Herbicide Carryover: Bioassay

Last year, a grower that I work with was noticing some unusual growth habits and symptoms of his various vegetables. Many of these symptoms look almost like viral diseases of vegetable crops. However, the problems were seen on many different crops and they were widespread - nearly every plant was affected. This is not common of plant diseases caused by viruses.

It was discovered that he was using manure that he received for free from a neighbors horse operation. The manure was determined to be contaminated with the herbicide picloram (trade name Grazon), that is common in pasture management.

Here are some pictures of the damage.

Chlorotic (yellowing) lettuce damaged by herbicide.

Chlorotic (yellowing) peas damaged by herbicide.

Spinach leaf distortion and puckering due to herbicide damage.

Discoloration of tomato leaves caused by herbicide damage.

There is documentation that shows that this active ingredient can remain in the soil, causing problems for three years!

Today I went out and we took some samples for a bioassay to determine if there are still damaging levels of picloram in the soil.

First we walked through the contaminated field and took soil cores every few feet throughout the plot.

Then we mixed the soil all together and put it in small pots.

We planted three bean seeds in each pot.

Of course we included a control. Some soil from a field where manure was not applied.

The pots were set on separate trays and placed in different spots in the greenhouse.

Now, we wait. In three weeks, we will inspect the seedlings. If they do not germinate or the true leaves show symptoms we will know there is still residue of the herbicide present.

For the grower's sake, let's hope that these beans grow perfectly!
I will keep you updated on the progress.

Before you plant your garden this year, please be aware of the possible dangers of using manure. Question your source about herbicide use. To learn more about herbicide carryover, read about. Dr. Jeanine Davis's extension article, Herbicide Carryover in Hay, Manure and Grass Clippings.

**UPDATE MAY 2010**
New Herbicide Carryover publication now available!

Dr. Jeanine Davis just posted an article about this topic on her site, NC Alternative Crops Blog.

**I'd like to thank Amanda and Phillip for their assistance with the diagnosis of this problem last summer. You two are the greatest!**