PLANT INJURIES AND THEIR CONTROL

OBJECTIVES

Upon completion of this chapter, you should be able to

• state the major causes of injury to plants.

• characterize insects and groups of pathogens as plant pests.

• list common symptoms of injured plants.

• list the principles of pest control.

• describe the types of pesticides and their safe use.

• explain the concept of integrated pest management.

KEY TERMS

biotic

oviparous

girdling

infectious

parasite

hypersensitivity

plant pathogens

nematodes

protectants

infestation

viroids

eradicants

phylum

mollicutes

herbicides

spiracles

conidia

nonselective herbicides

ovipositor

endoparasitic

selective herbicides

metamorphosis

ectoparasitic

microinjection

larva

cuticle

phages

pupa

buccal spear

compost tea

nymph

inoculum

instars

agent of dissemination

ovoviviparous

etiolation

PLANT INJURIES AND THEIR CAUSES_______________

Anything that impairs the healthy growth and maturation of a plant may be regarded as an injurious agent. Injuries may be natural, resulting from the interaction of plants with the many components of the earth’s environment. They may also be unnatural, resulting from the activi­ties of humans. For example, when heavy snow breaks a limb of a tree or an insect chews holes in a leaf, those injuries could have occurred without any action by people. However, when a plant suffers from excess fertilization or from being sprayed with a harsh chemical, then the injuries can be attributed to errors by humans. When viewed in this manner, many of the conditions that we regard as injuries are just nor­mal interactions between the plants, animals, and other components of the natural world. It is when people place value on plants for reasons of their aesthetic appearance and/or economic importance that those interactions are labeled as injuries and regarded as undesirable and problematic.

Sometimes scientists and others do not agree on the specific catego­rization of an injury. Is it a disease, an insect problem, an environmental condition, or should they all be included under one umbrella term, such as disease? Regardless of the categorization, scientists and horticulture practitioners agree that injuries to plants can be caused by both biotic (living) and abiotic (non-living) agents. In addition, those agents may be either infectious (capable of spreading from plant to plant) or noninfec­tious (not capable of spreading). As examples, if a lawnmower gouges the bark of a tree, the cause would be abiotic and noninfectious. Other adjacent trees would likely not experience the same injury. However, if a plant in the center of a crowded greenhouse bench developed mildew, it is probable that the plants around it, and perhaps throughout the entire greenhouse, would eventually contract the same mildew injury. The cause would be both biotic and infectious. Reviewing Table 6-1 will help in distinguishing among the many causes of plant injuries.

To fully understand the table, several additional terms need explana­tion. As described above, an agent that is infectious is capable of spread­ing from plant to plant. Such agents are usually living within the host plant and are using the plant as a source of nourishment. Those agents are collectively termed plant pathogens. Pathogens are biotic and infec­tious and include the bacteria, fungi, nematodes, viruses, viroids, mol – licutes, and some parasitic plants. Many forms of these pathogens as well as insects and weed seeds will persist in the soil where host plants are growing or even collect on the surface of the plants awaiting the opportunity to infect them. The presence of injurious agents on or in the nearby environment of desirable plants is termed infestation, the agents are regarded as infestious, and the plants and/or their environ­ment would be infested.

The parasitic insects, fungi, bacteria, viruses, and nematodes, as well as the weeds are so distinctive as biotic entities that the studies of them have evolved into specialized branches of science.

• Entomology is the study of insects.

• Bacteriology is the study of bacteria.

• Mycology is the study of fungi.

Causes of Plant Injuries

Agent of Injury

Biotic

Abiotic

Infectious?

Infestious?

Insects

Yes

No

Usually

Often

Mites

Yes

No

Yes

Fungi

Yes

No

Yes

Often

Bacteria

Yes

No

Yes

Often

Viruses

Yes

No

Yes

Viroids

Unclear

Unclear

Yes

Mollicutes

Yes

No

Yes

Sometimes

Nematodes

Yes

No

Yes

Yes

Weeds

Yes

No

No

Yes

Slugs and snails

Yes

No

No

Yes

Parasitic plants

Yes

No

Sometimes

Yes

Rodents, rabbits, deer, other animals

Yes

No

No

No

Nutrient deficiency

No

Yes

No

No

Temperature extremes

No

Yes

No

No

Chemical burns from fertilizer and pesticides

No

Yes

No

No

Lawn mowers and other mechanical tools

No

Yes

No

No

Vandalism

Probably

No

No

No

Water drought

No

Yes

No

No

Pollution

No

Yes

No

No

TABLE 6-1.

PLANT INJURIES AND THEIR CONTROLVirology is the study of viruses

• Nematology is the study of nematodes

• Plant pathology is the study of plant diseases, their causes, and their controls.

• Weed science is the study of weeds and their control

INSECTS_______________________________________

Regarded by nearly all biologists as the most interesting, economically significant, and certainly most prolific of all members of the animal kingdom, insects are a study in contrasts. For millions of years, insects have benefited and beleaguered human civilization. Essential as scaven­gers, pollinators, and reducers of organic matter to earth mold, insects also despoil food crops, invade our homes, attack our bodies and those of other animals, and lay waste our agricultural products.

Insects are primarily terrestrial animals that range from the arctic to the antarctic and from the depths of the earth to the mountain tops. They constitute over two-thirds of all the animal species on the planet and are surpassed only by microbes in sheer numbers of individuals.