phytotoxicity symptoms on blue spruce

Conifers with a blue cast to their needles like Colorado blue spruce can irreversibly lose their bluish color if sprayed with horticulture oil.

Updated: February 28, 2023

What is phytotoxicity?

Phytotoxicity is the misuse or misapplication of an insecticide, herbicide, or fungicide that causes damage to plant tissues.

  • Symptoms on broadleaved shrubs include leaf spotting, scorch or dead tissue, and can be confused with insect or mite damage, diseases, other abiotic problems such as nutrient deficiencies, or environmental conditions.
  • Injury symptoms on evergreens may include needle spotting or banding, tip browning, or curling of new growth. Needles of injured plants may turn yellow, followed by needle browning. 
phytotoxicity symptoms on leaves
Phytotoxicity symptoms. Photo: University of Maryland


phytotoxicity symptoms on blue spruce
Phytotoxicity on blue spruce from horticultural oil (turns blue needles green)

Possible reasons for phytotoxicity damage

  • High temperatures, in general, will increase the likely hood of injury from pesticides (insecticides and fungicides especially soaps, oils, and sulfur compounds). High temperatures and humidity can increase the chance of injury from sulfur-containing pesticides.
  • Damage may also occur due to wind drift onto nontarget or sensitive plants. A pattern of damage to nontargeted plants may be visible over a wide area such as adjacent yards. Spray applications should be applied during calm, dry, and cool conditions.
  • Most pesticides are best applied below 85 F. Always check label directions for cautions. Also, check label cautions about combinations of pesticides.
  • Cool damp weather may increase the chance of injury by copper fungicides
  • Insect injury and frost damage predispose plants to chemical damage.
  • Stressed, especially drought-stressed plants are more prone to phytotoxicity damage. 
  • Phytotoxicity may also result if incompatible chemicals were applied at the same time.

Additional resources 

Phytotoxicity | Penn State

Rev. 2020