Plant Disease Control No. 1
Revised January 2001
An Introduction To Plant Diseases
A plant disease is broadly defined as any condition in which a plant exhibits some malfunction or abnormality in its growth or development. The distinction between healthy and diseased plants may be extremely fine, but usually alterations in the physiological or morphological development are such that the unhealthy or diseased individual is obvious by appearance. If a plant does not produce or develop according to normal expectations, it is considered diseased.
Plant diseases are not new and are often mentioned in ancient records. It appears that plant diseases that have always plagued man. There are plagues, blights and blasts in the Bible and Saint Anthony's fire is believed to have occurred when people ate poison bread made from wheat infected with the ergot fungus. The great Irish potato famine in the mid-1800's was caused by the destruction of the potato crop by the late blight fungus. The American chestnut has virtually been eliminated in the U.S. by chestnut blight, and the American elm is now similarly threatened by elm disease. Crop losses were subjects for speculation through the ages and scientists were confused as to their cause. Most of our understanding of plant disease causes and control was developed in the past 150 years.
CAUSES OF PLANT DISEASES
Plant diseases are generally grouped into two categories based on their cause.
1. Nonparasitic diseases result from unfavorable growing conditions such as nutrient deficiencies or excesses, extremes of temperature, water excesses or deficiencies, toxic chemicals (weed killers or air pollution) or mechanical injury. These diseases cannot be transmitted to healthy plants, and control depends solely on correcting the condition causing the disease.
2. Parasitic diseases are caused by infection of the host plant by microorganisms or other pathogens which derive their food by growing on or in other plants. The most common causes of parasitic diseases are fungi, bacteria, phytoplasmas, viruses, nematodes, and parasitic seed plants.
Fungi are microscopic primitive microbes that lack a green coloring (chlorophyll) and therefore must obtain nutrients by growing on other plants or other organic sources. To most people, edible mushrooms, toadstools, mildew, and mold are the best known representatives of this group. There are about 100,000 different species of fungi and fortunately most of them are not harmful. Most fungi reproduce by spores instead of seeds. Many of the fungi are chemically controlled with fungicides.
Bacteria are much smaller than fungi and generally even smaller than most fungus spores. They are one-celled organisms that reproduce by simple fission at an astonishing rate. They can divide every 20-60 minutes. At this rate, it is possible for one cell to give rise to about 17,000,000 cells in 24 hours. The bacteria which cause plant disease are all rod shaped. Many if them have flagella, which are threadlike appendages that propel the cell in liquid medium. Bacteria are controlled chemically with bactericides.
Viruses (and viroids) are the smallest known disease-producing agents capable of being transmitted from plant to plant. They are so small they cannot be seen with the light microscope but can be observed with an electron microscope but can be observed with an electron microscope. They are composed of nucleic acid (the genetic material) and a simple protein coat. Many of the viruses which cause plant diseases are spread by insects; usually aphids or leafhoppers. They are also a serious problem in plants that are propagated by cuttings, bulbs or roots, because the virus is carried along in the propagating material. There are no means of chemical control for virus diseases.
Phytoplasmas (formerly mycoplasmas)cause many diseases of plants which were once thought to be caused by viruses. Phytoplasmas are very small bacterium-like organisms which are sensitive to some antibiotics. It has only been since 1967 that certain diseases were shown to be caused by phytoplasmas. This breakthrough was very important because, unlike virus diseases, the phytoplasma diseases can be treated with antibiotics. The phytoplasmas are usually spread by leafhoppers and Psyllids during feeding procedures.
Nematodes are small round worms that reproduce by eggs. The number of eggs produced by the female nematode and the number of generations per season depends largely on soil temperature. Therefore, nematodes are usually more of a problem in warm climates. Most nematodes feed on the roots, but a few attack stems, leaves , and flowers. Chemical control of nematodes is accomplished by volatile and highly toxic fumigants or systemic nematicides.
The life cycle of all disease causing organisms are greatly influenced by environmental conditions. Moisture and temperature are probably the most important factors affecting the presence and severity of plant diseases. The mere presence of the pathogen on the plant does not necessarily mean disease will occur. The host must be susceptible and the environment favorable for disease development. Because most plant pathogens require moisture on the surface of the host and rain is not common during the growing season in Utah, we more frequently encounter root and soil borne diseases. These types of diseases are difficult to diagnose since the damage occurs underground with the aboveground symptoms being quite variable.
HOW ARE PLANT DISEASES IDENTIFIED?
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 fact is to know the name of the plant that is affected. A comparison can then be made with lists of known plant diseases to see if a similar condition has been reported on that plant. See Plant Disease Control Bulletin No. 3, Aids in Diagnosing Plant Diseases.
How the disease develops 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 throughout the area or only in one or two spots? Parasitic disease usually do not affect a large percentage of the plants in the early stages, but start in one area and gradually spread to adjacent plants. 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 to 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 some leaves every year.
HOW ARE PLANT DISEASES CONTROLLED?
The key to controlling parasitic diseases is to introduce an obstacle into the life cycle of the disease organism. This obstacle may be the use of disease-resistant plants on which the pathogen cannot grow. Environmental conditions may be created or modified so that are unfavorable for disease development, or it may involve the application of a protective chemical on plant or to the soil. See Plant Disease Control Bulletin No. 2, Major Groups of Chemicals for Plant Disease Control.
Sherman V. Thomson/Retired
Listing of commercial products implies no endorsement by the authors or the Utah State Cooperative Extension Service. Criticism of products not listed is neither implied nor intended. Persons using such products are responsible for their use according to the current label directions of the manufacturer. Pesticide labels are legal documents, and it is a violation of federal and state laws to use a pesticide inconsistent with its labeling. The pesticide applicator is legally responsible for its prope
use. Always read and follow the label.