Diagnosing Plant Diseases

Diagnosing Plant Diseases

Like many garden problems, plant diseases are easier to control when detected early. However, unlike insect offenders, pathogens are much too small to see without magnification. More often than not, you will not know that your plants are infected until they begin to show symptoms of a bacterial/viral attack. Environmental and cultural problems such as frost injury, air pollution, or nutrient imbalances, can also be tricky to diagnose as the conditions that cause them are seldom tangible.

Below is a step-by-step guide to diagnosing disease in your plant:

First, you must identify your plant. As obvious as it sounds, this simple yet crucial step can put you surprisingly close to an accurate diagnosis in only a few minutes. Many popular garden plants are commonly attacked by easy-to-identify diseases and disorders, like black spots on roses, powdery mildew on lilacs, smut on corn, or blossom end rot on tomatoes.
If you are unsure of the plant in question, ask a fellow gardener, local nursery, or even your local Cooperative Extension Service for assistance. Even if identifying the plant does not help you in its diagnosis, it can prove to be important later on when you are deciding on a control measure. For example, some plants are sensitive to soap or oil-based sprays, so you would want to use a different type of control on these plants to avoid causing even more damage.
Take a good look. Notate anything abnormal that you see about the affected plant. What parts look to be affected or potentially problematic, such as the leaves, stems, flowers, or fruits? If the plant parts are spotted or discolored, jot down the color, size, and general shape of the patches. If leaves are also affected, is it the old or new leaves? Is the plant shorter than similar plants around it? Was it planted recently, or has it been in your landscape for many years? Anything you know or notice about your plant-no matter how minor-may help you or consultant make or confirm a diagnosis of the problem at hand.
Consider your environment. Extreme weather conditions, such as strong winds, hail, and waterlogged/dry soils can give plants an unhealthy appearance that resembles many symptoms of disease or disorder. Make a note of any unusual weather conditions or patterns that you can remember. It is always a good idea to write down normal weather occurrences that may cause stress to the plant, such as frosts, heavy rains, or dry spells. Also consider whether or not your plants have been exposed to pollutants like acid rain, herbicide drift, or road salt.
Rule out insect pests. Some insects can cause damage that very closely resembles plant disease/disorder symptoms. When in doubt, use a magnifying lens to check for insect signs like webbing or droppings. Symptoms that are particularly unusual might mean that damage has resulted from two or more pathogens or pests; open wounds from insects or other damage make easy-access entrances for plant pathogens.
Do some research. Discuss the problem with neighbors who have similar plants and check reference books in your local library.
Consult the experts. If a particular problem has you stumped, submit samples to your local or state plant disease diagnostic laboratory. Personnel at Cooperative Extension Service offices can provide you with the proper forms, mailing supplies, and instructions for sending plant samples by mail. Even local botanical gardens and nurseries may be willing to assist you in identifying your plant problems.

For additional assistance in identifying plant disease, see our list of Common Signs & Symptoms for images and detailed descriptions of commonly seen ailments.

Diagnosing Plant Diseases