Marshalling troops to defend against soilborne disease

Dr Graham Stirling with Eastwind Fruits manager David Giumelli

Farmers hear a lot about the bad things that live in our soils. But there are plenty of good things too. These beneficial organisms are battling against pests for supremacy of our soils.

Have you ever heard of nematode-trapping fungi? Or big predatory nematodes that feed on smaller pest nematodes? Or megastigmatid mites that can eat more than 20 nematodes a day?

We usually only hear about the bad nematodes and fungi, but there is a whole army of bugs in the soil, including good nematodes and fungi, that are beneficial to plants. Beneficial soil organisms compete with pests for space around our plant roots, so It’s a battle you want the good guys to win.

Somebody who perhaps understands this battle more than most is Queesland nematologist Dr Graham Stirling. Dr Stirling has studied this fascinating microscopic world for the past 46 years and was recently invited by South West Catchments Council (SWCC) to speak with orchardists and potato growers about soil biology.

“It’s a matter of having a picture of the whole system, not just the pests,” Dr Stirling said.

“So even though you’ve got pathogens that try to destroy roots, you’ve also got organisms that will help you keep them

under control.”

One beneficial organism is mycorrhizal fungi. These fungi grow in plant roots and help plants access water and phosphorus when soil levels are below excessive levels. Another critical role is keeping pests away from plant roots.

“There’s certainly scientific evidence which suggests that you’ll get less of those pathogens invading plant tissue when the mycorrhizae are there first,” he said.

Unfortunately, some agricultural practices are harmful to mycorrhizae.

“One of the problems is that we tend to over-use phosphorus in our systems, and the mycorrhizae tends to drop away. Tillage also knocks fungi around.”

Soil biology is dependent on soil carbon. Dr Stirling said organisms can be sustained by carbon inputs from plant residues and root exudates. These inputs fuel the growth of bacteria and fungi, which then become a food source for beneficial nematodes, mites and other soil organisms.

“Plants exude carbon out of their root system to feeds soil organisms,” he said. What they are trying to get in return is a good strong biology around their roots to protect against the pathogens.

“It’s something that has evolved over millions of years. So a lot of the battle to get a healthy biology is to have plants in the system all the time.”

Dr Stirling said a strong soil biological community can also improve water infiltration and soil drainage.

And while most farmers know that some bacteria can fix nitrogen, soil organisms are also crucial to holding and mineralizing nutrients.

South West Catchments Council has produced three videos from Graham’s presentation which can be found at

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