Cooperative Extension Service 
Institute of Food and Agricultural Sciences
Hendry County Extension P. O. Box 68 LaBelle, Florida 33975-0068 Phone (863) 674-4092

Hendry County Horticulture News

Disease Control Aided by Knowledge of the Enemy

Harmful insects and some of the other pests that attack fruits and vegetables generally can be detected with the unaided eye.  But most disease organisms are too small to be seen even with a magnifying glass. Unless you have a microscope, you may never get a close-up look at the fungus that causes early blight of tomatoes or any of the other disease pests that are often limiting factors in fruit, flower and vegetable production.

Because disease pests are so small, many home gardeners tend to ignore them or underestimate their importance and may not know much about them.  Consequently, more emphasis is often placed on insect and weed control, and gardeners sometimes fail to carry out an effective disease control program.

You will do a better job of controlling disease pests if you know more about your opponents.  Plant diseases are caused by four classes or types of microorganisms:  fungi, bacteria, viruses and nematodes.

Fungi are simple plants; however, unlike our familiar green plants, they cannot make their own food from water, carbon dioxide, and the sun's energy.  Fungi must obtain their food from organic matter of dead plants or from living plants (maybe your citrus, tomato, zinnia, or favorite rose bush).  When the latter occurs, the symptoms of disease are produced on the host plant.  When diseases are bad enough, poor quality fruit, flowers and vegetables, along with reduced yields, are the end result.

Many fungi are mold like organisms existing in the form of microscopic threads.  Most are transmitted from plant to plant by tiny seed like bodies called spores, which are carried by air currents, splashing water, or tools.  Common examples of diseases caused by fungi include late blight of tomato (remember those decaying sunken spots on your tomatoes ), fusarium wilt of melons, powdery mildew on flowering annuals, greasy spot and alternaria on citrus, or any of the dozens of fungus diseases that are annual problems for gardeners.

Bacteria are microscopic one celled organisms.  The forms causing plant diseases are rod like or cylindrical in shape and reproduce by fission. This means that one bacterium can divide to form two new cells, and these two can spit to form four cells.  In this manner, millions of bacteria may be produced in a short period of time unless controlled. Bacteria are carried from place to place by insects, rain, and tools.  Many vegetable gardeners are familiar with bacterial spot of pepper and tomato.  This disease tends to be a serious problem during the fall when this bacterial disease causes small dark sunken spots on pepper and tomato leaves and fruits.

Viruses are sub microscopic, cylindrical or spherical bodies.  They are not living organisms, at least not in the same sense as bacteria and fungi, but rather particles composed of nucleic acids and other compounds that are similar to the chemical make-up of chromosomes of the host cells. When introduced into plant cells, the virus particles multiply, often causing severe ill effects to the plant. An example of a common virus disease which strikes many home gardens each season is the squash mosaic virus which causes yellow squash to form large green blotches.

Virus particles may be transmitted by infected transplants, seeds, transmission of sap from one plant to another through pruning, insects, or even nematodes.  There is even one form of virus (tobacco mosaic virus) that can be transmitted by cigarette or cigar smokers.

Nematodes are thread-like round worms.  Those attacking trees and garden plants are microscopic in size and live in the small roots of their host plants or the surrounding soil.  Their mouth parts contain a hypodermic or needle like structure (stylet) used to penetrate the cells of roots.  With it, they deliver a digestive juice that predigests the root cell materials, which are then sucked into the nematode.  Somewhat in the same manner that we enjoy a milkshake!

When nematode populations are high, their feeding can result in stunted plant growth.  Some cause the plant to produce galls; other nematodes cause root lesions, which can be entry points for the fungi that cause root rot.

Nematodes are transmitted in a number of different ways.  One of the most common means is through infected nursery stock and vegetable transplants.  It's a good idea to buy inspected and certified disease- and nematode free planting stock from a reputable nursery, or take particular care if you produce your own planting stock.  Soil adhering to tilling equipment and by the movement of soil water and surface water run-off are also common ways nematodes are moved around.

Although these disease pests may exist in your garden, they don't have to wreck your vegetable or flower garden.  Check with the Hendry County Extension Office for tips on ways to keep diseases under control.  Many people believe that control of plant diseases always means the use of chemicals, but many diseases can be controlled by cultural practices.  Good luck and good gardening.

Gene McAvoy is the horticulture agent with the Hendry County Extension Service.  Direct your horticulture questions to PO Box 68, LaBelle, FL 33975,  e-mail - or phone 863-674-4092 or  863-983-1598.  You are also welcome to visit the Hendry County Extension Office at 225 Pratt Blvd., LaBelle.   Office hours are from 8:00 - 5:00.

The Institute of Food and Agricultural Sciences is an Equal Employment Opportunity - Affirmative Action Employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap or national origin.

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