Just like people, plants can get sick from infections or from unhealthy living conditions. And just like people, sick plants don't produce or grow well. While all gardeners encounter plant diseases at some point, understanding how to approach, treat and, most importantly, prevent problems from occurring helps you to have healthy, beautiful plants and shrubs.
The University of Missouri Extension defines a plant disease as a response (a change in the plant brought about by an environmental condition), a pathogen or a combination of the two. The response is what you observe as the disease state, such as the inability to produce flowers. In order for a plant disease to take hold, it requires a suitable host plant, a pathogen and environmental conditions favorable to the pathogen or that make the plant susceptible to infection.
Plant diseases may be caused by fungi, bacteria, viruses, organisms such as nematodes, or abiotic factors such as the nutritional deficiency that contributes to blossom-end rot. Fungal infections are the most common diseases caused by a living organism and include mildews, rusts and smuts. Diseases perpetuate when an infectious organism persists in soil or leaf litter over the winter and reinfects the plant come spring. Understanding the cycle of a particular disease helps provide clues to correcting it.
Different diseases present different symptoms, ranging from obvious changes in appearance, like the sooty mold that affects many tree and shrub species, to destruction of parts of the plant, such as seeds destroyed by rust, to disruptions in normal physiological function, seen in diseases like micronutrient chlorosis. Some diseases do little beyond make the plant discolored or unsightly. Others may inhibit seed, fruit and flower production or kill the plant outright.
Treatment of a disease condition requires accurate diagnosis. The University of Missouri Extension recommends five steps when diagnosing a plant disease. First, accurately identify the species of the host plant and learn about what is normal for the plant and what diseases commonly afflict it. Attempt to determine if the disease is caused by an organism or by a nonliving entity, such as a nutrient imbalance. Living organisms typically affect only plants of a particular family or species, while nonliving factors may be present in multiple species located in a particular area. Finally, observe the plant closely for symptoms such as changes in color or lesions. If identification eludes you, contact your local extension office for further diagnostic help.
Treatment follows accurate identification of the problem, and while treatment is required in many cases, prevention is often easier and less costly. Healthy plants grown in environments that meet all of their needs undergo less stress and are less susceptible to infection. If you know a particular disease is problematic in your area, seek out resistant cultivars. Practices like crop rotation, improving soil fertility and attracting beneficial insects further help plants resist infection.