Plant Problem Diagnostic
Herbicide Damage on Tomatoes
The short answer
If your tomato's newest leaves are cupping, curling, and taking on a narrow, twisted, fern-like look, it could be herbicide damage rather than disease. Two common routes are spray drift from a nearby lawn or field, and — increasingly — compost, manure, hay, or grass clippings contaminated with persistent 'aminopyralid'-class herbicides that survive composting and even passage through an animal's gut.
This pattern is easy to misread as a virus, so before you rip out plants, try to rule out the inputs: think about recent nearby spraying and where your compost, manure, or mulch came from. These growth-regulator herbicides affect the youngest growth first. If you suspect contaminated compost or aren't sure, your local Extension office can help you confirm and advise — a simple bioassay with a sensitive plant is one way to test suspect material.
What the damage looks like
Growth-regulator herbicides show up in the new growth: youngest leaves cup and curl, growing tips twist, plants stunt, and foliage can take on a fern-like, ferny appearance.
Because it hits new growth and distorts leaves, it's often confused with a virus. The distinction matters — a virus means removing the plant, while herbicide injury may be an input problem you can trace and avoid next time.
Trace the source, then decide
Spray drift is one path; the other is contaminated organic matter — manure, compost, hay, straw, or grass clippings carrying persistent aminopyralid-type residues that can affect sensitive plants like tomatoes, beans, and peas at very low levels.
Rule out your inputs before acting: where did the compost, manure, or mulch come from, and was anything sprayed nearby? If you're unsure, don't guess — contact your Extension office, which can guide a bioassay test of suspect compost and advise on whether affected plants may outgrow mild exposure.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- Oregon State University Extension — Aminopyralid Residues in Compost — Persistent aminopyralid herbicides survive composting and animal digestion; sensitive plants like tomatoes show cupped/curled, fern-like new growth and stunting at very low residue levels.
- NC State Cooperative Extension — Aminopyralids in Compost — Contaminated compost/manure causes leaf cupping, twisted growing points and fern-like appearance in the youngest leaves; a bioassay can test suspect material.