Pesticide burn, or phytotoxicity, is caused by misuse or misapplication of chemicals on plants. Symptoms included leaf spots, blotches, scorch, or tip burn. Symptoms are sometimes confused with disease, insect or mite damage or problems caused by environmental conditions.
Pesticide burn may also occur when pesticides are sprayed on stressed plants. Stresses, such as drought, disease, insect injury, and frost damage predispose plants to chemical damage. Even non-toxic sprays, such as insecticidal soaps and horticultural oils can result in pesticide burn when sprayed on injured or sensitive plants.
Phytotoxicity frequently occurs when pesticides are sprayed under adverse weather conditions. High temperatures, in general, will increase the possibility of injury from pesticides (insecticides and fungicides especially soaps, oils and sulfur compounds). High temperatures and humidity can increase the chance of injury from horticultural oils or sulfur-containing pesticides. Cool damp weather may increase the chance of injury by copper fungicides. Phytotoxicity may also result when incompatible chemicals, such as oil and carboxamide compounds, are applied at the same time. Damage may also occur due to wind drift onto nontarget or sensitive plants. Spray applications should be applied during calm, dry and cool conditions. Most pesticides are best applied below 85 F.
If pesticide use is warranted, be sure to apply chemicals according to label directions. Always check label directions for cautions on sensitive plants or cautions about combinations of pesticides. The plant you wish to spray should be listed on the label of the pesticide. When insects or diseases seriously damage annuals, perennials or herbs, cut off the damaged parts or consider replacing the plants instead of spraying a pesticide.