Chemical Injury
Chemical Abiotic disorder
Last updated
This profile contains basic abiotic disorder data. Regional field notes and expert review are in progress.
What Causes It
Chemical injury covers a broad class of non-pathogenic damage caused by contact with or uptake of substances that disrupt plant metabolism. The mechanism varies with the chemical: herbicides interfere with specific metabolic pathways (amino acid synthesis, photosynthesis, cell division) and produce characteristic distortion or chlorosis patterns; fungicides and insecticides can phytotoxically burn leaf tissue when applied off-label, in heat, or on sensitive species; cleaning products, spray drift, and industrial chemicals can cause contact burns that look like freeze or scorch injury. The pathway is usually foliar absorption, root uptake from contaminated soil, or bark contact on green stems. (Source: PNW Plant Disease Management Handbook, Rose - Chemical Injury; Blueberry - Chemical Injury.)
Quick Reference
Symptoms
Symptoms vary widely by chemical and dose. Common patterns: cupping, curling, strapping, or distortion of new leaves; chlorosis and bleaching of leaf tissue; small leaves and shortened internodes on new growth; necrosis of leaf margins or interveinal tissue; entire plant death in severe cases. Glyphosate-specific: bright yellow new growth in the season of application, or proliferation of small shoots with needle-like leaves and underdeveloped blossoms the following spring. Fungicide burn: marginal necrosis, speckling, or bleaching that appears within days of application, often after an application made during hot weather or outside the label rate. The plant may outgrow the injury if the dose was sublethal. Symptoms may not appear until the following growing season, particularly for fall applications of systemic herbicides. (Source: PNW Plant Disease Management Handbook, Rose - Chemical Injury.)
Chemical injury typically affects new growth (where meristems are actively dividing and most sensitive) rather than mature tissue. Directional patterns tracking drift (damage on the side of a plant facing a sprayed area) are common. Uniform damage across a block of plants points to an application event rather than a pathogen. Shoot distortion and strapping indicate growth-regulator-class herbicides; bright yellow new growth indicates glyphosate; marginal burn after a hot-weather spray indicates fungicide or insecticide phytotoxicity.
Timeline: Acute contact injury appears within hours to days of exposure. Systemic herbicide injury (glyphosate, growth regulators) may not appear until the next flush of new growth, and glyphosate in particular can persist visibly through multiple flushes. Fall applications of systemic herbicides near woody plants often do not become visible until the following spring bud break.
Triggers & Conditions
Off-label herbicide or pesticide application; spray drift from adjacent applications (particularly during warm still weather when vapor drift extends reach); direct overspray onto sensitive species; soil contamination from persistent herbicides in mulch, compost, or manure; contact between green bark and herbicide spray; root uptake from contaminated irrigation water; accidental fertilizer or de-icer contact. Glyphosate in particular is absorbed through green wood of canes, so cane contact at any season can translocate. (Source: PNW Plant Disease Management Handbook, Rose - Chemical Injury.)
Active growth flushes are the most vulnerable period for most chemicals. Dormant-season applications of systemic herbicides can be absorbed through green cane bark. Warm calm weather extends spray drift exposure zones. Newly planted stock is more vulnerable than established plants.
The most common chemical injury cases on Puget Sound landscapes are: (1) glyphosate drift onto ornamental shrub beds from adjacent lawn or driveway spot-treatment, showing up the following spring as distorted rhododendron or rose growth; (2) broadleaf herbicide drift from lawn weed-and-feed products onto tomato, bean, and ornamental foliage during warm calm weather when vapor drift extends the exposure zone; (3) fungicide burn on rhododendrons and camellias when copper or sulfur products are applied during hot sunny weather; and (4) contaminated bark mulch or compost containing persistent herbicide residues (clopyralid, aminopyralid) from treated hay or grass clippings, which can affect beds for one to several growing seasons after application. Always collect the application history before sampling for lab diagnosis.
Management
Prevention
- Read and follow pesticide labels exactly
- Avoid herbicide applications in close proximity to ornamental plants
- Do not apply pesticides during hot or windy weather
- Verify compost and mulch sources are free of persistent herbicides
- Shield sensitive plants during nearby applications
Mitigation
- Rinse contaminated foliage with clean water immediately
- Irrigate to dilute soil contamination
Separate ornamental beds from turfgrass and driveway spot-treatment zones by physical distance or hardscape. Do not site vegetable gardens downwind of frequent herbicide applications. Use integrated pest management to reduce reliance on chemical controls.
Plant Tolerance
All landscape plants can be injured by chemicals at sufficient dose. Sensitivity varies dramatically by species and by chemical class: some plants tolerate specific herbicides used for selective weed control, while others are acutely sensitive to the same products.
More Tolerant
- Plants labeled for the specific product being used
- Mature woody plants with thick bark and waxy cuticles
More Sensitive
- Newly planted stock of any species
- Plants with thin leaf cuticles (Japanese maple, redbud, dogwood)
- Ornamental vegetables and fruit trees during active growth
- Roses during the growing season (glyphosate particularly)
- Tomatoes and related Solanaceae (broadleaf herbicide drift)
Chemical class and mode of action, plant species and current growth stage, application rate and coverage, ambient temperature at and after application, adjuvants used in the tank mix, and time since the last application of any kind. Hot weather dramatically increases phytotoxicity for most fungicides and oils.
Secondary Effects
Sublethal doses can predispose plants to secondary stress, winter injury, and opportunistic pests.
Persistent soil herbicides can affect replanting for one to several seasons after contamination.
Distorted growth from growth-regulator herbicides may persist through multiple flushes before the plant recovers.