Phytotoxicity: The unexpected danger

When growers or landscape managers use a chemical to solve a pest or nutritional problem, the last result they want or expect is plant injury due to the application of the chemical. However, phytotoxicity-chemical injury to plants-is a concern and for good reason. A chemical that is safe on all plants under all conditions does not exist.

Phytotoxicity shows itself in many ways. Damage may appear as yellowing, death or destruction of tissues, stunting, growth retardation, abnormal growth or defoliation.

The use of new chemicals-or using a familiar product in a new way-brings uncertainty to a chemical application. Phytotoxicity can result from a variety of scenarios including: * Adverse reaction due to sensitivity of a particular species or variety of plant to a particular chemical * Use of excessive rates or too many applications at close intervals * Improper tank mixtures * Poor application of the chemical * Environmental conditions at the time of application. Understanding the various reasons for plants' phytotoxic response to a chemical application can lead to safer, more effective use of chemicals in plant-management programs.

Plant sensitivity Perhaps the most frequently observed phytotoxic damage is due to the fact that not all plants get along with all chemicals. For example, malathion, a commonly used insecticide, is phytotoxic to Hibiscus, Lantana, petunias, white pine (Pinus strobus), maples and many species of fern. Sulfur is toxic to Viburnum spp. Horticultural oils can harm tender new growth and foliage of particularly sensitive species such as mountain ash, beech and birch. Atrazine, a commonly used herbicide, selectively controls weeds in St. Augustinegrass but may damage ryegrass.

To avoid this type of phytotoxic damage, carefully read the label of all chemicals before you apply them. Years of research go into the development of these labels, and most list plants that are sensitive to the product. On some labels, the manufacturer lists only the plants that you can safely spray and thereby is released of legal responsibility if you treat other plant species with the product.

It is not feasible for chemical manufacturers to test all products on every species and cultivar used by the ornamental plant industry. If you have no information on the safety of a product on a particular plant species or variety, test a small number of plants with the product before treating an entire crop. Check the plants 2 and 7 to 10 days after application to evaluate any damage. Note any phytotoxic reaction of plants to chemical application in your spray records and keep that information for future reference. Your personal spray records can be an excellent source of information on phytotoxicity for your particular situation.

Excessive applications Phytotoxicity can also result from excessive rates or too many applications. Many pesticides formulated as emulsifiable concentrates carry precautionary statements regarding excessive rates or repeated applications at close intervals. At recommended rates, most pesticides labeled for use on ornamental plants and turfgrass cause no phytotoxic damage. However, if you increase the rates of these products to two, three or four times the recommended rate, the potential for damage greatly increases.

Even applications of horticultural oils or insecticidal soaps can damage sensitive plants if you apply them too heavily. You can safely apply oils and insecticidal soaps to most plants at or below a 2-percent rate. However, when application rates exceed 2 percent, phytotoxic damage is much more likely. Again, to prevent this type of phytotoxic damage, read the label for specific instructions on rates and proper application intervals.

Tank mixes Mixing insecticides, miticides, fungicides or fertilizers in one tank may save time, but it can be risky. You often can trace phytotoxic injury back to the application of a combination of chemicals. If one or more of the products are slightly phytotoxic, the mixture of products, even when all its constituents are within label rates, can multiply the potential for damage. Refer to the label for information on the compatibility of various products in tank mixes.

Test new tank-mix combinations on a small number of plants before treating an entire crop. Even if you perform a "jar test" for incompatibility, chemical changes that could result in phytotoxicity might not be visible. Keep records of successful tank mixes for future reference.

Drift and off-target application Phytotoxic injury also can result from sloppy application or drift onto non-target plants. To avoid this type of injury, make sure that spray equipment is in good condition and properly calibrated. Clean sprayers thoroughly between chemical applications. Maintain separate sprayers for application of herbicides and soil sterilants, and do not use these sprayers to apply other types of pesticides. Even a minute quantity of herbicide left in a sprayer can severely damage plants. Also, make sure that the spray mix receives frequent agitation during application. If chemicals settle in the spray tank, the application rate of spray mix drawn from the bottom of the tank will greatly exceed the recommended rate. To avoid treating non-target plants, make applications on calm days when the potential for drift is minimal.

Perhaps the most difficult phytotoxic injury to predict is that due to conditions at the time of application. You may discover that a chemical with a history of safe use on your plants suddenly causes injury. This type of injury usually stems from treatment of a particularly susceptible stage of growth, plant stresses or weather conditions at the time of application.

Again, read labels for specific application instructions. Chemical labels often carry warnings to avoid application during periods of excessive heat or cold. Some warn against treatment when flowers are present or when bracts are beginning to show color. Remember that, with most plants, flowers and bracts are much more sensitive tissues than leaves. In addition, make sure plants are not water-stressed during chemical application-water-stressed plants can be particularly susceptible to chemical injury.

Dr. Beverly Sparks is professor of entomology and extension entomologist with the University of Georgia-Athens.

* Read the label to ensure the chemical is registered for use on the plant. Varieties or cultivars of the same species can react differently to a chemical. * Use the chemical at the recommended rate and refer to the label for information on minimum intervals between applications. * Avoid unfamiliar tank mixes until you've made a test application. Keep records on phytotoxic reactions to tank mixes for future reference. * If unfamiliar with a product, test it on a small number of plants before treating an entire crop. * Maintain a separate sprayer for application of herbicides and soil sterilants. Do not use these sprayers to apply other types of pesticides. * Keep all sprayers in good working order and properly calibrated. * Frequently agitate the sprayer during application. * Avoid chemical application during periods of excessively low or high temperatures or to plants exposed to intense sunlight. * Do not apply chemicals to water-stressed plants. Make sure plants are turgid (not wilted) during application. * Build on your knowledge of the chemicals used in your operation by keeping accurate records of all applications. Make note of phytotoxic reactions for future reference.

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