Certain insecticides or spray mixtures cause plant damage called phytotoxic reaction. Pesticide labels usually mention sensitive plant species and cultivars. However, most plant listings on product labels are not binding and allow for these products to be used on other ornamental plants. The sensitivity of unlisted plants to the product or tank mixture is unknown.

Sensitive flowering periods and stressed plants. Plants usually are more sensitive to pesticides after they begin to bloom, and foliar sprays should be avoided or used with caution at that time. During bloom, the careful use of smokes or certain aerosol products may be preferable in the greenhouse.

Plants under water and/or temperature stress are also more prone to phytotoxic reactions. Avoid spraying when the weather is extremely hot and sunny. Spray in the mornings when possible, preferably between 6 and 10 a.m. When air or plant tissue temperature is about 90 degrees or higher, damage will likely occur. On bright, sunny days, leaf tissue temperature may be 5 to 15 degrees higher than that of the surrounding air, thus increasing the possibility of injury.

Pesticide formulations and application methods. Wettable powders are usually safer for plants than emulsifiable concentrates since they do not contain emulsifiers and solvents. The disadvantage of wettable powders is the objectionable residue left on the foliage by some products. Almost all aerosol and fogging formulations will cause phytotoxicity if applied at less than the recommended distance between nozzle and plant. A distance of 18 to 20 inches usually is recommended.

Tank mixtures. Mixtures of insecticides, miticides or fungicides may cause plant injury when the use of one of the materials alone would not. Pesticides should not be tank mixed unless this use appears on their labels, or unless the grower knows first-hand that the mix will not injure his crop.

Phytotoxicity testing. When using a product, tank mixture or application method for the first time, test it on a small group of plants and observe them for several days. With foliar applications the new growth is most likely to show phytotoxic damage. With soil drenches damaged root tissue may show up as plant stunting or slow decline, with the older, not younger,leaves damaged.

Symptoms of phytotoxic reactions. Plants react to pesticides in a number of ways:

* Chlorosis – appears as spots or as tip, margin or leaf yellowing.
* Leaf distortion – appears as curling, crinkling or cupping of the leaf.
* Stunting – The entire plant is reduced in size or certain parts (fruit, flowers, roots) are smaller while the rest of the plant appears normal.
* Abnormal growth – Excessive growth on either certain parts (aerial roots, suckering) or the entire plant.

To avoid phytotoxicity, study the label and any brochures that are available concerning the particular pesticide to be used. Pay attention to dosage rates, application instructions and phytotoxicity information. Do not overdose. Use a clean sprayer after each use. Do not use sprayers in which herbicides have been used. Don’t apply a pesticide to plants that are stressed. Never spray plants when they are in need of water, since wilted or dry plants are extremely sensitive to spray injury. Don’t apply pesticides when the temperature is either very high or very low, or when the spray will not dry quickly. Under cool, humid conditions plants will remain wet, which increases the possibility of injury and disease. This is one of the reasons greenhouse plants are more likely to be damaged.