Iowa weed scientist wades into glyphosate controversy at Ag Days

Is glyphosate the greatest thing since sliced bread or an agronomic catastrophe waiting to happen?

There’s not solid evidence for the latter, but there is some cause for concern, according to a presentation by agronomy expert Bob Hartzler at Ag Days.

“Based on my experience in Iowa, where we have 22 million acres of Roundup Ready corn and soybeans, we have not seen any problems that we can correlate with the widespread use of glyphosate and plant disease and micronutrients,” said the extension weed specialist and professor of agronomy from Iowa State University.

“That doesn’t mean it doesn’t cause problems. We’re at near-crisis levels in terms of glyphosate-resistant weeds.”

Looming over his presentation were public statements made by “that person” – Don Huber, an emeritus professor of plant pathology from Purdue University – who Hartzler described as the “flag-bearer” for those portraying glyphosate as “a bad thing.”

A case “could be made” for the retired professor’s concerns about glyphosate’s role in new and emergent crop diseases such as fusarium and Goss’s wilt, said Hartzler, who knew Huber briefly during his undergraduate days.

“It’s well documented that plants that are susceptible to glyphosate — when exposed to a sublethal dose — that increases the risk of disease developing in that plant,” said Hartzler.

However, the insertion of a foreign gene into Roundup Ready crops such as corn and soybeans keeps a critical pathway of immunity open for such plants, thus preventing disease, he added.

Hartzler also addressed Huber’s claim that the strong chelating effect of glyphosate immobilizes micronutrients such as manganese in plant tissues leading to more disease outbreaks.

“Again, there has been a lot of research conducted and none of it has shown that glyphosate ties up micronutrients in plants,” he said.

One area that may have “greater credibility,” noted Hartzler, is the “well-documented” suspicion that the herbicide’s tendency to be translocated through the plant to the root zone may be influencing pathogen activity in the rhizosphere.

University of Missouri microbiologist Robert Kremer’s research has “clearly documented” that spraying Roundup Ready soybeans with glyphosate causes increased colonization of the roots by fusarium species, he said.

“Glyphosate does increase the number of disease organisms growing on the roots, but what Dr. Kremer has never shown is increased disease incidence,” said Hartzler, adding that outbreaks of disease require “unique conditions.”

He also pointed to long-term plot research that seems to show the real culprit in sudden death syndrome outbreaks may, in fact, be genetic weaknesses in the source hybrids used to create Roundup Ready crop varieties, not the genetic modification itself.

“You could look at this data and say glyphosate is actually protecting us from disease,” he said.

A study of fusarium in barley conducted in Saskatchewan and Quebec found production of fusarium inoculum and mycotoxins in the grain seemed to be more related to the susceptibility of the cultivar grown than the application of glyphosate, he said.

In response to a question about glyphosate’s reported tendency to become resurrected in soil after an application of phosphorus fertilizer, Hartzler said all the studies of this phenomenon were done under lab conditions, not field conditions.

Hartzler, who said he’s considered a “tree-hugger type” by his peers, admitted that as “just a weed scientist” he often gets “really nervous” when discussing plant pathology in the public sphere. He said he agreed to address the glyphosate controversy after being approached by officials at the University of Iowa who were concerned about the effect Huber’s sensational allegations might have on exports from the Midwest Corn-Soy Belt.

“They talked to the plant pathologists and they said they didn’t want to touch it with a ten-foot pole,” said Hartzler. “The majority of plant pathologists say there’s nothing there.”

Still, audiences are often hostile to his defence of the technology, he said, adding he’s been asked “what he will tell his children when Huber is eventually proven right.”

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