PLANT DISEASES

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

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

Egg

Egg

T

T

Several nymph and instar

Larva

stages. Each resembles the adult

• Enlarges via instars and molts,

but lacks wings and genital

but bears no resemblances to

appendages, Each molt brings

the adult

the resemblance closer

• Worm-like or grub-like in

to the adult

appearance

T

• Chewing mouth parts

Adult

T

• Fully developed wings

Pupa

• Fully developed genital

• Distinctive changes in

appendages

appearance and physiology

• Assorted morphological

• Inactive, does not feed

modifications

T

• Examples:

Adult

Aphids, Grasshoppers,

• Fully developed morphologically

Squash Bugs

and physiologically • Diverse appearances: most

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.