LSU AgCenter's Weekly Message

Distorted Leaves on Vegetables Might Indicate Herbicide Damage

Recently, I’ve seen three cases in which herbicide-contaminated manure or compost apparently caused injury to tomato plants. This is not a new problem, but it’s not often that I run into it with this frequency.

There’s a group of herbicides called synthetic auxins. Auxin is a naturally occurring and very important plant hormone, but, like many substances, this type of chemical is detrimental when plants are exposed in excessively high concentrations. Plants injured by herbicides in this group often exhibit distortion of leaves and other plant parts.

Synthetic auxin herbicides are generally used to selectively kill broadleaf weeds growing among grasses. The relatively common 2,4-D and dicamba herbicides are in this group but are not as long-lasting in soil and other materials as some of the newer chemicals. (We do sometimes see similar damage from 2,4-D or dicamba when they drift onto non-target plants.)

Herbicides containing picloram, clopyralid, aminopyralid, or aminocyclopyrachlor have greater potential to cause damage because of how persistent they are. This longevity is helpful to those who are trying to manage broadleaf weeds, but if appropriate precautions aren’t taken, it can be a problem for people growing vegetables and other broadleaf plants.

There’s a handful of products containing one or more of these four ingredients that are used on other sites, such as non-residential turfgrass, but we most often see problems associated with products, such as Grazon P+D, that are used on pastures or hayfields.

A common scenario through which vegetables are injured goes like this: An herbicide containing picloram, clopyralid, aminopyralid, or aminocyclopyrachlor is used on a hayfield or pasture. Horses or cattle eat the hay or pasture grass, and manure from the animals is then used in a vegetable garden.

Other ways injury can occur include the following: (1) Hay treated with one of these herbicides is used as mulch for a vegetable garden, or aged or composted hay is mixed into the soil as an organic amendment. (2) People plant vegetables on land where one of the herbicides was applied in the past. (I saw damage to vegetables in a field where, I was told, one of these herbicides had been applied approximately five years prior to when vegetables were planted.)

If you’re a vegetable grower, ask people from whom you get manure, hay, or compost containing one of these if they know what herbicides were applied to the hay or pasture grass that the animals ate.

If you have manure or compost and want to check to make sure it’s okay before using it, or if you’re concerned that you’ve somehow contaminated an area where you want to grow vegetables, what can you do? If you know what herbicide was used, you can check its label and see if it tells how long you’ll need to wait before it’s safe to plant certain things. If you don’t know what (if anything) was used, you can do a bioassay. This involves planting some pea or bean seeds and watching to see what happens. Information about how to do this can be found in the North Carolina Cooperative Extension publications “Herbicide Carryover in Hay, Manure, Compost, and Grass Clippings: Caution to Hay Producers, Livestock Owners, Farmers, and Home Gardeners” and “Manage Compost and Soil Contaminated with Broadleaf Herbicides in Residential, School, and Community Gardens.”

Other things can cause vegetable leaves to be misshapen. Certain plant viruses cause this, although many of these also cause some kind of discoloration, too, while the leaves of plants affected by synthetic auxin herbicide contamination tend to remain green. If you’d like help determining the cause of distorted vegetables leaves or another plant problem, you can contact me or the Extension agent in your area.

Let me know if you have questions.

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Dr. Mary Helen Ferguson is an Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at mhferguson@agcenter.lsu.edu or 985-277-1850 (Hammond) or 985-839-7855 (Franklinton).