Interactions between herbicides and plant diseases aren’t something you read a lot about. However, a persistent rumor that links glyphosate and the mysterious soybean disease called sudden disease syndrome (SDS) begs discovery.
SDS hit many parts of the Midwest hard this past fall. The herbicide rumor also resurfaced with the disease, says Iowa State plant pathologist X.B. Yang. “This issue came up more than a decade ago when Roundup Ready soybeans hit the market,” Yang notes. “Our plot data shows labeled rates of glyphosate had similar levels of SDS as plots that received Pursuit, the popular herbicide prior to the Roundup Ready system.”
However, Yang notes that plots receiving twice the labeled glyphosate rate did exhibit an increased incidence of SDS. He suggests growers who have upped rates to tackle tough weed problems may want to consider this as they manage SDS.
“In 2010, high SDS levels were found in both Roundup Ready and non-Roundup Ready soybean fields,” he adds.
Iowa State University agronomist Robert Hartzler says both positive and negative interactions between herbicides and plant diseases are well documented. Glyphosate is known to predispose many plants to pathogens due to its inhibition of the shikimic acid pathway. Phytoalexins, which are compounds produced by plants to defend against pathogens, are products of this pathway.
Hartzler explains that glyphosate resistant crops crops gain their resistance to glyphosate by insertion of a gene for an insensitive target site (EPSPS). Since glyphosate does not bind to the transgenic enzyme, the shikimic acid pathway functions normally. “The effects of glyphosate on phytoalexin accumulation in plants should be minimal in glyphosate resistant crops,” says Hartzler.
Hartzler says he’s not sure why high rates of glyphosate might result in more incidence of SDS. “Even though glyphosate won’t disrupt the EPSPS enzyme in Roundup Ready plants, the glyphosate is an ‘exotic’ molecule to the plant,” he says. “The plant will utilize resources to metabolize the glyphosate or isolate it from the cytoplasm.
“Anytime you divert resources from the plant’s normal processes, it predisposes the plant to other stresses, including pathogens. At higher rates, it is possible that some of the interactions with micronutrients that have been proposed may come into play.”
Listen in as University of Illinois plant pathologist Carl Bradley discusses the symptoms of SDS: