Plant Pathology Infobytes

April 19, 1999
Your Garden - Is There Unwanted Life Out There?

Harmful insects and some of the other pests that attack Mississippi fruits and vegetables generally can be detected with the unaided eye. But what about disease organisms? Most are too small to be seen even with a magnifying glass.

This means that unless you have a microscope at the end of one of your tomato rows, you'll never get a closeup look at the fungus that causes early blight of tomatoes or any of the other disease pests that are often limiting factors in fruit and vegetable production.

Because disease pests are so small, most home gardeners tend to 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.

There's time to work out a game plan for controlling disease pests for the 1999 season, but you'll do a better job 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 plants; however, unlike our familiar green plants, they cannot make their own food from water, carbon dioxide, and the sun's energy. The fungi must obtain their food from organic matter of dead plants or from living plants (maybe your tomato crop, squash, or peach crop!). When the latter occurs, the symptoms of disease are produced on the host plant. When diseases are bad enough, poor quality vegetables and fruit, 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 lima bean anthracnose (remember those reddish areas on lima bean pods?), root rot of okra, rust of sweet corn, bitter rot of apple, brown rot of peach, or any of the dozens of fungus diseases that are annual problems for us in Mississippi.

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. Most orchardists are familiar with fireblight. This disease tends to be a serious problem during wet springs when this bacterial disease causes a "scorched" appearance in the crowns of susceptible pear and apple varieties.

Viruses are submicroscopic, 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 are multiplied, often causing severe ill effects to the plant. An example of a common virus disease which strikes many home gardens each season is a strain of 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 mouthparts 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. That's why 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. Also, soil adhering to tilling equipment and by the movement of soil water and surface water run-off are common ways nematodes are moved around.

Although these disease pests may exist in your garden or orchard, they don't have to wreck your vegetable or fruit crop yields. Check with the County Extension Office for free brochures and information sheets on ways to keep diseases under control.


Written by the late Dr. Frank Killebrew

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