You may have observed plants with twisted leaves, cupped foliage, distorted growth, or unexplained stunting. The immediate question is often, "Did herbicide drift cause this?"

Sometimes the answer is yes. Sometimes it is not.

Pest management programs in current corn and soybean production systems commonly use dicamba and 2,4-D for weed control. Their use raises concerns about potential off-target movement for produce and fruit growers, vineyard managers, and homeowners. Before concluding that a distorted plant was injured by herbicide drift, it is important to understand what drift symptoms actually look like, what they do not look like, and the steps growers should take when concerns arise.

Not every distorted plant is herbicide drift

One of the most common mistakes during the growing season is assuming that any unusual growth pattern must be caused by herbicides. Many other problems can resemble drift injury, including:

• Heat stress
• Physiological leaf roll
• Broad mite damage
• Nutrient disorders
• Contaminated compost
• Insect feeding
• Environmental stress
• Fasciation

For that reason, it is critical to rule out any other causes prior to assuming herbicide drift damage. Symptom diagnosis should always consider the entire field history, crop stage, weather conditions, neighboring farm(s) herbicide application history, and distribution of symptoms. You don't have to figure this out on your own, MU Extension specialists are there to help diagnose symptoms.

Some crops are more sensitive than others

Research at University of Missouri has shown that plant species differ tremendously in their sensitivity to synthetic auxin herbicides such as dicamba and 2,4-D. Among perennial species evaluated in Missouri, grapevine was consistently one of the most sensitive plants, while hydrangea was among the least sensitive. Among vegetable crops, tomato, watermelon, and cantaloupe were highly sensitive to driftable fractions of dicamba and 2,4-D. Two neighboring plant species exposed to the same drift event may show different responses.

The diagram below shows how different plant species respond to dicamba and 2,4-D. The diagram can help narrow down potential culprits. For example, dicamba is the more likely culprit if you see injuries on an apple tree. Sensitive plants may serve as indicators. A grower may notice injury on their tomato plants and check for symptoms in plants surrounding their farm to narrow down the potential culprits.

What to do if you suspect drift?

If herbicide drift is suspected, avoid making immediate assumptions or taking irreversible actions. Focus on narrowing down the issue, documenting evidence, and keeping records.

Write down the date you first noticed symptoms and flag affected plants. Take photographs (make sure to set the date when using point and shoot cameras) of the entire plants, individual leaves, field edges, patterns across the planting area, and adjacent vegetation. Take notes of crop growth stage, weather conditions, and other potential stress factors. Have conversations with neighbors to make them aware of issues and determine if they have made herbicide applications.

Growers can benefit from a second opinion from extension services. Many issues may initially appear to be herbicide related but can turn out to be related to diseases, insect pests, nutritional problems, environmental stress, or carryover in compost or hay.

Food safety considerations

There is confusion about the difference between plant injury, pesticide residue, food safety and marketability. A plant can show obvious herbicide injury while containing extremely low pesticide residues. Visible symptoms alone do not determine whether a crop is safe, legal, or marketable.

The U.S. Environmental Protection Agency establishes pesticide tolerances, which are maximum legal residue levels permitted on food commodities. The tolerances can be found in the Code of Federal Regulations 40 CFR Part 180 (§ 180.142 for 2,4-D and § 180.227 for dicamba). Each pesticide-crop combination has its own regulatory framework. If EPA has not set a tolerance or tolerance exception for a specific crop, then any detectable residue in that food is illegal and legally classified as adulterated (a zero-tolerance standard). Zero tolerance exists because safety thresholds for human consumption may not have been determined for that commodity.

When drift is suspected and harvest is approaching, growers should consult extension specialists, Missouri Department of Agriculture (MDA), and organic certifiers (if applicable) before making any marketing decisions. Extension specialists can direct you to laboratories that can test for residues.

Maintaining good relationships with neighbors

In many situations, a respectful conversation can resolve misunderstandings before they become disputes. Most neighboring farmers share similar goals: producing a quality crop, protecting their livelihoods, being good stewards, and maintaining community relationships.

Communication is often most effective when concerns are discussed early and professionally. A conversation may reveal what products were applied, when they were applied, weather conditions during application, or whether another explanation is more likely.

Maintaining positive relationships is particularly important in Missouri's diverse agricultural communities where crop producers, produce growers, orchards, vineyards, and livestock operations often work side by side.

When should you contact the Missouri Department of Agriculture?

If you believe pesticide misuse may have occurred and concerns cannot be resolved informally, the Missouri Department of Agriculture can investigate alleged pesticide misuse incidents. MDA administers pesticide licensing, inspections, and complaint investigations under Missouri pesticide regulations.

Growers should consider contacting MDA when significant injury is observed, evidence suggests a recent pesticide application may be involved, crop losses are likely, documentation has been collected, and questions remain unresolved. Prompt reporting is important because evidence becomes more difficult to collect as time passes.

Final thoughts

Herbicide drift is a real concern for specialty crop growers, but not every distorted plant has been injured by off-target herbicides. Good diagnosis starts with careful observation, documentation, and communication.

The goal should not be to immediately prove drift occurred. The goal should be to accurately determine what happened, protect the crop when possible, preserve relationships when practical, and ensure that decisions are based on evidence rather than assumptions. With a systemic approach, growers can make better decisions and avoid unnecessary conflicts while protecting both their crops and their businesses.