Herbicide 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
Non-target herbicide exposure disrupts plant growth by interfering with specific metabolic pathways. Growth regulator herbicides (2,4-D, dicamba, picloram, clopyralid, aminopyralid) mimic natural auxins and cause uncontrolled cell elongation and distortion in new growth, producing the characteristic cupping, strapping, and epinasty. Glyphosate inhibits the shikimate pathway and blocks aromatic amino acid synthesis, producing bright yellow new growth followed by distorted needle-like leaves. Photosystem-II inhibitors (diuron, atrazine) block photosynthesis and produce interveinal chlorosis and marginal necrosis. Contact herbicides (glufosinate, diquat) burn tissue on direct contact. Exposure occurs through spray drift, vapor drift (especially during warm still weather), root uptake from contaminated soil, and persistent residues in mulch, compost, or manure. (Source: PNW Plant Disease Management Handbook, Rose - Chemical Injury; general horticultural knowledge.)
Quick Reference
Symptoms
Growth regulator herbicides (2,4-D, dicamba): leaf cupping, strapping (leaves become long and narrow), twisting, parallel venation on normally netted-vein species, and distortion of new growth while older leaves appear normal. Glyphosate: bright yellow leaves on new growth in the season of application, or proliferation of small shoots with needle-like leaves and underdeveloped blossoms the following spring; glyphosate can be absorbed through green cane bark so rose canes contacted at any season can translocate. Contact herbicides: sudden bronzing, bleaching, or necrosis of tissue that contacted the spray. Photosystem-II inhibitors: interveinal chlorosis, marginal necrosis, whole-plant yellowing. Systemic herbicides may not show symptoms until the next flush of new growth. (Source: PNW Plant Disease Management Handbook.)
Distortion of new growth without visible pathogen structures is the signature of growth-regulator injury. Bright yellow new growth is the signature of glyphosate. Directional exposure tracking a known spray source or application event strongly supports the diagnosis. Persistence across multiple growth flushes after a fall herbicide application is characteristic of systemic products.
Timeline: Contact injury appears within hours to days. Growth-regulator injury appears within days to weeks as the new flush emerges. Glyphosate symptoms may not appear until the following spring if applied in fall. Persistent soil residues from compost contamination can affect growth for multiple seasons.
Triggers & Conditions
Application of herbicide on or near sensitive ornamentals; spray drift during wind; vapor drift during warm still weather; contaminated mulch, compost, or manure introducing persistent herbicide residues to soil; accidental application of the wrong product; contact of green cane wood with glyphosate at any time of year. (Source: PNW Plant Disease Management Handbook, Rose - Chemical Injury.)
During active growth flushes for most products. Fall glyphosate applications on woody plants can translocate without visible symptoms and show the following spring. Warm weather increases vapor drift from 2,4-D and similar products.
The most common Puget Sound cases are (1) lawn weed-and-feed products containing 2,4-D or dicamba drifting onto tomatoes, peppers, and adjacent ornamental beds during warm calm spring weather, (2) glyphosate spot-treatment of driveway weeds drifting onto rhododendron, rose, and hydrangea foliage, (3) contaminated hay mulch or municipal compost carrying clopyralid or aminopyralid residues that persist in landscape beds for one or more growing seasons, and (4) line-clearance herbicide applications on utility rights-of-way drifting onto nearby landscapes. Always collect a full application history for the property and surrounding parcels before assuming a pathogen is responsible for distorted new growth.
Management
Prevention
- Avoid broadcasting herbicides near sensitive landscape plantings
- Do not apply during warm or windy weather
- Verify mulch and compost sources
- Clean application equipment between products
Mitigation
- Rinse contaminated foliage immediately
- Irrigate heavily to dilute soil contamination
Separate vegetable gardens and sensitive ornamentals from turfgrass and driveway treatment zones. Use integrated pest management to reduce herbicide use. Install physical drift barriers on parcel edges facing treated areas.
Plant Tolerance
Broadleaf ornamentals and vegetables are acutely sensitive to growth-regulator herbicides. Roses are documented as sensitive to glyphosate. Grasses tolerate broadleaf herbicides but are damaged by glyphosate and other nonselective products.
More Tolerant
- Turfgrasses (tolerant of most broadleaf herbicides used on lawns)
- Established plants labeled for the specific product in use
More Sensitive
- Tomato, pepper, and other Solanaceae (2,4-D, dicamba)
- Grape (2,4-D drift causes distortion at extreme distances)
- Rose (glyphosate)
- Beans, peas, and legumes (clopyralid, picloram residues)
- Newly planted ornamentals of any species
Herbicide mode of action, plant species and current growth stage, exposure route, and cumulative dose. Warm weather amplifies vapor drift.
Secondary Effects
Persistent soil contamination from clopyralid and aminopyralid can affect bean, tomato, sunflower, and pepper crops for multiple seasons.
Sublethal injury weakens plants and predisposes them to other stresses.
Rose canes damaged by glyphosate in fall may show symptoms only the next spring, delaying diagnosis.