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What Causes Cupped Leaves Other than Dicamba?

Many theories abound regarding reasons for damage symptoms, but peer-reviewed studies backing them are scant.

Illinois is a state that’s been in the middle of off-target dicamba reports in recent years. This year, there’s been some good news on that front. As of July 16, just 11 dicamba-related complaints had been filed with the Illinois Department of Agriculture (IDOA). The bad news? At the end of July 31, IODA reported 191 dicamba complaints. 
IDOA also indicated there has been an increase in the number of phone calls during which the caller wanted IODA to know of a damage incident but was unwilling to file an official complaint.  Many callers indicated they wanted to have the incident recorded but didn’t want to “get their neighbor” in trouble with a formal complaint. 
Below, Aaron Hager, University of Illinois Extension weeds specialist, gives his take on the off-target dicamba situation in Illinois. 

The widespread occurrence of dicamba symptoms on soybean for the third straight year is by itself troubling, but perhaps even more troubling are the feeble theories being promulgated to explain the cause of the soybean leaf cupping.  I addressed these in past articles, but it appears science continues to be sacrificed for sales.  So, we’ll take some time to work through these once again.

Cupped Soybean Leaf Causes

Last week, several commercial retail applicators shared with me that company representatives were promulgating multiple causes of cupped soybean leaves.  In particular, dry weather stress, Liberty + AMS (ammonium sulfate), and tank contamination are purported to cause soybean leaf cupping.  Furthermore, some postulate multiple causes are involved when entire fields are uniformly cupped.

I would argue dicamba is the only cause of the widespread leaf cupping symptom, but with advancing age and graying hair I hope to be open-minded enough to recognize I might have overlooked other possible causes.  Therefore, I want to address these “multiple causes” with the hope industry representatives can help provide answers or direct me to the published literature where I can find answers to the questions I will pose.

1. Dry Weather Stress

An interesting hypothesis that, if true, likely has been previously researched.  However, I have not been able to locate any peer-reviewed literature that specify or describe the exact (or any, for that matter) environmental conditions that induce the cupping now so prevalent.  

If these conditions exist, one would speculate they could be replicated under controlled conditions to confirm their impact on symptom development. Granted, I have not attempted to complete a thorough review of the agronomy literature, so I hope someone in industry can help answer the following questions:

Please explain how “environmental stress” can induce this widespread leaf cupping?  What occurs to the physiology of the soybean plant that results in leaf cupping under this alleged environmental stress? Furthermore, please explain how this environmental stress can start and stop at a property line? And while you’re at it, please explain how the dicamba-resistance trait somehow protects dicamba-resistant varieties from this strange and apparently selective environmental stress?

2. Ammonium Sulfate (AMS)

I had hoped this one was laid to rest last year, but it appears that it is being tossed around again in 2019.  Last year, I posted two comments about AMS and leaf cupping on my Twitter account but since some folks don’t use Twitter I thought I would share my 2018 comments herein:
“I’ve evaluated soybean plots for 25 years at the University of Illinois and never recall observing AMS causing leaf cupping symptoms similar to those caused by dicamba. But now that’s the reason for cupped Liberty Link (LL) soybean? Wonder why AMS isn’t causing similar symptoms on non-LL soybean?”

The purpose of the second Twitter post was to share the results of a replicated field experiment in which we evaluated soybean leaf cupping following application of different rates of AMS.  

“Speculation abounds that AMS causes leaf cupping in LL soybean. Here are results of a recent UI trial: 43 ounces of Liberty combined with 3 rates of AMS or a “contamination” rate of dicamba. No leaf cupping from any AMS treatment. No leaf cupping in any other UI LL experiments.”

Again, in my fervent attempt to be open-minded, I hope industry can provide answers to these questions:
Millions of Illinois soybean acres have been sprayed with glyphosate + AMS since 1996 without the widespread leaf cupping common since 2017. Why, then, did AMS all of the sudden cause widespread leaf cupping beginning in 2017 and continuing in 2018 and 2019?  Can anyone please provide peer-reviewed data generated over multiple environments that speculate, suggest, or even clearly demonstrate leaf cupping is caused by AMS?

3. Tank Contamination

Yes, tank contamination occurs, and I have seen instances of cupping where contaminated application equipment clearly was responsible. This avenue of exposure was somewhat common in 2017, but less common in 2018 and appears to be even less common in 2019 as applicators gained an enhanced appreciation for sprayer hygiene.  

Ag retailers, to their credit, have invested substantial money and time into segregating mixing systems, tender vehicles, and in many cases installed direct injection systems on their sprayers to keep dicamba isolated from other pesticide mixtures. They have also extensively contacted growers and mapped the traits of soybean in a concerted effort to manage wind, buffers and “do not spray” label requirements to protect sensitive soybean and other sensitive crops and areas.  

If that’s not enough, before applying dicamba in 2019, approximately 3,000 licensed Illinois operators completed the process to become licensed applicators thanks to the new label mandate instituted by U.S. Environmental Protection Agency (EPA) for the revised label.  

It’s logical to conclude that each of these 3,000 applicators operates a spray rig, so is industry suggesting that hundreds of agrichemical facilities and thousands of tender trucks and application equipment in Illinois are contaminated?  Does anyone have physical evidence of this, or is it just more speculation?  If contamination is the cause of even half the instances of soybean leaf cupping, commercial applicators might question the prudence and legal ramifications of applying a product that seemingly cannot be removed from their chemical formulation, transportation and application equipment.

The frequency of entire soybean fields demonstrating uniform cupping symptoms is not suggestive of “multiple causes,” but rather of exposure through volatility of dicamba previously applied.  Multiple university weed science programs have demonstrated volatility of these dicamba formulations can be detected for up to four days after application, yet these results seem to be largely ignored by industry as another possible source of exposure.  Registrants never have referred to these products as non-volatile, but rather with lower volatility than other formulations.  

As many have stated previously, low volatility is not synonymous to no volatility.  However, industry continues to promulgate theories and speculations that have few (if any) data in their support.  I’m curious if industry will be more transparent with their customers about issues of volatility and recognize this as a potential source of exposure.  

Or, perhaps not.

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