Fertilizers, fungicides, insecticides, and plant growth regulators can cause chemical damage to plants (phytotoxicity) if they are misapplied or used when environmental conditions or cultural practices predispose plants to phytotoxicity. Desirable plants exposed to herbicides are at the greatest risk of phytotoxicity because herbicides are specifically designed to kill plants.
Identification
Phytotoxicity from herbicides or other chemicals can cause symptoms resembling damage from drought or water deficit, excessive soil salinity, and leaf-spot pathogens. Phytotoxicity is difficult to diagnose without knowing the affected plant's chemical-treatment and cultural-care history. Many of the same techniques used to diagnose herbicide damage can be used to diagnose phytotoxicity due to other chemicals.
Disorder development
Damage is possible from applications during or just before hot weather (about 90°F) or if plants are stressed from water deficit. The risk of phytotoxicity is increased by
- excess rates
- conditions that do not favor rapid drying of spray
- treating during certain growth stages e.g., young plants or during flowering
- not following all of the product's label directions
Treating plants not on the product label and using spray equipment that previously applied herbicides also can cause phytotoxicity.
Horticultural (narrow-range) oil can remove the desirable bluish cast on foliage of certain spruces. Oil application is not recommended on a few genera of sensitive plants, on any species that is drought stressed, during hot weather, or in mixes with chlorothalonil, sulfur, and certain other fungicides.
Damage
Phytotoxicity symptoms include
- brown to yellow leaf spots
- chlorosis or necrosis of leaf margins, interveinal areas, or entire leaves
- leaf curling and stunting
- premature leaf drop
In some cases plants will be stunted, growth will be atypically slow. In the worst cases, desirable plants will die.
Solutions
Avoid phytotoxicity by applying pesticides and other chemicals carefully, as directed on the product label. If phytotoxicity is suspected, learn the affected plant's cultural-care history, what chemicals were used, how and when they were applied, and their rates. Apply the diagnostic methods discussed for herbicide phytotoxicity.
If phytotoxicity occurs, provide damaged plants with good care, especially appropriate irrigation. Modify cultural methods and the plant growing environment to maximize plant health and minimize the need for pesticide use.
Use nonpesticide alternatives whenever feasible, such as for foliage-feeding insects. Many types of broad-spectrum insecticides (carbamates, organophosphates, and pyrethroids) applied during warm weather can induce outbreaks of mites, causing foliage to appear burned and possibly drop prematurely.
After phytotoxicity occurs, avoid further injurious practices. Plants may later produce new, uninjured growth. Consult Abiotic Disorders of Landscape Plants for more information.
