Malformed soybean leaves
by Bob Hartzler

Updated June 20, 2000 - Calls concerning the appearance of cupped leaves in soybeans have started.  Persons generally are interested in either: a) what caused the response, or b) what impact the abnormal growth will have on yields.  This article will describe what factors may induce this common problem, the second aspect is discussed in an accompaning article.  

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 due to drift or contaminated sprayers. 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.   Undoubtably the introduction of several new package mixes containing dicamba will lead to a few problems of drift or contaminated sprayers by unsuspecting users of these products.   Even though these products contain less dicamba than the stand-alone products (Banvel, Clarity), problems will still arise due to the high sensitivity of soybeans to dicamba.  Thus, the same precautions need to be followed whether applying Banvel/Clarity (0.25 - 0.50 lb dicamba/acre) or Northstar (0.12 lb dicamba/acre).

The number of problems associated with leaf cupping has increased with the increase in postemergence applications in soybean. Roundup Ready soybeans seem to be as likely to develop the response as traditional varieties.  While sprayer contamination with dicamba or 2,4-D often is responsible, it has become apparent that 'growth regulator type symptoms' can develop in the absence of growth regulator herbicides.  Leaf cupping has been observed following applications of all types of herbicides; thus, the response does not appear to be related to the herbicide's specific mode of action. The response may be due to the inert ingredients in the herbicide formulation, the herbicide itself, 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 frequently develops symptoms and there will not be any indication of a "drift" pattern.  Another thing to look for is the presence of growth regulator symptoms on susceptible weeds in the field.   Velvetleaf is very responsive to low levels of dicamba.  If soybeans are exhibiting cupped leaves but velvetleaf in the field appears normal there is a good likelihood that the cupping is an environomental response rather than one caused by a growth regulator herbicide.

Soybean leaf cupping near unaffected velvetleaf

According to Garren Benson, former extension corn and soybean specialist, soybeans did not develop cupped leaves in the absence of a growth regulator herbicide 15 to 20 years ago. Due to the high pH soils found in much of Iowa, dicamba has been a leading herbicide for nearly 20 years, thus an increase in dicamba useage is probably not the cause of the malformed leaves.  A possible explanation is that current genetics found in soybean are prone to this response. I like to say that today's higher yielding varieties are more temperamental than older varities, and this results in their hormones getting out of balance occassionally and creating a growth regulator herbicide-type response.  However, there are no data to support this theory.

When dicamba or some other growth regulator herbicide are not involved, soybean plants typically resume normal growth shortly after the cupped leaves are observed. Frequently two or three leaves will develop symptoms and then normal growth resumes.  We do not believe that soybean yield should be impacted 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.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University