Contributor: J.R. Baker, Extension Entomology Specialist
For example, dimethoate (Cygon) is very useful for insect and mite control on azalea, camellia, boxwood, and a few other plants. However, dimethoate can cause all the leaves to drop from Chinese holly and Elaeagnus if applied in the spring.
Contributing Factors
Sensitive Plants. Some plants are inherently sensitive to pesticides in general. The true ferns, English ivy, palms, and the bracts of poinsettias are extremely sensitive to pesticides.
Tank Mixes. Tank mixes conserve labor and energy by the application of multiple pesticides for simultaneous control of a variety of pests. Insecticides, miticides, and fungicides are sometimes mixed together and applied at fairly close intervals without harm to plants, particularly during the fall, winter and spring. Additional spreaders or stickers may be added to these tank mixes without apparent injury to plants during the cooler parts of the year. However, in hot weather, tank mixes are much more likely to cause phytotoxicity than are applications of single-ingredient pesticides.
Adjuvants. Wetting agents, spreaders, emulsifiers, dispersing agents, foaming agents and foaming suppressants, penetrants, stickers, correctants and attractants are used to improve the efficacy of pesticides. Because all pesticides are formulated with adjuvants already added, the danger of a phytotoxic reaction from tank mixing pesticides becomes apparent.
Solvent. Xylene and other petroleum distillates are used to dissolve and dilute the active ingredients of emulsifiable concentrates. Chrysanthemums and other ornamental crops are sensitive to xylene. Pesticides containing xylene cause malformation of the leaves.
Water Stress. Plants that are under moisture stress are much more likely to be damaged by pesticides than plants which are not. It is always a good idea to water plants before applying a pesticide. In a greenhouse, if the formulation is to be applied with a smoke generator, irrigation should be done far enough in advance of the application so that the foliage has a chance to dry. If some smoke generators are set off near wet plants, the water on the leaves may absorb enough of the pesticide smoke to burn the leaves as the water evaporates--and the pesticide thereby becomes more concentrated.
Heat and Light. The combination of heat, light and wet pesticide residue can cause phytotoxicity. Placing many species of house plants into direct sunlight without preconditioning or acclimatizing the plants over a few weeks may cause sun-scald injury by itself (much less adding the stress of the wet pesticide to the tender plant). Phytotoxicity is most likely to occur in greenhouses where growers use tank mixes in hot weather.
Excessive Use of Pesticides
Excessive rates of pesticides or very short intervals between pesticide applications may result in a burning of petals and margins of leaves (common for spray injury) or yellowing of the leaves (soaps), puckering (xylene injury) or plant growth regulator type of injury. Fortunately, many ornamentals are tough enough to take a lot of abuse, but plant damage eventually may occur. In most cases, excessive amounts of pesticides do not control a greater percentage of insects than the labeled amount. In general, if insects seem to be unaffected by the labeled rate, it is probably time to change pesticides.
When applying pesticides, the label should be read to determine whether the pesticide is appropriate for the pest and plant and to determine the proper rate for application. In general, it is best to apply pesticides in part the cool of the day so that the residue dries before the plants are subjected to hot sunlight. It is also good to irrigate plants before applying pesticides to lessen the chances of pesticide injury.
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