Is Soybean Leaf Cupping a Roundup Ready Problem?

Soybean leaf puckering, cupping, and wrinkling was found in many fields this season. These are characteristic symptoms of growth regulator herbicide damage. Leaves were malformed and in severe cases the tips of the leaves turned yellow or brown. In some cases, this puckering developed following the application of Roundup on RR (Roundup Ready) soybeans. Some producers have asked, "Is this a Roundup Ready soybean problem?"

In severe cases, hormone herbicide injury is most often to blame. Leaf cupping on soybeans has been observed with dicamba rates as low as 1/10 000 of field use rates. Dicamba spray drift, volatilization, or sprayer contamination can be discerned from sprayer patterns, overlap, or damage proximity to corn fields. Soybeans are somewhat more tolerant to dicamba during the early vegetative stages than in later development. As they mature and approach the reproductive stage they become more sensitive to growth regulator herbicides. Since RR soybeans are frequently sprayed later in the season, sprayer contamination may be more problematic in these later sprayed fields. The surfactant in Roundup is an excellent spray tank cleaner and may pull out small amounts of residual chemical remaining in the spray equipment. A simple water rinse, even three times, before applying Roundup to RR soybeans does not appear to be enough to properly clean out residual herbicides in the spray tank.

What about leaf cupping that develops when herbicide contamination is ruled out? For example, when the field has not been sprayed with a postemergent herbicide and no corn fields are nearby? Or those cases when cupping does not develop until 4 or 5 weeks after herbicide application? Definitive answers become more elusive. Leaf cupping may be a physiological response to accumulated stress factors. Under multiple stress conditions, the natural hormonal balance within the plant is disrupted. One or two stresses may not be enough to produce symptoms, but when many stresses accumulate puckering occurs. Moisture stress, high temperatures, poor soils, herbicide application, sprayer contamination or cold stress may interact to produce a level of overall physiological stress to induce leaf malformation. Cupping may be observed after the application of a postemergent herbicide. An additional stress to already stressed plants, and enough to induce cupping.

In greenhouse trials extremely low amounts of dicamba have been applied to RR soybeans to create physiological stress. A relatively small amount of cupping was observed. When Roundup Ultra was applied between 1 and 3 days later significantly more cupping developed compared to the dicamba alone. But the same symptoms developed when Classic, Basagran, or Fusilade were applied, instead of Roundup Ultra, after the initial dicamba application. Roundup is not the only herbicide that will induce these symptoms. And this problem is not isolated to Roundup Ready varieties. Leaf cupping has been observed in both conventional and RR varieties for a number of years.

There is no easy answer to explain every case of leaf puckering. Each field is unique, with its own set of problems. In those cases of leaf malformation when sprayer contamination has been ruled out, plants usually outgrow the problem and no yield loss is expected. However, upon close examination most severe puckering cases can be traced back to dicamba. The soybean variety, the dose of dicamba, and the time of exposure all play a role in potential yield loss. Light to moderate dicamba injury does not generally reduce yield significantly. Ultimately, the best thing producers can do to avoid this problem is to take extra care to clean out spray equipment before applying Roundup, or other post-emerge herbicides.

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