An imbalance in nitrogen and manganese levels within corn can cause a stalk to feed on the nutrients in its own cell walls, inviting such diseases as Gibberella stalk rot. The pinkish area on the stalk is the Gibberella fungus. Purdue plant pathologist Don Huber says acres of crops fail to reach yield potential every year because they can't draw sufficient micronutrients from the soil to fend off diseases. (Photo by Don Huber)
Glyphosate can increase plant disease
In our previous issue we looked at evidence that over-reliance on glyphosate can increase nutrient deficiencies. In this article we consider evidence that it can also increase plant susceptibility to diseases.
Dr Don Huber, professor emeritus at Purdue University, USA, reports (Huber 2010) that glyphosate stimulates numerous plant pathogens, including “ubiquitous bacterial and fungal root, crown, and stalk rotting fungi; vascular colonizing organisms that disrupt nutrient transport to cause wilt and die-back; and root nibblers that impair access or uptake of soil nutrients.” He lists over 25 organisms, including those responsible for take-all and crown rot of cereals (Gaeumannomyces graminis), rice blast (Magnaporthe grisea) and various Fusarium diseases.
How glyphosate increases susceptibility to disease
Glyphosate has at least two effects in increasing plant susceptibility to disease. Its normal mode of action as a herbicide is to block a biochemical pathway in plants call the shikimic acid pathway. This pathway has several roles, including the synthesis of three crucial amino acids, without which the plant dies. The shikimic acid pathway is also a critical supplier of most plant defence chemicals and actions. Hence, glyphosate blocks a plant’s “immune system”, leaving it susceptible to soil-dwelling pathogens.
The second effect is to tie up micronutrients in the soil, making them unavailable to plants, which use them as components in the enzymes that fight disease organisms.
Consequences for Roundup Ready crops too
Because glyphosate blocks much of the plant’s immune system, this has possible implications even for Round Ready crops. And because glyphosate can remain in soil for many months (chemically bound to soil particles, from where it can be re-released by phosphate fertiliser), all plants are placed at risk of either direct glyphosate toxicity or (indirect) disease attack.
Possible remedies
Huber (Johal and Huber 2009) recommends two simple strategies that could be used to protect crops from glyphosate-induced disease:
- Reduce the rate of glyphosate application – trials show that much lower rates than recommended are just as effective at killing weeds.
- Apply critical micronutrients 2 weeks before glyphosate is sprayed.
Further strategies include developing new Roundup Ready genes that detoxify glyphosate in the plant, new soil microorganisms that break down glyphosate in the soil, and the development of tolerant rhizobia to inoculate legumes.
Further reading
Huber DM. 2010. Ag chemical and crop nutrient interactions – current update. Botany and Plant Pathology, Purdue University, Indiana, USA.
Johal GR, Huber DM. 2009. Glyphosate effects on diseases of plants. Eur J Agron 31: 144–152.