Scientists from Purdue University in Arizona have suggested that ‘superweeds,’ weeds which have developed resistance to one or more common pesticides, believed to be a side effect of cultivating herbicide tolerant genetically modified crops, may have developed from naturally stronger individual plants, more resistant to other diseases.

Whereas most laboratory tests to understand glyphosate resistance are conducted in sterile soil, void of soil microbes, the Purdue team theorised that these microorganisms might play a role in glyphosate resistance.  

The team grew giant ragweed, horseweed and common lambsquarter in both sterile soil and field soil and subjected them to glyphosate. In each soil, strains of weeds both susceptible and resistant to glyphosate were tested. Jessica Schafer, a botanist and plant pathologist explained the scientists’ hypothesis, "The soil you're growing the plants in is important to the results; if we're growing in a sterile media, we could get some false positive results because the plants are more tolerant to glyphosate in those conditions."

Both versions of giant ragweed were damaged more from the glyphosate in field soil. The susceptible version of common lambsquarter was also more heavily damaged in field soil. Horseweed fared the same no matter which soil or strain - susceptible or resistant. The results revealed that microbes can play an important role in the activity of glyphosate, presumably by invading the glyphosate-weakened plants. The results also suggest that glyphosate-resistant weeds may be more resistant to disease pressure as well.

Steve Hallett, a professor of weed science at Purdue commented, "Soil microbes can be major contributors to how glyphosate is able to affect plants. We may be selecting not only for glyphosate resistance, but inadvertently selecting for weeds that have disease resistance as well."

Bill Johnson, also a professor of weed science, said, "Dirt is a living organism. It's important to know how all the pieces interact."

During testing, the ‘ability to withstand glyphosate’ was considered to be a plant’s dry shoot and root weight after testing. The sterile soil used in the study came from field soil exposed to gamma radiation to kill microbes and bacteria. The irradiated soil was tested to ensure that its nutrients were not diminished. The team said further research will be carried out into how fungi in the soil affect root development, both with and without glyphosate.

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