No. 15/July 3, 1998
Plant! Emerge! Grow! Pucker??
We have already received several calls about soybean plants that are demonstrating strange injury symptoms, usually described as "puckering." With the conditions of the 1998 growing season, it's not surprising that soybean plants are currently demonstrating this condition. The article that follows was written for the Proceedings of the 1998 Illinois Agricultural Pesticides Conference and is reproduced here to try and shed some light on the occurrence of puckered soybeans.
Soybean plants demonstrating injury symptoms commonly described as puckering were somewhat common and apparently widely distributed during the 1997 growing season. The 1997 season was not the first time the problem was observed, but it may have been as widespread during 1997 as any in recent memory. This phenomenon was not restricted to soybean fields in Illinois, as weed scientists in Iowa and Indiana also reported instances of puckered soybeans. The symptoms that were frequently reported include
The most difficult issue with respect to puckered soybeans is identifying the cause or causal agent. Several theories have been put forward by weed scientists across several states in the north-central region and are presented here: It is very unlikely that only one of these possibilities will explain the cause of puckered soybeans in all instances.
- extreme cupping of trifoliolate leaves, usually most pronounced on the upper trifoliolates
- veins of affected leaves tending to assume a parallel orientation instead of the usual netlike venation pattern
- tips of cupped leaves with parallel veins often brown
- plants stunted, as compared to plants not demonstrating the aforementioned symptoms; these plants may remain stunted the remainder of the season, but this does not always happen.
1. The plants were exposed to a growth-regulator herbicide.
The growth-regulator herbicides tend to mimic the effects of endogenous plant hormones, in particular, auxins. Plant hormones control many growth and development processes. These hormones are physiologically active within the plant at extremely low concentrations (parts per million or billion); exposing a soybean plant to a synthetic hormone (a plant-growthregulator herbicide) can induce a wide range of responses within the plant, from slight morphological modifications (leaf abnormalities, for example) to plant death. The degree or severity of response partly depends upon the concentration of herbicide the plant was exposed to, as well as environmental conditions and crop variety. The literature has manyreferences to research on the response of various crops to exposure of sub-lethal amounts of various growth-regulator herbicides. Most of these studies were conducted more than 20 years ago, but the symptoms of exposure these studies describe were very similar to those encountered during the 1997 growing season.
How much (what concentration) growth regulator does it take to induce symptoms? Dicotyledonous plants vary in their sensitivity to growth-regulator herbicides. In field demonstration studies, leaf cupping/distortion was observed in soybean plants exposed to Banvel at 1/10,000 of the field-use rate. The stage of plant growth at which exposure occurs also can influence the amount of injury induced. Several studies report that soybeans were more tolerant to exposure to growth regulators when in early vegetative development, as compared to larger plants nearing the reproductive stage.
The herbicide most often discussed or implicated in the cupping response of soybeans is dicamba (Banvel, Clarity, Marksman, etc.). How would the plants be exposed to this corn herbicide? Three possible avenues of exposure are discussed next.
1a. Residues remaining in/on the spray equipment from previous applications in corn fields are detached and applied with the soybean herbicide at low concentrations.
Labels of products containing dicamba provide techniques for cleaning application equipment to remove residues. The procedure from the label of Banvel states to:
(1) Hose down thoroughly the inside as well as outside surfaces of equipment while filling the spray tank half full of water. Flush by operating the sprayer until the system is purged of the rinse water.
(2) Fill tank with water while adding one quart of household ammonia for every 25 gallons of water. Operate the pump to circulate the ammonia solution through the sprayer system for 15 to 20 minutes and discharge a small amount of the ammonia solution through the boom and nozzles. Let the solution stand for several hours, preferably overnight.
(3) Flush the solution out of the spray tank through the boom.
(4) Remove the nozzles and screens and flush the system with two full tanks of water.
If these cleaning procedures are not followed exactly, how much residue would remain in the application equipment and would it be enough to cause injury to soybeans? Many producers and applicators who reported puckered soybeans in 1997 indicated that the symptoms appeared to follow the spray equipment "to the row." Drift (discussed next) generally does not stop at a selected row in a field. Rather, there is often the feathering effect: Symptoms are most severe on the side of the field closest to the source of drift and lessen with increasing distance. Unfortunately, failure to clean the application equipment thoroughly does not appear to explain the reported cases of "...the soybeans sprayed with the first load puckered, those sprayed with the second and third loads are fine, but the ones sprayed with the fourth load puckered" when all other factors are held relatively constant.
1b. Herbicide vapors on the plant or soil surface move out of the treated area and are absorbed by soybeans (vapor drift).
The volatility of a herbicide is a function of several factors: those related to the formulation of the herbicide and those related to prevailing environmental conditions. Vapor pressure is a measure of the tendency of a herbicide to volatilize. As the vapor pressure of a herbicide increases, the potential for volatility also increases. Ester formulations of 2,4-D are generally much more volatile than amine formulations. Banvel is formulated as the dimethylamine salt of dicamba, Clarity as the diglycolamine salt, and Marksman as the potassium salt. Each of these salt formulations differs in its potential to volatilize. With respect to environmental conditions, volatility tends to increase as soil moisture and temperature increase. As soil moisture decreases, the amount of herbicide adsorbed to soil particles can increase and thus reduce the amount of herbicide available to volatilize.
1c. Physical drift of spray particles during the actual application process.
This cause of exposure may be the easiest to identify based on field observations. The labels of many postemergence herbicides have statements regarding windspeed and drift. Most specify that applications should not be made when windspeed is in excess of X miles per hour or the wind is moving toward a sensitive crop.
2. The soybean plant is expressing a physiological response to somewhat adverse growing conditions.
This theory generally attempts to exclude exposure to a growth-regulating herbicide in the explanation. Rather, soybeans express puckered symptoms due to environmental factors that have an adverse impact on their growth. Very few components in the puckered-soybean "equation" have held consistent over the past several years, except that most cases are not noticed or reported until after the first few days when air temperatures exceed 90 degrees. Soybeans may be entering into a phase of very rapid growth and development, and some speculate that this phase may disrupt the hormonal balance within the plant. This theory exists because there have been cases of puckered soybeans that were not sprayed with any postemergence herbicide and no corn fields were nearby.
3. The response is induced by a postemergence herbicide application.
The majority of samples received at the Plant Clinic demonstrating puckering were previously treated with a postemergence herbicide, usually a translocated herbicide such as Pursuit, Classic, Pinnacle, Concert, or Roundup Ultra. Many of these applications include spray additives such as oil concentrates (petroleum or vegetable base) and an ammonium nitrogen fertilizer (28% UAN or ammonium sulfate). How can these applications induce puckering? Some explanations include:
3a. Translocated herbicides move into the apical meristem, the location of hormonal control, and disrupt the hormone balance of the plant. Following the disruption of hormonal balance, the plant exhibits some response like leaf cupping or puckering.
3b. The spray additives are able to remove dicamba residues from the spray equipment (see 1a above).
3c. If 28% UAN was used, the level of biuret may be high enough to induce the response.
So what exactly causes puckered soybeans? In short, no data definitively explain every case of puckered soybeans. It is unlikely that one "blanket" explanation exists: Each case may be unique. Data exist that describe the response of soybeans to exposure to growth-regulator herbicides, but other factors may also be at work.
Aaron Hager (firstname.lastname@example.org) and Marshal McGlamery (mmcglame@ uiuc.edu), Crop Sciences, (217)333-4424