Proper ID of Plant Diseases

Dr. P. Allen Hammer

Greenhouse production problems don’t go away by watching. Plant disease problems certainly won’t go away by watching them, but equally important, disease problems most often can also not be controlled unless one knows the correct identity of the disease. With experience and training, a grower can often identify the most likely disease, particularly with easily identifiable diseases (i.e. powdery mildew). However, those leaf spot and root rot diseases are often difficult for growers to identify correctly even with lots of experience.

The reason correct identity of a disease is so important is because most chemical controls are specific for the particular disease or at least a particular chemical is more effective in controlling a specific disease. I’m amazed at how often-unknown diseases are treated with a chemical or multiple chemicals without proper identification. When a grower observes a suspected disease problem in the greenhouse, the very first action is to make sure the disease is properly identified. The first action is not to reach for a chemical from the chemical storage. Applying the wrong chemical wastes money and simply causes additional delay in treating the real disease.

There are numerou

s university diagnostic labs in North America that can be used to identify specific diseases. Almost every state in the USA has a diagnostic lab. Go to www.apsnet.org/members/directories/Pages/UnivDiagnosticians.aspx for a complete list of labs, as well as their websites for additional information. If you’re not 100% sure of the specific plant disease, quickly send a plant sample to a lab for proper diagnosis. I would also urge you to carefully follow the lab instruction on collecting, packing and sending the sample to the lab. Improper handling of samples often delays diagnosis and creates lots of frustration for both the grower and the lab. Any extra bit of time in handling a sample correctly will pay large dividends in quickly controlling a greenhouse disease problem.

A good example showing the importance of identifying the specific disease is shown in the calibrachoa basket (Figure 1), which shows a “strange” symptom to many growers. Why would a disease kill one plant and the other plant in the same basket seemingly keeps growing without disease symptoms? Washing the roots does show some decay on the underground stem and roots of the diseased plant, but does little to identify the specific disease (Figure 2). With experience, I would quickly “guess” the disease is Thielaviopsis, but I still always want a pathologist to confirm my guess. Controlling Thielaviopsis can present challenges for growers, therefore, it’s most important to be 100% sure of the disease. A University Diagnostic Lab confirmed the diseased calibrachoa in the photo as Thielaviopsis.

I encourage growers to use a diagnosis lab to identify unknown diseases and confirm the identity of diseases you know from experience. I bet you might be surprised by a lab report. Disease problems are easier and less costly to control when the grower knows the specific disease they’re controlling. GT

Dr. P. Allen Hammer is a retired professor of floriculture at Purdue University, West Lafayette, Indiana, and is now in product development and support for Dümmen USA.