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Prepared by John R. Hartman, Extension Plant Pathologist Adapted from Master Gardener materials - University of Kentucky, University of Georgia and Virginia Tech Edited by Gail Ruhl, Purdue University INTRODUCTION Plant diseases in the landscape and garden are very important and can be a significant source of frustration and loss to the gardener. There are about30,000 diseases of economic importance in the U.S. Plant pathology is the study of the biotic and abiotic agents that cause disease in plants; of the mechanisms by which these causal agents induce disease in pants and of the methods of preventing or controlling disease and reducing the damage caused. PLANT DISEASES IN HISTORY Certain diseases have had tremendous impacts on our society. Perhapsforemost among these is Phytophthora late blight which caused the potato famine (1845) in Ireland. It is estimated that 1.5 million Irish died from starvation and just as many immigrated to the United States. Two forest tree diseases which caused great economic losses in America are Dutch elm disease and chestnut blight. Both were introduced accidentally to the United States and while the formercontinues its destruction, the latter completely destroyed valuable trees in the Appalachians. These examples are prominent because they caused so much damage. In reality, total crop loss due to plant disease is rare. Most disease loss in the garden is due to endemic diseases. DISEASE DEFINED Diseases result from more or less continuous irritation of the plant tissues by a primary causal agent.Disease is a process that takes time, is physiological in nature, abnormal, and detrimental. Diseases cause damage by reducing yield and/or quality of plants and/or plant products. TYPES OF PLANT DISEASES There are two types of plant diseases: those whose primary causal agents are biotic (infectious), and those that are abiotic (not infectious). The causal agent of infectious diseases is called thepathogen, and the susceptible plant the suscept. Diseases caused by microorganisms or microbes, are infectious. Diseases caused by parasitic plants are also infectious. Diseases may involve more than one causal agent and often involve secondary causal agents. Noninfectious (Abiotic) Diseases Examples of abiotic diseases include: o o o Nutrient Deficiencies -- A lack of essential elements such as ironor zinc may cause plant foliage to yellow. Lack of or Excess Soil Moisture -- A plant can become dehydrated during drought periods, and may suffocate when poor drainage cuts out oxygen around the roots. Too Low or Too High Temperature -- Plants grown out of their adapted habitat can be injured or killed by extremes in temperature.


Edited by Ruhl 10-10-07

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AirPollution -- Ozone, sulfur dioxide and automobile exhaust fumes can injure plants. Soil Acidity or Alkalinity -- Adverse soil pH can injure plants Mechanical Damage - Girdling from roots, nylon twine or wire; injury from construction

Biotic Diseases Biotic (infectious) diseases occur when a host plant is invaded by a living organism. Most of these organisms are microbes, and can also bereferred to as parasites which attack plants. A host is a plant which has been invaded by a parasite. A parasite is an organism which obtains its nutrients from living organisms, often plants. In the process of feeding, the parasite not only consumes plant tissue, which weakens the host, but also produces toxins, enzymes, and growth regulating substances which disturb the normal metabolic processes inthe plant. In some cases the parasite actually blocks the movement of food and water in the plant's conducting tissue. Any of these disorders caused by a parasite will result in a diseased plant. Microbes are the major biotic pathogens of plants. The four major groups of microbial plant pathogens are fungi, bacteria, nematodes, and viruses. Less commonly, phytoplasmas (bacteria-like) and viroids...
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