Malformed soybean leaves

In the past two weeks there have been numerous reports of soybean plants developing symptoms characteristic of damage from growth regulator herbicides. In some cases the symptoms, which include leaf cupping and distorted veins, developed shortly after postemergence herbicides were applied to the field; in other situations there was no apparent reason for the response.

Soybean leaf cupping can be caused by numerous factors.

Leaf cupping in soybean is not well understood. Growth regulator herbicides (such as 2,4-D and dicamba) will trigger this response if these herbicides drift onto soybean, or if the sprayer was contaminated with these materials. The first step in dealing with this situation is to rule out the possibility of the soybean coming in contact with a growth regulator herbicide.

The number of problems associated with leaf cupping has increased with the increase in postemergence applications in soybean. While sprayer contamination with dicamba or 2,4-D sometimes is responsible, it is apparent that the stress associated with the herbicide application may induce this response. Leaf cupping has been observed following applications of all types of herbicides, thus, the response does not appear to be related to the mode of action of the herbicide. The response may be due to the ingredients in the herbicide formulation, the herbicide, or the spray additives used with the herbicide.

Soybean plants also may develop cupped leaves in the absence of herbicide applications. This most commonly occurs during conditions of rapid growth. Apparently under these conditions, the balance of naturally occurring hormones in the plant is disrupted, resulting in symptoms characteristic of growth regulator herbicide damage. When this situation develops, the entire field will likely demonstrate symptoms and there will not be any indication of a "drift" pattern.

When dicamba is not involved, soybean plants typically resume normal growth shortly after the cupped leaves are observed. We do not believe that soybean yield should be impacted significantly under these situations. The potential for a yield response is greater when a growth regulator herbicide is involved, however, it is impossible to determine the extent of yield loss by examining symptoms that develop after the exposure. The only reliable method of determining a yield response is comparing the yield of the injured soybean to an area of the same field that is unaffected by the herbicide. In many situations, a valid comparison is not available to help determine the cost of the herbicide damage.

This article originally appeared on page 131 of the IC-478(16) -- July 7, 1997 issue.

Updated 07/06/1997 - 1:00pm