Sunn hemp trialled in cotton fields to combat reniform nematodes
By Megan Hughes
ABC Rural
Topic:Agricultural Pest Control
Sunn Hemp is being trialled to control reniform nematodes. (Supplied: Department of Primary Industries)
In short:
A cotton-growing region is battling a microscopic soil pest that is decimating cotton crops.
Scientists have found that a new legume — called sunn hemp — may reduce the impact of reniform nematodes.
What’s next?
The trials are ongoing for the next few years to determine its success.
For almost a century, farmers in the Dawson Valley have been successfully growing cotton, but that success has allowed a microscopic pest to flourish.
Reniform nematodes are microscopic parasites that live in the roots of plants, sucking out nutrients before laying their eggs.
And they love cotton.
Queensland Department of Primary Industries (DPI) plant pathologist Linda Smith said reniform nematodes stunted the cotton plant.
“The [nematode breeding] cycle goes on over and over and over through the season,” Dr Smith said.
Reniform nematodes, pictured, look like small leaves growing out of a cotton root. (Supplied: Department of Primary Industries)
Growers facing the pest are urged to plant something different — like sorghum or corn — for up to 18 months to reduce the nematode population.
But the valley is unique, and for growers like Andrew French who planted two summer sorghum crops, that means two seasons without a decent income.
“When you’re running a business, you have to grow what makes the best return,” he said.
“Unfortunately, half-ordinary cotton is a better return than a bumper crop of sorghum or wheat.”
Now, researchers hope they have an answer — one that means cotton fields can take a break, without taking a break.
Linda Smith, Andrew French, and Paul Grundy at a crop trial in Theodore in central Queensland. (ABC Rural: Megan Hughes)
Farmers often plant a break crop in between their main crops to return nutrients to the soil and reduce pest pressure.
But those crops do not always make money and can mean valuable land is unproductive for years.
DPI principal scientist Paul Grundy said the Dawson Valley’s long cotton-growing history had enhanced the problem.
“This area has been a back-to-back cotton-producing area for a very long time,” Dr Grundy said.
“When you have back-to-back cotton for a long time, you just get a build-up of other disease-causing organisms.”
Dr Grundy in a cotton crop. (ABC Southern Queensland: Nathan Morris)
While growers are careful not to spread the pest between farms, for many, taking cotton out for even one season is not a viable option.
So instead, the DPI team is trialling a new crop to break the cycle, one that grows fast and suppresses nematodes without losing a cotton season.
Cotton planted as part of the sunn hemp trial. (ABC Rural: Megan Hughes)
Turning to the sunn
Sunn hemp is a fast-growing legume with bright yellow flowers that can be used to make fibre or feed livestock.
It is widely used in Brazil and other countries as a break crop, but Dr Smith said while research clearly showed planting it suppressed nematodes, no-one had defined how.
“It’s reducing the population or it’s preventing them from at least reproducing,” she said.
A trial plot of sunn hemp. (Supplied: Department of Primary Industries)
Dr Grundy said the major benefit for growers was the plant grew fast, taking just two to three months to fully develop.
“The purpose of this trial is to see how much suppression we can get of this particular nematode species,” he said.
The farming system trial compares plots of land planted with back-to-back cotton to those that have some with sunn hemp planted in between cotton crops.
If it works, growers will be able to plant and harvest it and still have time to plant a profitable cotton crop.
Dr Grundy said the early results were promising but the trial needed another two-and-a-half years to confirm the data and determine its effectiveness.
The microscopic pest is sucking the life out of cotton crops in central Queensland. (ABC News: Hamish Harty)
Diverse solutions for tough season
Alongside the sunn hemp trial, the team was also testing a new cotton variety developed by the CSIRO that was showing signs it resisted reniform nematodes.
Andrew French said even though it was years away from being available to growers, he was excited about its prospects because even if sunn hemp proved successful, adding it to the rotation would still be challenging.
“It’s really hard to get a sacrificial crop in between production crops,” he said.
“Our area is summer crop-based, we grow something every year … so how do you make that fit?
“But whatever the challenge is, it looks like it’s 100 per cent worth it because it’s a fairly dramatic improvement.”
Reniform nematodes live in the soil and feed on plant nutrients by attaching themselves to roots. (Supplied: Department of Primary Industries)
Farms across the region are also battling another root-sucking soil pest called symphyla, and black root rot that also stunts plant growth.
Dr Smith said the combined stresses massively impacted the crop’s success.
“The plant just can’t cope with all of those antagonists at the one time,” she said.
But she hoped the sunn hemp trial would at least remove one enemy, improving the crop’s resilience overall.
“By removing that pressure of the nematode, maybe those other things are not going to have such a significant impact.”
ABC Rural RoundUp newsletter
After years planting cotton, a growing region has built up a soil pest that sucks the life out of crops — but there may be a solution on the horizon. Read More
