of these may be infectious diseases and others may be noninfectious environmental problems. Table 1 gives examples of nonspecific symptoms and possible causes.
The only way to achieve a correct determination of the causes of a plant problem when beginning with a nonspecific symptom is to try and find more symptoms and information. Gather more data in order to produce a symptom profile. Even if the set of symptoms includes nothing more than a group of individually nonspecific conditions, it might still lead to a correct diagnosis.
Suppose you start with yellow leaves. You could increase the symptom profile by: 1) noting where on the plant the yellow leaves are, 2) when they first appeared, 3) what unusual cultural practice might have occurred, 4) what other leaf conditions you see (browning, leaf drop, leaf size change), 5) what soil conditions are present (wet, dry, compacted, acidic, salty). Perhaps the help of an outside diagnostic clinic or a soil testing service will be needed. The result will be a greater ability to correctly diagnose the cause of the symptom and plan a solution.
There is no substitute for thorough field observation. When confronted with a plant health problem, be prepared to look at the plant and the surrounding environment closely. A good way to proceed is to answer a series of questions. Remember that each question can have one or more answers. The following paragraph outlines an approach that has worked for many.
First, properly identify the plant. With this knowledge you can determine the cultural requirements for the plant. Knowing the plant type will also enable you to anticipate what specific diseases to expect and when to expect them. For instance, if you have a dead branch on a tree, you might expect first fireblight if it's a crabapple or Verticillium wilt if it's a Norway maple. Gather reference material and be familiar with the symptom profile to be expected. Remember, pictures in texts often are out of perspective because they are selected for their ease of showing one problem. The plant you are observing may exhibit symptoms of several diseases, environmentally induced stresses and insect damage all at once.
Other questions to ask during a diagnosis include the following: