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Plant Diseases

Plant Diseases

History of plant pathology

Causes of plant disease

Bacteria

Fungi

Viruses and viroids

Nematodes

Other causes of plant diseases

Disease cycles

Control

Resources

Like human beings and other animals, plants are subject to diseases. In order to maintain a sufficient food supply for the worlds population, it is necessary for those involved in plant growth and management to find ways to combat plant diseases that are capable of destroying crops on a large scale. There are many branches of science that participate in the control of plant diseases. Among them are biochemistry, bio-technology, soil science, genetics and plant breeding, meteorology, mycology (fungi), nematology (nematodes), virology (viruses), and weed science. Chemistry, physics, and statistics also play a role in the scientific maintenance of plant health. The study of plant diseases is called plant pathology.

The most common diseases of cultivated plants are bacterial wilt, chestnut blight, potato late blight, rice blast, coffee rust, stem rust, downy mildew, ergot, root knot, and tobacco mosaic. This is a small list of the more than 50,000 diseases that attack plants. Diseases can be categorized as annihilating, devastating, limiting, or debilitating. As the term suggests, annihilating diseases can totally wipe out a crop, whereas a devastating plant disease may be severe for a time and then subside. Debilitating diseases weaken crops when they attack them successively over time and limiting diseases reduce the viability of growing the target crop, thereby reducing its economic value. Plant diseases are identified by both common and scientific names. The scientific name identifies both the genus and the species of the disease-causing agent.

For the past 50 years, the ability to combat plant diseases through the use of modern farm management methods, fertilization of crops, irrigation techniques, and pest control have made it possible for the United States to produce enough food to feed its population and to have surpluses for export. However, the use of pesticides, fungicides, herbicides, fertilizers and other chemicals to control plant diseases and increase crop yields also poses significant environmental risks. Air, water, and soil can become saturated with chemicals that can be harmful to human and ecosystem health.

History of plant pathology

While early civilizations were well aware that plants were attacked by diseases, it was not until the invention of the first microscope that people began to understand the real causes of these diseases. There are references in the Bible to blights, blasts, and mildews. Aristotle wrote about plant diseases in 350 BC and Theophrastus (372-287 BC) theorized about cereal and other plant diseases. During the Middle Ages in Europe, ergot fungus infected grain and Shakespeare mentions wheat mildew in one of his plays.

After Anton von Leeuwenhoek constructed a microscope in 1683, he was able to view organisms, including protozoa and bacteria, not visible to the naked eye. In the eighteenth century, Duhumel de Monceau described a fungus disease and demonstrated that it could be passed from plant to plant, but his discovery was largely ignored. About this same time, nematodes were described by several English scientists and by 1755 the treatment of seeds to prevent a wheat disease was known.

In the nineteenth century, Ireland suffered a devastating potato famine due to a fungus that caused late blight of potatoes. At this time, scientists began to take a closer look at plant diseases. Heinrich Anton DeBary, known as the father of modern plant pathology, published a book identifying fungi as the cause of a variety of plant diseases. Until this time, it was commonly believed that plant diseases arose spontaneously from decay and that the fungi were caused by this spontaneously generated disease. DeBary supplanted this theory of spontaneously generated diseases with the germ theory of disease. Throughout the rest of the nineteenth century scientists working in many different countries, including Julian Gotthelf Kühn, Oscar Brefeld, Robert Hartig, Thomas J. Burrill, Robert Koch, Louis Pasteur, R. J. Petri, Pierre Millardet, Erwin F. Smith, Adolph Mayer, Dimitri Ivanovski, Martinus Beijerinck, and Hatsuzo Hashimoto, made important discoveries about specific diseases that attacked targeted crops.

During the twentieth century advances were made in the study of nematodes. In 1935 W. M. Stanley was awarded a Nobel Prize for his work with the tobacco mosaic virus. By 1939, virus particles could be seen under the new electron microscope. In the 1940s fungicides were developed and in the 1950s nematicides were produced. In the 1960s Japanese scientist Y. Doi discovered mycoplasmas, organisms that resemble bacteria but lack a rigid cell wall, and in 1971, T. O. Diener discovered viroids, organisms smaller than viruses.

Causes of plant disease

Plant diseases can be infectious (transmitted from plant to plant) or noninfectious. Noninfectious diseases are usually referred to as disorders. Common plant disorders are caused by deficiencies in plant nutrients, by waterlogged or polluted soil, and by polluted air. Too little (or too much) water or improper nutrition can cause plants to grow poorly. Plants can also be stressed by weather that is too hot or too cold, by too little or too much light, and by heavy winds. Pollution from automobiles and industry, and the excessive application of herbicides (for weed control) can also cause noninfectious plant disorders.

Infectious plant diseases are caused by pathogens, living microorganisms that infect a plant and deprive it of nutrients. Bacteria, fungi, nematodes, mycoplasmas, viruses and viroids are the living agents that cause plant diseases. Nematodes are the largest of these agents, while viruses and viroids are the smallest. None of these pathogens are visible to the naked eye, but the diseases they cause can be detected by the symptoms of wilting, yellowing, stunting, and abnormal growth patterns.

Bacteria

Some plant diseases are caused by rod-shaped bacteria. The bacteria enter the plant through natural openings, like the stomata of the leaves, or through wounds in the plant tissue. Once inside, the bacteria plug up the plants vascular system (the vessels that carry water and nutrients) and cause the plant to wilt. Other common symptoms of bacterial disease include rotting and swollen plant tissues. Bacteria can be spread by water, insects, infected soil, or contaminated tools. Bacterial wilt attacks many vegetables including corn and tomatoes, and flowers. Crown gall, another bacterial plant disease, weakens and stunts plants in the rose family and other flowers. Fireblight attacks apple, pear, and many other ornamental and shade trees.

Fungi

About 80% of plant diseases can be traced to fungi, which have a great capacity to reproduce themselves both sexually and asexually. Fungi can grow on living or dead plant tissue and can survive in a dormant stage until conditions become favorable for their proliferation. They can penetrate plant tissue or grow on the plants surface. Fungal spores, which act like seeds, are spread by wind, water, soil, and animals to other plants. Warm, humid conditions promote fungal growth. While many fungi play useful roles in plant growth, especially by forming mycorrhizal associations with the plants roots, others cause such common plant diseases as anthracnose, late blight, apple scab, club root, black spot, damping off, and powdery mildew. Many fungi can attack are variety of plants, but some are specific to particular plants.

The list of fungi and the plants they infect is a long one. Black spot attacks roses, while brown rot damages stone fruits. Damping off is harmful to seeds and young plants. Downy mildew attacks flowers, some fruits, and most vegetables. Gray mold begins on plant debris and then moves on to attack flowers, fruits, and vegetables. Oak root fungus and oak wilt are particularly damaging to oaks and fruit trees. Peach leaf curl targets peaches and nectarines. Powdery mildew, rust, sooty mold, and southern blight attack a wide variety of plants, including grasses. Texas root rot and water mold root rot can also infect many different plants. Verticillium wilt targets tomatoes, potatoes, and strawberries.

Viruses and viroids

The viruses and viroids that attack plants are the hardest pathogens to control. Destroying the infected plants is usually the best control method, since chemicals to inactivate plant viruses and viroids have not proven effective. While more than 300 plant viruses have been identified, new strains continually appear because these organisms are capable of mutating. The symptoms of viral infection include yellowing, stunted growth in some part of the plant, and plant malformations like leaf rolls and uncharacteristically narrow leaf growth. The mosaic viruses can infect many plants. Plants infected with this virus have mottled or streaked leaves; infected fruit trees produce poor fruit and a small yield.

Nematodes

Nematodes are tiny microscopic animals with wormlike bodies and long, needlelike structures called stylets that suck nutrients from plant cells. They lay eggs that hatch as larvae and go through four stages before becoming adults. Nematodes have a 30-day life cycle, but they can remain in a dormant state for more than 30 years. Nematicides are chemicals used to control nematode infestations. Marigolds are resistant to nematodes and are often planted to help eliminate them from infected soil.

Nematodes primarily attack plant roots, but they may also destroy other parts of the plant either internally or externally. They thrive in warm, sandy, moist soil and attack a variety of plants including corn, lettuce, potatoes, tomatoes, alfalfa, rye, and onions. However, all nematodes are not harmful to plants. Some are actually used to control other plant pests such as cutworms, armyworms, and beetle grubs.

Other causes of plant diseases

Mycoplasmas are single-celled organisms that lack rigid cell walls and are contained within layered cell membranes. They are responsible for the group of plant diseases called yellow diseases and are spread by insects such as the leafhopper.

Parasitic plants, such as mistletoe, cannot get their nutrients from the soil, but must attach themselves to other plants and use nutrients from the host plant to survive. They weaken the wood of their host trees and deform the branches.

Disease cycles

An equilateral disease triangle is often used to illustrate the conditions required for plant diseases to occur. The base of the triangle is the host and the two equal sides represent the environment and the pathogen. When all three factors combine, then disease can occur. Pathogens need plants in order to grow because they cannot produce their own nutrients. When a plant is vulnerable to a pathogen and the environmental conditions are right, the pathogen can infect the plant causing it to become diseased.

Plant disease control is achieved by changing the host plant, by destroying the pathogen or by changing the plants environment. The key to success in growing plants, whether in the home garden or commercially, is to change one or more of the three factors necessary to produce disease. Disease-resistant plants and enrichment of soil nutrients are two ways of altering the disease triangle.

Weather is one environmental factor in the plant disease triangle that is impossible to control. When weather conditions favor the pathogen and the plant is susceptible to the pathogen, disease can occur. Weather forecasting provides some help; satellites monitor weather patterns and provide farmers with some advance warning when conditions favorable to disease development are likely to occur. Battery-powered microcomputers and microenvironmental monitors are place in orchards or fields to monitor temperature, rainfall, light levels, wind, and humidity. These monitors provide farmers with information that helps them determine the measures they need to take to reduce crop loss due to disease.

Control

Control of plant disease begins with good soil management. The best soil for most plants is loamy, with good drainage and aeration. This minimizes diseases that attack the roots and allows the roots to feed nutrients from the soil to the rest of the plant. Organic methods, such as the addition of compost, can improve soil quality, and fertilizers can be added to the soil to enrich the nutrient base. Soil pH measures the degree of acidity or alkalinity of the soil. Gardeners and farmers must be aware of the pH needs of their plants, since the right pH balance can help reduce susceptibility to disease, especially root diseases like club root or black root rot.

Other important factors in the control of plant disease are the selection of disease-resistant plants (cultivars), proper watering, protection of plants from extreme weather conditions, and rotation of crops. Disposal of infected plants is important in the control of diseases, as is the careful maintenance of tools and equipment used in farming and gardening. Many plant diseases can easily be spread by hand and by contact with infected tools, as well as by wind, rain, and soil contamination. Plant diseases can also be spread by seeds, and by transplants and cuttings; careful attention to the presence of disease in seeds, transplants, and cuttings can avoid the spread of pathogens.

Crop rotation is an important part of reducing plant diseases. Pathogens that favor a specific crop are deprived of their preferred host when crops are rotated. This reduces the virulence of the pathogen and is a natural way to reduce plant disease. Soil

KEY TERMS

Cultivar A distinct variety of a plant that has been bred for particular, agricultural or culinary attributes. Cultivars are not sufficiently distinct in the genetic sense to be considered to be subspecies.

Disease triangle The presence of a host plant, favorable environment, and a pathogen that is capable of causing disease.

Infectious plant diseases Disease caused by living agents (pathogens) that are able to spread to healthy plants.

Noninfectious plant diseases Usually called plant disorders, these conditions are caused by nonliving agents, such as soil pH, pesticides, fertilizers, pollution, or soil contamination.

Pathogen An organism able to cause disease in a host.

Plant pathology The study of plant diseases.

solarization is another natural method used by gardeners to reduce diseases.

Barriers or chemical applications to eliminate pests that may carry pathogens to plants are another method of disease control. The use of chemical pesticides has become standard practice among home gardeners and commercial growers alike. Among the organic chemicals used today are copper, lime-sulfur, Bordeaux mixture, fungicidal soap, and sulfur. After World War II, DDT, a synthetic insecticide, was used to destroy plant pests. Today, the use of this and a number of other pesticides has been banned or restricted because they were found to present hazards to the health of human, wildlife, and the environment.

See also Rusts and smuts.

Resources

BOOKS

The Epidemiology of Plant Diseases, edited by B.M. Cooke, D.G. Jones, and B. Kaye. New York: Springer, 2006.

Lucas, G.B., C L. Campbell, and L.T. Lucas. Introduction to Plant Diseases. Westport, CT: AVI Publishing, 1985.

Rappaport, Rosalynn. Controlling Crop Pests and Diseases. Warwickshire, U.K.: Practical Action Publishing, 2005.

Strange, Richard N. Introduction to Plant Pathology. New York: John Wiley & Sons, 2003.

Vita Richman

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Plant Pathology

Plant pathology

Plant pathology is the study of plant diseases. The most common causes of plant diseases are bacteria, fungi , algae, viruses, and roundworms. Plant diseases can drastically affect a country's economy. They may be responsible for the loss of up to ten percent of human crops each year. Plant pathology is also important in environmental studies. Many plants are especially sensitive to pollutants in the air and water. They can be used as "early warning systems" of pollution problems that may affect humans as well. For example, ozone at a concentration of only 0.005 parts per million produces noticeable changes in tobacco plants.

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