University of Vermont Extension
Department of Plant and Soil Science
News ArticleCONTROLLING PLANT DISEASES
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
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 (www.nepdn.org/home) are all good for
more information and assistance.
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