The impacts of glyphosate (Round-UpR) on soil biology and micronutrients

Glyphosate's herbicidal action stems from its ability to strongly chelate Manganese (Mn), which prevents the plant from using Mn in important metabolic pathways. The glyphosate resistance gene in genetically modified crops therefore reduces not only the ability of the plant to use Mn, but also Mn uptake. It also reduces the plants ability to withstand disease, because without Mn, a plants ability to induce disease resistance and protect itself from pathogens is compromised. In fact, studies have shown that the effectiveness of glyphosate mainly results from the colonization of the roots of treated plants by Fusarium and Pythium, two common soil pathogens. In a sterile media, where no microorganisms were present, glyphosate treated weeds did not die and growth was only temporarily inhibited! Incidentally, one study soybean seedlings were inoculated with anthracnose and one day later treated with a 0.001 ml drop of glyphosate. This tiny dose of glyphosate reduced the plants defenses, by inhibiting Mn use, to the point that the disease was able to kill the plants. None of the plants that went untreated with glyphosate were killed by the disease.

Furthermore, glyphosate is known to be toxic to many microorganisms, and actually selects for microorganisms that are Mn oxidizers. As Mn is only available to plants in its reduced form (Mn+2) and not in its oxidized form (Mn+4), the proliferation of Mn oxidizing microorganisms in glyphosate treated soils exaggerates Mn deficiencies. This effect is cumulative and its impacts increase with frequency of application. The out of balance microbial community, and therefore the Mn deficiencies will also persist for at least a year after the last application. In fact Take-All in wheat, an uncommon disease when crop rotation is used, is more common after Round-Up Ready soybeans because of the inability of the crop to withstand disease pressure, again due to Mn deficiency. (An interesting side note is that oats can help a great deal in ameliorating the effects of glyphosate in the soil. Oats produce compounds in the rhizosphere that encourage Mn reducing microorganisms, helping to re-establish their populations more quickly. This could have important implications for farmers transitioning to organic production.) Many other diseases have been found to increase in cropping systems where glyphosate is used, such as apple canker, bean root rot and damping off, soybean root rot, white mold, sudden death syndrome and cyst nematode, as well as wheat scab and glume blotch.

Citation:
Johal, G.S. and D.M. Huber. Glyphosate effects on diseases of plants. 2009. European Journal of Agronomy 31:144-152.