AN INTRODUCTION TO PLANT DISEASES

A plant disease is broadly defined as any condition in which a plant differs in some way from a normal plant in either structure or function. The plant may be shorter, have more branches, or fewer leaves than normal­it differs in structure. Or, it may wilt and die prematurely, or not produce flowers or fruit­its function differs.

Plant diseases have had an important role in history. The holy fire in the Bible is believed to have been due to the ergot fungus in the heads of grain which made bread poisonous. The great famine in Ireland in the mid-1800s was caused by the destruction of the potato crop by late blight. The American chestnut has been virtually eliminated as a commercial tree in the United States by chestnut blight. The American elm is now similarly threatened by Dutch elm disease.

Fungi are plants that lack the green coloring (chlorophyll) found in seed-producing plants and therefore cannot manufacture their own food. There are between 50,000 and 100,000 different species of fungi of many types and sizes, but not all are harmful. Most are microscopic in size, but some, such as the mushrooms, are quite large. Fungi reproduce by spores.

Bacteria are very small, one-celled plants that reproduce by simple fission. They divide into two equal halves, each of which becomes a fully developed bacterium.

Viruses are so small they cannot be seen with an ordinary microscope. Many of the viruses that cause plant diseases are transmitted from one plant to another by insects, usually aphids or leafhoppers. Viruses are also very serious problems in plants that are propagated by bulbs, roots, and cuttings because the virus is easily carried along in the propagating material. Nematodes are small eel-shaped worms that reproduce by eggs. The number of eggs produced by one female nematode and the number of generations in a season depends largely on soil temperature. Therefore, nematodes are usually more of a problem in warmer areas of the country. Most nematodes feed on the roots and lower stem of plants, but a few attack the leaves and flowers.

The life cycles of all disease organisms are greatly influenced by environmental conditions. Temperature and moisture are probably the most important factors that affect the severity of plant diseases. They not only influence the activities of the disease organism, but also affect how easily a plant becomes diseased and the way the disease develops.

The extremely large number of plant diseases (about 50,000 in the United States) makes it impossible for any one person to be familiar with all of them. However, there are certain facts that help in the identification of plant diseases. The most important is to know the name of the plant that is affected. One can then check known plant diseases to see if a similar condition has ever been reported on that plant.

How the disease developed is also important. If it occurred in a very short time (overnight), it is probably not a parasitic disease, but is more likely due to some unfavorable environmental condition or chemical. How is the condition distributed in the planting? Is it general over the area or only in one or two spots? Parasitic diseases usually do not affect a large percentage of the plants in the early stages, but start in one area and gradually spread to the other plants. And, parasitic diseases usually do not affect several different kinds of plants in one area at one time, even though some disease organisms can attack many different plants.

Knowing the past history of the planting and what chemicals have been applied may help identify a problem.

A knowledge of the growing requirements and habits of the affected plant also helps understand certain conditions. There is often a tendency for gardeners to immediately suspect a disease when a leaf turns yellow or brown. But, it may just be a normal response. Even evergreens lose their leaves eventually.