University of Vermont Extension 
Department of Plant and Soil Science


Dr. Leonard Perry, Extension Professor
University of Vermont

Whether you have houseplants, flowers, summer vegetables, or fruits, you’ll invariably encounter plant diseases.  Safe and successful control begins with understanding some important plant disease terms.
First, realize that the term “disease” refers the abnormal functioning of a plant, and can be caused by either some infectious organism (“pathogen”), or by an environmental situation that isn’t suited for or doesn’t agree with the plant.  Many plants that appear diseased are more often subject to an environmental problem, such as too much light (burning leaves), or too wet soil (roots can suffocate with no oxygen).  Even if an environmental problem doesn’t injure a plant seriously, it may lead to a “secondary” infection by a disease.  An example of this is roots, stressed from too much water, being susceptible to root rots.
There are several types of organisms that can cause diseases.  Fungi are perhaps the most common.  Seen under a microscope they are usually in filaments, have a certain type of cell walls, and reproduce by spores.  The white powdery mildew on leaves of phlox, or the gray mold (commonly also known by its scientific name of “botrytis”) on old flowers, are examples of fungi.  Fungicides, whether they are synthetic or organic, are the chemicals that kill fungi.
There are many types of fungi.  One commonly seen in gardens is the rust fungi-- fungi that produce sexual spores (reproduce with mating) on an external structure.  Their name is descriptive as they are rusty in color.  Beginning usually on lower leaves, as on hollyhocks, they move up the plant.  When leaves are severely infected they usually shrivel and fall off.  While weakening plants and making them unattractive this, similar to powdery mildew, usually can be tolerated by plants from year to year (much more so than gardeners may tolerate them!).
Bacteria are microscopic, only have one cell, and reproduce by the cell wall dividing (“fission”).  These can affect most plant parts, and move internally throughout the plant (“systemic”) before external symptoms are noticed.  Some of these disease symptoms they cause include leaf spots, scabs, and wilts.  One nice feature of such disease terms is that they are quite descriptive of what you see.
Chemicals used to kill these are called bactericides.  Since bacteria are quite mobile and move quickly and easily among objects, it is important to use very clean tools and practices if these are about.  Clean tools thoroughly with a sanitizing product after they touch infected plants. 
Virus organisms are even smaller—submicroscopic—and not even cells, but rather cell genetic ingredients (RNA or DNA) surrounded by protein.  They are parasites, meaning they live in close association with another organism (such as plants), on which they depend for nutrition.  They may not even be a “pathogen” or disease-producing organism. 
Streaking or patterns in leaves of flowers and foliage is caused by viruses.  Tobacco mosaic virus, cucumber mosaic virus, or streaking in flowers such as roses and dahlias are examples of this highly mobile disease. It is really important to use good sanitation to avoid viruses, as there are no chemical treatments, no “viricides”, and the usual products for diseases will not work on them. The only control, if needed, is to destroy the plants.
An even smaller organism than the virus is a “viroid”, consisting just of a single strand of genetic RNA material.  Examples of these are the spindle tuber viroid of potatoes, a stunt viroid of chrysanthemums, and others in economic crops such as citrus, coconuts, and avocados.
Sanitation has been mentioned more than once, and merely refers to keeping surfaces clean and removing any potentially infected plant material.  Related terms usually are used loosely, “disinfecting” referring to cleaning or ridding an already infected plant of an organism. 
“Disinfesting” kills organisms that have not yet caused disease, such as on inanimate objects as surfaces and tools.  When wiping off tools with a cleansing agent such as diluted bleach water (9 parts bleach to one part water), this is technically disinfesting.  If you don’t want to use bleach due to its caustic nature and potentially harmful fumes, a household cleaning product (such as Lysol) can be used, one part of the product to two parts water.  Ideally, allow objects (such as pots) or surfaces to soak for at least 10 minutes before rinsing with water.  Such cleaning of containers is especially important for starting seeds, as the seedlings can be quite susceptible to diseases such as “damping off” (sudden toppling of stems at the soil surface). 
Another sanitation term often used incorrectly with objects or soil is sterilization.  This refers to killing all living organisms, good and bad, as is done with extreme heat (180 degrees F or more).  Pasteurization (140 degrees F commonly with potting soils) for 30 minutes will kill bad disease organisms, but not the good organisms that a plant needs for growth, and that may fight the bad organisms.
Although there are many more terms relating to plant diseases (“phytopathology”), these few will help you to get started on figuring out possible causes of a plant problem, proper remedies, and prevention of future problems.  Often an online search, including of images, for problems of your plant or crop may be all you need for an identification.  But consider other options and environmental conditions, too, as often a symptom may have several possible causes. Trained professionals at local garden centers, state master gardener helplines, or university plant diagnostic clinics ( are all good for more information and assistance. 

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