7.7 Diagnosing Herbicide Injury & Off-Target Damage
Key Takeaways
- Symptom pattern in the field is the first diagnostic: drift shows a gradient decreasing away from the source, carryover follows soil type and old field boundaries, tank contamination is uniform across the whole treated area, and misapplication follows the sprayer pattern.
- Synthetic auxin injury shows as epinasty, stem twisting, leaf cupping and strapping, and on rice as parrot-beaked or blanked panicles; symptoms appear on new growth days after exposure.
- ALS-inhibitor injury shows as stunting, purpling, chlorotic new growth and dead root tips; glyphosate shows as chlorosis at growing points and general yellowing developing over one to two weeks.
- Vapor drift is distinguished from particle drift by timing and pattern: particle drift arrives during or immediately after application with a sharp gradient, while volatilization causes damage hours or days later over broader, less directional areas.
- Many non-chemical problems mimic herbicide injury - nutrient deficiency, compaction, flooding, disease, nematodes and weather - so document, photograph, sample and get a diagnosis before concluding drift.
7.7 Diagnosing Herbicide Injury & Off-Target Damage
Core Concept: When a neighbour reports damage, the question is never simply "did drift happen." It is what pattern does the damage make, which chemistry does the symptom match, and what else could produce the same picture. Getting this right protects an innocent applicator as effectively as it identifies a guilty one - and in Arkansas, where the Department investigates drift complaints as a matter of course, it is a professional survival skill.
Step 1: Read the Pattern
The spatial arrangement of injury is the most informative single piece of evidence, and it is available before any laboratory work.
| Pattern observed | Most likely cause |
|---|---|
| Gradient - severe at the field edge nearest a treated field, decreasing with distance | Particle drift from that source |
| Broad, diffuse, non-directional injury appearing days later, often worst in low areas or under a canopy | Vapor drift / volatilization |
| Uniform across the entire sprayed area, matching the boundary of one applicator's pass | Tank contamination or a misapplied product |
| Follows the sprayer pattern - stripes at boom width, doubled overlap strips, headland concentration | Application error: nozzle plugging, overlap, incorrect rate, or a rate controller failure |
| Follows soil type, eroded knolls, high-pH or low-organic-matter spots, or old field boundaries | Herbicide carryover from a previous season |
| Follows water movement - low spots, tailwater channels, the inlet end of an irrigated field | Runoff, irrigation water contamination, or a tailwater problem |
| Random scattered plants with no spatial logic | Usually not herbicide - look at disease, insects, or a seed or fertility issue |
Step 2: Match the Symptom to a Herbicide Group
| Group | Signature symptoms |
|---|---|
| Synthetic auxins (2,4-D, dicamba, HRAC Group 4) | Epinasty - leaves and petioles bending and twisting; cupping of new leaves; strapping and parallel venation on new growth; stem twisting and callus; on rice, parrot-beaked heads and blanked panicles. Symptoms appear on new growth several days after exposure |
| ALS inhibitors (Group 2) | Stunting, purpling of foliage, chlorotic or yellow new growth, shortened internodes, dead or stubby root tips. Slow to appear |
| EPSPS inhibitor (glyphosate, Group 9) | Chlorosis beginning at growing points, general yellowing, slow decline over 1 to 2 weeks; on the following crop, white or bleached new growth from carryover in some species |
| Photosystem II inhibitors (atrazine, metribuzin, Group 5 and 7) | Interveinal chlorosis followed by necrosis on older leaves first, often beginning at leaf margins and tips |
| PPO inhibitors (Group 14) | Rapid contact burn - bronzing, speckling, necrotic spots where spray droplets landed. Fast, and usually confined to contacted tissue |
| ACCase inhibitors (Group 1) | Grass-specific: chlorosis and death of the growing point, which pulls out easily from the whorl; broadleaves unaffected |
| Bleachers (Groups 12, 13, 27) | White or bleached new tissue |
| Root-inhibiting dinitroanilines (Group 3) | Stunted, club-shaped root tips, poor stand, swollen hypocotyls |
Two useful discriminators:
- Older leaves affected first points at a mobile, root-absorbed product moving in the transpiration stream - typical of PS II inhibitors.
- New growth affected first points at a phloem-mobile or growing-point-active product - typical of auxins, ALS inhibitors and glyphosate.
Step 3: Particle Drift Versus Vapor Drift
| Particle drift | Vapor drift (volatilization) | |
|---|---|---|
| Timing of exposure | During or immediately after application | Hours to days after, as residue volatilizes off treated surfaces |
| Weather driver | Wind speed, droplet size, boom height, inversion | Temperature and humidity after application |
| Spatial pattern | Directional, with a sharp gradient from the source | Broad, less directional, can affect areas upwind of the original application |
| Physical evidence | Deposit on cards or foliage near the edge | Often no deposit; injury only |
| Arkansas relevance | Addressed by nozzle, boom height, wind and buffer rules | The reason high-volatile esters of Class F products are prohibited and the reason the Class H rules exist |
A complaint describing symptoms that appeared three days after a neighbour sprayed, over a wide area, with no clear gradient, is a volatilization pattern - and it means the applicator's wind log at the time of application, while necessary, will not by itself resolve the case.
Step 4: Rule Out the Look-Alikes
Before concluding herbicide injury, consider:
- Nutrient deficiency - interveinal chlorosis from iron or manganese deficiency mimics PS II injury closely; check pH and tissue tests.
- Compaction and root restriction - stunting and uneven growth following traffic patterns.
- Flooding and saturation - yellowing, stunting and root death in low areas, extremely common in Delta fields.
- Disease and nematodes - stunting and chlorosis in patches following soil type.
- Weather - cold injury, hail, wind whipping, and salt from irrigation water.
- Fertilizer burn - marginal necrosis where granules contacted foliage.
- Seed treatment or in-furrow injury - symptoms confined to the seed row.
Step 5: Document Properly
Whether you are the affected party or the accused applicator, the record is what decides the case:
- Photograph immediately, with wide shots showing the field context and close-ups of symptoms, plus something for scale. Include the boundary and the direction to the suspected source.
- Map the affected area - extent, gradient, and distance from the suspected source.
- Sample symptomatic and non-symptomatic tissue separately, with chain-of-custody discipline, and keep samples cool.
- Preserve your tank samples, the spray log, weather records, GPS or flight telemetry, and product containers.
- Notify the affected landowner and report to the Department's Pesticide Section.
- Do not alter records. Altering an application record after the fact is a separate and far more serious problem than the underlying drift.
[!WARNING] Exam Trap: Uniform Injury Across a Whole Field Is Not Drift Drift produces a gradient. Injury that is even across an entire treated field, stopping precisely at the boundary of one applicator's work, points to tank contamination or a misapplication - which is why the Class E and Class F cleanout standard requires no detectable concentration of the previous product in the next tank load, and provides a sampling valve to prove it.
A cotton grower reports leaf cupping and stem twisting that appeared three days after a neighbour applied an auxin herbicide to a field a half mile upwind. The injury is spread broadly across 60 acres with no clear gradient from the shared boundary, and it is worst in a low-lying area. What does this pattern most likely indicate?
A soybean field shows interveinal chlorosis progressing to necrosis, beginning on the older, lower leaves and starting at leaf margins and tips. Which herbicide group is the most likely cause, and what does the symptom distribution tell you?
An applicator is accused of causing drift injury on an adjoining field. Which set of actions best protects the applicator and supports a correct determination?