The control of insects, pathogens, and weeds depends on how successfully the horticulturist applies one or more of the four basic principles of control.
Exclusion Exclusion is the first principle of control. It includes all the measures designed to keep a pest from becoming established in an area. The area may be a single flat of soil, a greenhouse bench, a lawn, a nursery field, a geographic region, or even an entire country. The measures vary depending on the type of pest.
• The use of pest-free propagative stock is essential in the commercial production of horticultural crops. Growers must select their sources of supply carefully. Certain sources have excellent reputations as suppliers of disease – and insect-free cuttings and weed-free seed.
Certified seeds and plants are produced under the direction of an agency that provides documentation of their varietal purity, viability, moisture content, and the absence of weed seed, insects, and pathogens.
• The treatment of propagative material can kill pathogens and insects. Immersing bulbs in a fungicidal dip, treating seeds with heat, disinfesting propagative tools, and disinfecting cuttings prior to placing them in a propagation bench are all ways to eliminate pests that may be in or on those materials.
• Plant quarantines are restrictions on the production, movement, or very existence of certain plants or plant products in an area. The restrictions are imposed by a legislative authority and enforced in an effort to prevent the introduction or spread of a pest in an area.
Eradication Eradication is the principle that seeks to remove or eliminate pests that are already in, on, or near plants in infested areas. The
measures of eradication attempt to reduce the quantity of pathogenic
inoculum, insects and their eggs, and weeds and their seeds.
• Isolating and destroying individual infected plants is timeconsuming and expensive, yet it has its place in ornamental plant production. In agronomic production, it is usually too costly. When the removal of infected plants accompanies another job function such as watering or disbudding, the cost of control is reduced. Nevertheless, isolating and destroying are more effective for the greenhouse pot crop grower than for the sod producer.
• Hand-pulling and cultivation are directed against weed pests in nursery fields and landscape plantings.
• Destruction of alternative hosts such as weeds can also aid pest control. The alternate hosts may allow completion of the life cycle of a fungus. They may also harbor insect vectors that transmit virus pathogens.
• Crop rotation and soil treatments are both methods of eradicating pests that persist in the soil. After a nursery crop has been dug from a field, a season of planting with a cover crop such as barley or sudangrass will not only improve soil structure but also may allow the cover crop to compete with and smother troublesome weeds.
Soil treatment in the form of either heat or chemicals will eradicate pathogens, insects, and weeds. Greenhouse soils are commonly pasteurized by steam heat or hot water available from the heating system (Figure 6-15a and b). Nursery fields and landscape plantings can use chemical eradicants effectively.
• Destroying host parts that display evidence of insect or disease damage can reduce the amount of inoculum available for dissemination to nearby hosts. As growers tend their crops, they have the opportunity to eradicate potentially problematic host parts with little additional expense in time and money.
• Removing infested refuse eliminates a site for the overwintering of pathogens or insects. It makes a good case for the elimination of fallen leaves and grass clippings from landscapes.
Chemical sprays, dusts, and drenches are generally the most expensive methods of eradication. The products, collectively termed eradicants, strive to kill the pathogen before it can infect the host; kill the insect before it can do much damage or reproduce; or kill the weed before or shortly after it emerges. Chemical control will be discussed in more detail later in the chapter.
Protection Protection is the principle of control that sets up a barrier between the host plants and the pests to which they are susceptible. It is a shielding endeavor that can be accomplished either through manipulating the plants’ growing environment or by applying chemicals.
• Manipulating the environment is an attempt to create conditions for growth more favorable to the ornamental host than to the pest. An obvious example of a shield offering protection is the growth of plants inside a screened area to protect the host from insects and the pathogens they may be carrying. Other examples of environmental alteration include modifications of the moisture level to avoid creating a prolonged period of heavy moisture in which pathogens can thrive; alteration of the soil pH to attain a pH at which the host will grow better than the pest; modification of the nutrient level
of the soil to produce healthy, actively growing plants that are less susceptible to injury by pests; and maintenance of cool temperatures to slow the metabolic rate of both hosts and pests, thereby reducing the extent of damage. When cut flowers are kept in refrigerated storage by a florist, the principle of protection is being applied.
• Chemical sprays and dusts can be applied to seeds, foliage, and wounds of plants to place a barrier between the host and the insect or pathogens. In this case, it is essential that the chemical be applied before the pests arrive. It then kills them after they arrive.
Resistance Resistance is the fourth principle of control. It is an attempt to change the plant’s morphology and/or genotype so that it will suffer less from diseases or insects. Resistance may originate naturally with a plant population or it may be developed through research by plant hybridizers.
Resistance in plants is not like immunity in animals. That is, plants do not produce antibodies as animals can. Instead, plant resistance is based either on the physical structure of the plant or cell hypersensitivity. For example, physical features such as leaf pubescence or thick cuticle may discourage penetration by fungal spores or insects merely by holding them at a distance from potential sites of infection. Hypersensitivity is the extraordinary reaction of a plant cell to invasion in which the cell dies so quickly that it fails to support further proliferation of the pest.
Resistance is seldom total. All plants have some degree of susceptibility, and that susceptibility often increases with time. The loss or lessening of resistance to a pest does not necessarily mean that the host plant has changed; it may mean that the pest has altered in a way that allows it to infect the plant. Considering the reproductive potential of most pests and the rapidity of their life cycles, it is not surprising that resistance, especially that developed by artificial hybridization, is often overcome by the natural mutation of insects and pathogens.