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Plant Pathology

  MG Manual Reference
Ch. 4, pp. iii - v

[ Introduction: history | diseases defined| conditions | symptoms | development | control | summary ]


Certain diseases have had tremendous impacts on our society. Perhaps foremost among these is Phytophthora late blight which caused the potato famine in Ireland (1845 - 1846). As a result, there was widespread famine in northern Europe and more than one and one half million people immigrated to the United States.
In 1879, a new disease, downy mildew of grape, was introduced into Europe from the United States, spread rapidly, and threatened to ruin the vineyards of Europe. A mixture of copper sulfate and lime, used initially to deter pilferers, was discovered to control the disease. This discovery became known as "Bordeaux mixture" and stimulated the study of the nature and control of plant diseases.
These examples are prominent because they caused so much damage. However, plant diseases cause variable amounts of damage from year to year, depending often on weather patterns.


Plant disease is the rule rather than the exception. Every plant has disease problems of one sort or another. Fortunately, plants either tolerate these maladies, or they are not very serious in most years. A plant disease is defined as any alteration (of a plant) that interferes with its normal structure or function and renders it unfit for its normal use.
Plant problems may be caused by either living or non-living factors. Living factors are infectious (they spread from plant to plant). These include pathogens (fungi, nematodes, bacteria, and viruses), insects, and other animals. Non-living factors that cause plant problems do not spread from plant to plant. They are caused by chemical, physical, or mechanical factors. Examples of these abiotic factors include nutrient deficiencies and water or temperature stress.
One must distinguish between infectious disease, caused by biotic agents, and noninfectious disease (abiotic agents).
Infectious organisms can be defined as follows:
an organism (plant) with no chlorophyll, that reproduces by means of structures called spores, and usually has filamentous growth; e.g., molds, yeasts, mushrooms.
a single-celled, microscopic organism with cell walls and no chlorophyll; reproduces by fission.
a microscopic, bacteria-like organism that lacks a cell wall, and therefore appears filamentous.
a submicroscopic, subcellular particle that requires a host cell in which to multiply; it is not known if a virus is a living or nonliving agent.
a virus-like particle that lacks the outer protein coat of a virus particle.
a microscopic roundworm, usually living in soil, which feeds on plant roots.
Parasitic seed plant
a higher plant with chlorophyll that lives parasitically on other plants, e.g., mistletoe.
Fungi and bacteria cause such plant diseases as leaf spot and fruit, stem, or root rot. Plant viruses, viroids, and mycoplasmas often cause growth distortion, stunting, and abnormal coloration. Nematodes can cause stunting and root distortion. Parasitic seed plants cause a general weakening of the host plant.


In order for disease to occur, three conditions must be met. First, it is necessary to have a susceptible host plant. Each species of plant is capable of being infected by only certain organisms (pathogens). The plant must be in a stage of development susceptible to infection by the disease agent.
The second requirement is the presence of an active pathogen in a stage of development conducive to infecting the host plant. If there is no pathogen present, there can be no disease.
The third condition is an environment suitable for the pathogen to infect the plant. Temperature and moisture are important factors. Humidity is the critical factor limiting the spread of many foliage diseases in Arizona. The diseases of major importance are those which attack below the soil line where moisture and temperature conditions are favorable. Viruses, however, are plant pathogens that are little affected by climate.

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