Phytotoxicity, or unintentional pesticide damage to plants, results in abnormal growth, foliar burn, leaf drop, and discolored, curled, and spotted leaves (ex. 2, 4-D herbicide injury to grape). If phytotoxicity is severe, the plant may die. Phytotoxicity often resembles such other problems as insect damage, plant disease, and poor growing conditions such as insufficient moisture and improper fertilization. The following contribute to accidental plant injury by pesticides:
Ornamental plants include herbaceous, semi-woody, and distinctly woody species. Generally, herbaceous plants (chrysanthemums, petunias, turfgrasses, etc.) are more susceptible to pesticide damage than woody plants, and the woody plants are more susceptible when growth is young and tender.
Accidental plant damage is most likely to occur with herbicides, since their function is to injure certain kinds of plants. For example, herbicides used to kill broad-leaved weeds in turfgrasses may also injure broad-leaved ornamental trees, shrubs, and flowers if not used properly. Insecticides and fungicides can cause injury to certain sensitive species of plants and also to normally tolerant plants under unusual conditions such as hot weather. The pesticide label is the best guide for the safe use of pesticides on a specific ornamental plant. If the pesticide is not known to be safe for use on a specific ornamental plant, it should not be used. Greenhouses present a special problem because phytotoxic vapors tend to be trapped in the closed environment, exposing all plants within the greenhouse to the pesticide.
The proximity of different plants with varying susceptibility to pesticide damage requires that commercial ornamental and turf pesticide applicators be especially aware of drift problems. Several steps can be taken to prevent damage to non-target plants.
Drift can be prevented or minimized by using methods described in a later section. If possible, select pesticides that are safe for both target and non-target plants. It may be necessary to place a barrier around the target plant or remove susceptible plants from the area (such as removing susceptible potted plants from a greenhouse). A decision may have to be made whether or not to apply a pesticide to the target plant if the benefit does not justify the hazard to nearby plants.
Persistence is an important part of pest control, since successful pest control requires a knowledge of the persistence period to make subsequent applications. A persistent chemical has an advantage for long-term pest control because fewer applications are needed.
The period of pesticide residual activity varies greatly from one class of pesticides to another. Persistence is directly related to the rate of application, soil type or texture, temperature, moisture conditions, rainfall, and other factors. Commercial applicators must be familiar with the persistence of each pesticide that may be applied to ornamentals and turf, especially where adjacent areas may be affected or where treated soil is used to grow other plants. When different plants are rotated in the same soil, phytotoxicity can be a problem because a pesticide used to control some pests on one plant may leave residues in the soil that will damage or kill another plant. Information on the persistence of a given pesticide can be found on the product label.
Turf and ornamental pesticides must often be applied in areas that humans, as well as pets and other domestic animals, frequently use. The pesticide applicator must be constantly alert to the hazard associated with this situation. Primarily, the problem is twofold: (1) hazardous amounts of pesticides must be prevented from drifting into non-target areas; and (2) humans, pets, and other animals must be prevented from contacting hazardous amounts of pesticides within the treated area.
Where several pesticides are available, the applicator should strongly consider the hazard and toxicity of the active ingredient in making a choice. Use formulations and methods of application that will minimize drift.
Air blast sprayers can be used in ornamental and turf pest control if very safe pesticides are used (toxicity to applicator and non-target plants is low). Air blast sprayers should never be used to apply herbicides.
Two types of drift are associated with pesticides. The most common, drift of spray droplets or dust particles, is directly affected by such things as spray pressure, nozzle opening size, wind velocity, and pesticide formulations. Lower sprayer pressures will result in larger droplet sizes and less drift potential. The same is accomplished with larger nozzle opening sizes. Movement of a chemical with low vapor pressure (or high volatility) termed “vapor drift,” is a second kind of drift. Vapors or gases can drift in harmful concentrations—even in the absence of wind. Fumigants, such as methyl bromide, must be confined so they will not drift from the treated area (proper sealing with a plastic tarp is essential). Some pesticide products are volatile, or capable of vaporizing from soil and leaf surfaces in potentially harmful concentrations after application. Herbicide vapor can severely damage and even kill desirable plants.
The potential for contamination of groundwater is an important consideration when choosing pesticides. Several products have groundwater advisory statements on their label. Such statements advise not to apply these products where the water table (groundwater) is close to the surface and where the soils are very permeable (well drained soils such as loamy sands). Refer to these statements and observe all precautions on the label when using these products. Protecting Humans, Pets, and Domestic Animals
Before applying a pesticide, the application site should be cleared of such things as toys, pet food dishes, bird feeders, and other articles. Pesticide residues on these articles can be a hazard.
Animals and humans should be kept from the area during pesticide application. (They should also be kept from the area of potential drift and run-off until the spray has dried or the dust has settled, regardless of the toxicity of the pesticide used.) Some pesticides may be potentially hazardous for a longer time; therefore, label directions concerning reentry should be closely followed.
Highly toxic systemic insecticides should not be used on plants just before they are sold to the public. It is possible that in the process of selecting a plant, customers may handle the soil and be exposed to the active chemical. In commercial operations, it is necessary to hold the plants until the chemical has lost its toxicity. Recently sprayed ornamental plants with showy flowers or edible fruits present a special hazard because they are likely to be handled or even eaten by children.
Earthworms are important, beneficial invertebrates in turfgrass, where their burrowing and feeding activity enhances soil structure and fertility and incorporates thatch and other plant residues into the soil. Certain pesticides can significantly reduce earthworm populations with long-term effects. Preservation of earthworms and other beneficial soil invertebrates may be critical to long-term stability of the turfgrass ecosystem. Worker Protection Standard
The U.S. Environmental Protection Agency (EPA) has issued final rules governing the protection of employees on farms, and in forests, nurseries, and greenhouses from occupational exposure to agricultural pesticides. The new Worker Protection Standard (WPS) covers:
In greenhouses, employers must ensure that workers and other persons do not enter specific areas during, and in some instances, after certain pesticide applications. In nurseries, employers must make sure that during certain pesticide applications, workers and other persons do not enter treated areas on the nursery. Employers affected by the WPS must post specific types of information at a central location, provide decontamination sites, provide emergency assistance if needed, provide pesticide safety training, give notice about pesticide applications, and provide personal protective equipment.
All pesticide products affected by the WPS will carry a statement under the Agricultural Use Requirements section of the labeling. This statement will instruct users affected by the WPS to comply with all provisions of the WPS. For more information about the WPS, please contact your local Cooperative Extension Service Office or the Division of Pesticides at (502) 564-7274.