Understanding and preventing herbicide injury

Overview

Not all plant injuries are caused by a pathogen like bacteria or fungi; some symptoms are caused by abiotic (non-living or environmental) factors, including herbicides. Herbicide damage can be challenging to diagnose because many of the symptoms can look like those caused by biotic factors. Symptoms are varied and depend on the type and amount of herbicide. Location of symptoms on the plant depends on the source of the herbicide.

Symptoms

Herbicide chemical class	WSSA Group	Potential symptoms of injury
Growth regulator	4	Epinasty*, tissue deformation, broadleaf plants affected
Pigment inhibitor	13, 27	Chlorosis**, white, bleached appearance
ALS inhibitor	2	Symptoms appear on new growth, chlorosis**, red veins, distorted leaves
PPO inhibitor	14	Necrotic*** lesions on leaves contacted by herbicide, broadleaves more sensitive than grasses
Glyphosate (EPSPS inhibitor)	9	Symptoms appear on foliage that develops following exposure, chlorosis**, distorted leaves
ACCase inhibitor	1	necrotic (dead) growing point within the stem of grasses, chlorosis**

*Epinasty: bending or twisting of plant parts downward and outward (https://www.merriam-webster.com/dictionary/epinasty)

**Yellowing

***Dark brown or black, dead plant tissue

Herbicide damage caused by a contact herbicide may have distinct droplet patterns. Herbicide injury symptoms may develop following application of herbicides either in your yard and garden or in surrounding areas. Depending on the herbicide, sensitivity of the plant, and environment, symptoms may develop within a few days, to more than a week after the application.

It is also possible for herbicide residues to remain in the soil, plant material, manure, and compost for up to a year or longer. There are significant differences among herbicides in how long they will remain active in soil and other materials. Herbicide carryover in the soil may result in weak emergence and seedling establishment in the area where the herbicide was applied the previous season. Herbicide residue in mulches or compost made from grass clippings, yard waste, and wood chips can cause injury symptoms following application of the mulch or compost product.

Various species showing symptoms of chemical exposure in the landscape (Euonymous-Left, Locust- Right)

Suspected herbicide damage on Amur Maple

Management

If you are applying pesticides, it is important to pay close attention to the instruction on the label, wind direction and wind speed. Other considerations include temperature, personal protective equipment (PPE), re-entry interval (REI), and pre-harvest interval (PHI). Equipment used for mixing and spraying must be cleaned thoroughly after the application of a chemical to prevent contamination. More information on reading herbicide labels can be found at https://extension.psu.edu/what-you-need-to-know-about-reading-a-pesticide-label. The Environmental Protection Agency (EPA) is also a good resource for pesticide information https://www.epa.gov/pesticides.

Carryover in the soil can be avoided by reading the label for information regarding the following crop and applying the appropriate labeled rate.

Maintaining plant health: planting location, planting depth, watering practices, nutrients

If plants are damaged by herbicide, they may or may not recover, depending on the severity of the damage. Preventing other stresses from harming the plant is the best way to minimize herbicide injury. Plant care includes watering during hot and dry periods, fertilizing if the soil is deficient, and scouting for other pests such as insects and disease.

Scouting

Symptoms will appear following the application of a chemical or addition of contaminated mulch or manure. Be aware of spraying occurring in surrounding fields, yards, and gardens, and watch your plants for symptoms of damage in the following days. Also watch for symptoms after you apply chemicals, manure, or mulch to your field, yard, or garden.

Prevention

Avoid using amendments if you suspect mulch or manure is or could be contaminated with herbicide.

We recommend running an assay early in spring, where you collect some of the compost or some soil from the land previously in a pasture or row crop that will be used to plant sensitive vegetable plants/crops. Take a representative sample of the compost and soil, probe different areas with a soil auger and mix thoroughly. Then transplant healthy tomato seedlings and evaluate the development of the plants for at least four weeks.

Chemical management

There are no chemicals sprays or nutrient application that can reverse herbicide damage.

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