Ministry of Agriculture

Food, Crop & Livestock Safety

Phytotoxicity

Phytotoxic means harmful or lethal to plants. Phytotoxicity is the degree to which a chemical or other compound is toxic to plants.

All types of pesticides can injure or kill plants. Herbicides are especially hazardous to plants because they are designed to kill or suppress plants. Some insecticides and fungicides can also harm plants.

Phytotoxic effects caused by herbicides can be from spray droplets, soil residues or vapours contacting sensitive plants. Plants can also be harmed by herbicides which move off target in water or soil or when sensitive crops are planted in fields too soon after a herbicide treatment was applied for a previous crop.

Phytotoxic properties of pesticides are usually associated with specific formulations (wettable powder, emulsifiable concentrate, granule, etc) or specific plants rather than groups of pesticides or plants. A pesticide label may indicate whether the pesticide could be phytotoxic, and may list plants or varieties that are sensitive.

Phytotoxic effects can range from slight burning or browning of leaves to death of the plant. Sometimes the damage appears as distorted leaves, fruit, flowers or stems. Damage symptoms vary with the pesticide and the type of plant that has been affected.

Glyphosate damage to apple tree Glyphosate drift damage to cucumber
Glyphosate damage to apple tree Glyphosate drift damage to cucumber
Dimethoate injury to cherry tree Kresoxim-methyl drift injury to cherry tree
Dimethoate injury to cherry tree Kresoxim-methyl drift injury to 'Sweetheart' cherry

Phytotoxicity is not necessarily caused by the active ingredient. Plant damage can also be caused by: the solvents in a formulation, impurities in spray water, using more pesticide than listed on the label, or poorly mixing the spray solutions. Condition of the plant at the time of treatment can affect phytotoxicity; stressed plants may be more susceptible. Environmental conditions such the temperature, humidity, and light can influence phytotoxicity. High temperatures can speed up pesticide degradation and volatilization, but may also result in increased phytotoxicity for some products. UV light rapidly breaks down many pesticides. Soil properties such as texture, temperature, moisture, microbial activity and pH also influence phytotoxicity. Higher pH soils are less binding and may increase photoxicity. High microbial activity can reduce phytotoxicity.

Minimize the risk of phytotoxic effects by:

  • reading and following label directions, especially the correct rates and timing, and being aware of potential weather effects,
  • avoiding application of pesticides when drift is likely to happen,
  • waiting for the correct planting times,
  • if unsure, conduct a simple field bioassay by treating only a few plants, before the treating whole block to check for phytotoxic effects, especially when growing new culitivars.