July 28, 2003
By Bob Hartzler
Source: Iowa State University Department of Agronomy
July 28, 2003 - I received the first report of cupped leaves on soybean last week. This is an annual occurrence in Iowa and surrounding states, and in spite of its prevalence we still do not fully understand all of the factors that may lead to this response. It is well known that growth regulator herbicides (dicamba; 2,4-D; clopyralid; etc.) can induce this response when they come in contact with soybean and other susceptible plants. 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 corresponded with the increase in postemergence applications to soybean in the mid-80's. Sprayer contamination with dicamba is probably the leading cause of this problem. Research has shown that 1/10,000 of the label rate of dicamba can induce this response. Due to the extremely low level of herbicide required to induce leaf cupping, residues capable of causing injury may reside in the sprayer even after thorough cleaning or after many loads of other herbicides have been run through the sprayer.
Although it is likely that growth regulator herbicides are responsible for most instances of injury, I am convinced that 'growth regulator-type symptoms' can develop in the absence of growth regulator herbicides. I believe that certain factors (environment, stress from herbicide applications) may disrupt the normal balance of hormones in the soybean and result in the type of abnormal growth normally associated with growth regulator herbicides. Although there are no studies to support this assumption, I base this conclusion on the increase in occurrence of the problem during the 1990's and the occurrence of the symptoms in fields not sprayed with postemergence fields.
The symptoms most commonly occur in fields within a week or two of the field being sprayed with postemergence herbicides. Due to the low amount of herbicide required to induce a response, it is difficult to completely rule out the possibility of a contaminated sprayer. However, I have seen enough situations where a sprayer dedicated to spraying soybean was used, and it seems unlikely in these situations that dicamba was present. Soybean also may develop cupped leaves in the absence of herbicide applications. This most commonly occurs during conditions of rapid growth. In these situations it is possible that dicamba contacted the soybean due to drift or volatilization. However, in many of these situations the symptoms develop long after the majority of corn fields have been treated, and this greatly reduces the likelihood that dicamba would move into the fields. When this situation develops, the entire field often demonstrates symptoms and there will not be any indication of a "drift" pattern.
Adding to the confusion is that this response was not observed 10 to 15 years ago. Garren Benson, former extension corn and soybean specialist, has stated that he doesn't recall soybeans developing cupped leaves in the absence of herbicide drift or other sources prior to the mid 1980's. Due to the high pH soils found in much of Iowa, dicamba has been a leading herbicide used in corn for over 25 years, so an increase in dicamba use is not likely the cause of the malformed leaves. A possible explanation is that current genetics found in soybean are prone to triggering this response. I like to say that today's higher yielding varieties are more temperamental than older varieties, and this results in their hormones getting out of balance and creating a growth regulator response. However, there is no data to support this theory.
When dicamba is 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.