When The Medicine Feeds The Problem

Do synthetic fertilisers and pesticides exacerbate pest and disease threats? A session at the Oxford Real Farming Conference looked at the science behind the claim

Written by Mike Abram

The use of synthetic nitrogen fertilisers is increasingly under the microscope, driven mostly by its impact on the environment and as a major contributor to greenhouse gas emissions on arable farms. But presentations at the Oxford Real Farming Conference also pointed how its use, and that of pesticides, might be exacerbating pest and disease problems by enhancing the nutritional quality of crops for those pests and pathogens. Nitrogen is the nutrient required in the highest quantities for plants, and was vital for crop growth and development, explained Daisy Martinez, a researcher at the University of Edinburgh.

“Plants take up nitrogen from the soil, and use it to synthesise amino acids – small, molecular building blocks – and then use it to build proteins. Proteins are the stuff of life alongside carbohydrates.”

But when the crop was flooded with too much soluble nitrogen, in the form of synthetic fertiliser, the concentration of amino acids expanded faster than the plant could make proteins, she said.

“So the plant is rich in amino acids, which are valuable nutrient sources for pests and pathogens. It’s like saying welcome to the banquet.”

A literature review of scientific papers found some insect pests laid more eggs on crops fertilised with high nitrogen rates, larvae developed more rapidly to a larger size and were more likely to survive and reproduce, she said.

“In the literature we found many examples of significantly denser populations of insect pests fertilised with high rates of nitrogen fertiliser compared with lower rates.”

Similar was true of fungal and bacterial pathogens, where crop disease severity measured by things like microbial colony-density, disease lesion area or spore production, increased where nitrogen was used at high rates, she said.

“These findings support the idea that intensive fertilisation with synthetic freely available nitrogen feed the problem with pest or pathogen damage by enriching the quality of our crops through the availability of nutrition.”

These findings supported the conclusions of a relatively littleknown French agronomist Francis Chaboussou (1908 – 1985).

But there was more to the story as just as humans have defences against diseases, so do plants, she said. These ranged from physical defences, such as waxy protected surface on leaves or strengthened cell walls, to producing defensive chemicals.

“Some of these chemicals are toxic and kill the pest or pathogen outright, while others make the crop less palatable, so the crop is not worth eating or impossible for the nutrients to be absorbed by the pathogen or pest.”

The chemicals could be split into two broad groups – nitrogen-containing or carbon-based compounds, she said. “The natural production of these chemicals can be enhanced or suppressed in response to nitrogen fertilisation.

“High nitrogen availability typically decreased production of the carbonbased defences, while stimulating the production of nitrogen-based defences. “But whether that is good or bad for the crop depends on the context, and on the crop. There are many of these compounds, and particular chemicals can be more or less crucial in different crops. “And it gets even more complicated as the pests or pathogens can be more or less sensitive to these chemicals as well so you might get an insect that is really deterred by one chemical but isn’t sensitive to another. It’s just a really complicated area,” she said. “But the main message is that nitrogen fertilisation doesn’t only just affect the crop’s nutritional quality, and therefore their susceptibility to pests and pathogens, it also affects their defences. “It is not possible to say more N is good or bad for defence, it depends on context, but it is an area that needs more research,” she concluded.

Do pesticides also enhance the nutritional quality of crops for pests and pathogens?

The research team also looked at whether applying pesticides also enhanced the nutritional quality of crops for their pests and pathogens. “This is a little paradoxical – pesticides are applied to crops to suppress, deter or kill pests or pathogens, and should be minimally harmful to crops in the process,” Daisy said.

The study’s hypothesis was that pesticides counterintuitively by their impact on the crop’s internal biochemistry might actually benefit pests and pathogens by improving their diet.

“We looked at several studies which showed applying pesticides resulted in several significant biochemical changes in the crop’s tissues. These changes are akin to others that are commonly seen in plants under stressful conditions. “The crop is able to cope with the stress – it doesn’t keel over, but the outcome of these metabolic shifts is the accumulation of amino acids, especially nitrogen rich ones. “And these are valuable, nitrogen-rich nutrition for pests and pathogens,” she explained.

“While these studies were not designed to test the effect of the biochemical changes on crop susceptibility to pests and pathogens, our tentative hypothesis is they might make use of this pesticide-stressed biochemical state in the crop for their own benefit.”