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.

• Virology 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.