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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. |
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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. |
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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. |
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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. |
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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. |
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One must distinguish between infectious disease, caused
by biotic agents, and noninfectious disease (abiotic agents). |
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Infectious organisms can be defined as follows: |
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- Fungus
- an organism (plant) with no chlorophyll, that reproduces by
means of structures called spores, and usually has filamentous
growth; e.g., molds, yeasts, mushrooms.
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- Bacterium
- a single-celled, microscopic organism with cell walls and
no chlorophyll; reproduces by fission.
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- Mycoplasma
- a microscopic, bacteria-like organism that lacks a cell
wall, and therefore appears filamentous.
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- Virus
- 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.
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- Viroid
- a virus-like particle that lacks the outer protein coat of
a virus particle.
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- Nematode
- a microscopic roundworm, usually living in soil, which
feeds on plant roots.
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- Parasitic seed plant
- a higher plant with chlorophyll that lives parasitically on
other plants, e.g., mistletoe.
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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. |
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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. |
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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. |
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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. |
