Use Care Applying Pesticides

Gardening season is in full swing, and many people are bringing plants to our office for disease or insect pest identification. Many of the plants are not diseased; they are suffering from some form of phytotoxicity.

Phytotoxicity – what’s that? Phytotoxicity is plant injury generally caused by a chemical applied on or around plants. By far, the most common of these chemicals is some form of pesticide.

The symptoms of phytotoxicity on growing plants include stunting or delay of plant growth; misshapen, curled or distorted plants, fruit or leaves; russeting or bronzing of leaves or fruit; dead spots or flecks on leaves; dead leaf tips or leaf margins; and dead areas between the veins of the leaves. Many of these symptoms sound very similar to disease or insect infestation symptoms, so how can you tell the difference?

The damage you see might be phytotoxicity if any of the following statements are true: if you see no signs of plant disease or insects that might have caused the damage, if you know chemicals have been applied in the area recently, if the symptoms show only on plants in a certain location in the landscape, if the symptoms do not spread from plant to plant, if the damage includes twisted stems and curled leaves.

A very common problem we see is damage to adjacent trees and shrubs after the application of weed-and-feed-type product to a lawn. The roots of the adjacent plants are under the lawn and will absorb the broadleaf weed killer. Sometimes these plants die, but more often the plants begin to look sickly, so homeowners suspect a disease or insect pest.

Another common error is applying pesticides when temperatures are too high. Most pesticides are labeled for application between 60 degrees and 80 degrees F. At temperatures above 80 degrees, sprayed pesticides can volatilize, turning into a gas. The gas form of the pesticide can then drift to non-target sites or plants. Apply pesticides early in the morning, when temperatures are lower and winds are light, to minimize the chance of damage. This will also protect pollinators, such as bees, which are not as active early in the morning.

Phytotoxicity can also occur when plants are under water and/or high temperature stress. Under normal conditions, the plant would not be damaged, but because they were stressed before the pesticide was applied, they are more likely to suffer damage. Avoid spraying pesticides when the weather is extremely hot and sunny. On bright sunny days, leaf tissue temperature may be 5 to 15 degrees higher than the surrounding air temperature.

If the pesticide is too concentrated, it may cause damage. Always check the label directions when mixing pesticides, or better yet, buy ready-to-use formulations to avoid mixing mishaps. Most pesticides are safe if applied properly. Read, understand and follow all label directions.

Melody Hefner is the Urban IPM and Pesticide Safety Education Program Assistant for University of Nevada Cooperative Extension. Have a gardening question? Ask a Master Gardener at 775-336-0265, or