Despite our best efforts at control, the United States loses an estimated 15 to 20 percent of crop productivity each year due to plant diseases. History is laden with accounts of starvation and pestilence resulting from crop failures that were themselves the result of widespread infection by plant pathogens. Plant diseases such as the potato blight in Ireland altered the destiny of an entire country. In our country, chestnut blight and Dutch elm disease have all but eliminated two important native North American tree species.
From the moment it begins as a seed, cutting, or other propagative structure, the plant is susceptible to injury and death from hundreds of pathogenic agents. Some affect the plant in its early stages of growth. Others are a threat to the maturing plant. Some pathogens affect the flowers while leaving the foliage unimpaired. Others bother only the
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Egg T Several instar stages. Each resembles the adult but is larger than the previous instars. T Adult • Primitive • Wingless • Examples: Springtails, Silverfish No metamorphosis |
Egg T Several nymph and instar stages, with distinctive changes in appearance and physiology. • Breathe through trached gills • Spend these stages in water T Adult • Fully developed wings • Breathe atmospheric oxygen • Assorted morphological modifications • Examples: Mayflies, Stoneflies, Dragonflies Incomplete metamorphosis |
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Egg |
Egg |
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T |
T |
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Several nymph and instar |
Larva |
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stages. Each resembles the adult |
• Enlarges via instars and molts, |
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but lacks wings and genital |
but bears no resemblances to |
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appendages, Each molt brings |
the adult |
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the resemblance closer |
• Worm-like or grub-like in |
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to the adult |
appearance |
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T |
• Chewing mouth parts |
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Adult |
T |
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• Fully developed wings |
Pupa |
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• Fully developed genital |
• Distinctive changes in |
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appendages |
appearance and physiology |
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• Assorted morphological |
• Inactive, does not feed |
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modifications |
T |
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• Examples: |
Adult |
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Aphids, Grasshoppers, |
• Fully developed morphologically |
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Squash Bugs |
and physiologically • Diverse appearances: most |
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Gradual metamorphosis |
common form of metamorphosis and represent most insects Complete metamorphosis |
figure 6-7. The four types of insect metamorphosis (Delmar/Cengage Learning) fruit. Still others injure any and all parts of the plant. To attain a basic understanding of plant diseases, you need to be familiar with both the causal agents (pathogens) and the diseases they create in their host plants.
