The basic requirements for growing healthy plants are relatively easy to identify. These include: good soil structure, texture and drainage; proper balance and amount of nutrients; acceptable soil pH; sufficient space for expanding roots and crowns; sufficient water, acceptable temperature and light; clean air; and relative freedom from pests and diseases. Significant fluctuations within or among any of the above mentioned environmental conditions can result in poor plant health.
Much of the information presented in this bulletin refers to the diagnosis and control of infectious plant diseases. However, it is important to remember that "sick" or unhealthy plants may not be infected with a plant pathogen. In fact, many plant health problems encountered in Ohio landscapes are not related to plant pathogens, insects or other pests. Unfavorable environmental conditions in addition to many other factors can lead to plant stress. Depending upon the level and duration of stress, plant health can be greatly affected. If stress is severe and long term, it can lead to visible "abnormal" or "undesirable" symptoms and, eventually, death of the plant. Understanding the importance of plant stress, combined with the ability to identify stressful situations in order to prevent or alleviate them, is essential to successful plant health management in the landscape.
Although we emphasize the importance of stress on plant health, it is important to remember that a stress-free, perfectly healthy plant can be attacked by many plant pathogens. If a healthy plant is highly susceptible and environmental conditions are favorable for disease development, the pathogen can infect and cause disease that damages or even kills the plant. Although most plant pathogens do not require a stressed or weakened plant in order to infect, stressed or weakened plants may be more susceptible to attack by some plant pathogens. Some plant pathogens do require some degree of stress or plant injury before they can infect and cause disease. Examples of this are species of the fungus Botryosphaeria. These fungi can infect a wide range of woody ornamentals, but are rarely a problem on vigorous, healthy plants. On crabapple, it generally becomes a problem (stem cankers) after the tree has been predisposed to infection by some sort of injury. A common form of predisposing injury in Ohio is cold weather or winter injury. This type of relationship is well documented for several plant diseases. Thus, an understanding of the factors that predispose plants to disease is critical to successful plant health management.