Glyphosate, the chemical underpinning the world's most productive farming systems, may becoming an agent of harm, a visiting US scientist believes.
"Glyphosate has been a very powerful tool for us in weed control, but it's been seriously abused by continued overuse," said veteran American plant pathologist, Dr Don Huber.
"I feel that's one of the main reasons that we're seeing a lot of other factors come to threaten the sustainability of our production."
Dr Huber links glyphosate to the increasing severity of diseases like fusarium and take-all, and the explosion of Goss's wilt of corn and Sudden Death Syndrome (SDS) in soybeans in America's mid-west.
Now retired from his career as a plant pathologist at Purdue University, but retaining the title of Professor Emeritus, Dr Huber is in Australia to air those concerns at the invitation of Owen McCarron, director of the IPM Masterclass series.
If it is allowed to accumulate in the soil, glyphosate doesn't just kill weeds, Dr Huber told Rural Press.
The chemical is a strong chelator, meaning that it can bind positively-charged mineral ions in the soil to its own molecules, making the mineral unavailable to plants. It is known to have an affinity for copper, zinc, manganese and molybedenum, among others.
"Glyphosate can make a number of elements unavailable for the plant to use, so there are many of the physiological functions of the plant that are compromised," Dr Huber said
"In that compromise period that plant becomes very susceptible to diseases, fungal diseases especially."
Glyphosate also affects important soil organisms in different ways, according to Dr Bob Kremer, a microbiologist with the USDA's Agricultural Research Service and adjunct professor at the University of Missouri.
In the soil, where it is carried by spray, rainfall or plant roots, the chemical is an energy source for some microbes - including those responsible for its degradation in the soil - but a killer of others.
Among the organisms that flourish in the presence of glyphosate appear to be certain strains of fusarium, which in European studies were shown to multiply in the presence of the compound, Dr Kremer told Rural Press.
That appears to line up with old Canadian research which found that wheat sown in fields that had been fallowed with glyphosate was more susceptible to fusarium head blight than control wheat plantings.
"(The researchers) hypothesised as the susceptible weeds died, it built up the fusarium populations and then when the wheat was planted later, there was a higher instance of fusarium head blight compared to fields that did not receive glyphosate treatment," Dr Kremer said.
Other organisms are suppressed by glyphosate, including the rhizobium bacteria reponsible for nodulation in legumes and the the algaes that are an important soil glue.
But Dr Kremer said the research needed to clarify these effects isn't being done. When he wants to interpret some of his own observations, he often has to look at research done decades ago.
And yet, he acknowledged, some of these processes, and glyphosate's chelation effect, have the potential to be highly damaging to crop profitability.
Dr Huber became interested in glyphosate when, after a long career in plant pathology, he and his colleagues saw crop diseases that had been adequately managed for decades suddenly burgeon out of control.
Goss's wilt of corn, for instance, was first discovered in the US in 1969, but only in the past few seasons it has emerged as a major pest of the Mid-West corn belt.
Dr Huber believes that genetic modification for glyphosate resistance contributes to disease vulnerability.
"Just the presence of the glyphosate resistance gene reduces the efficiency of the plant for many of the micronutrients - like manganese, iron - up to 30 or as much as 70 per cent, depending on the original variety," he said.
"When glyphosate is applied there will be an additional reduction in uptake and efficiency of micronutrients that are immobilised by the chemistry."
He is calling for "much more prudent use, and certainly much greater research to establish glyphosate's safety".
"There are a lot of indicators that it's not nearly as benign a product as we thought. With the growing residues that we're finding in our soils and crops and feedstocks, there's a very serious concern for the health and safety aspects of the products."
* Dr Huber will be talking in Bendigo, Vic. on September 5 and Corowa, NSW, on September 7. For more information call Oen McCarron on 0419 006 100 or email owen@ipmmasterclass.com
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