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Creating disease suppressive soils (22 May 2019)

Growers are increasingly changing the way they manage disease, by creating disease-suppressive systems as opposed to looking for silver bullets. It’s about prevention rather than cures.

The Australian cotton industry’s most pre-eminent pathologist, the late Stephen Allen explained that disease is the soil’s way of restoring natural diversity when we impose a monoculture on it. He said when we create a monoculture, the soil reacts by finding ways to take the monoculture out to return diversity to this living microcosm.

Brendon Warnock’s farm in the Namoi Valley, is a living example of Stephen’s words.

It’s no secret that fields in the Namoi are struggling with the effects of Verticillium wilt, a soil-borne disease that has varying degrees of severity, from leaf discolouration to death. The re-emergence and severity of this disease in the past four years, it would be fair to say, has take the cotton industry by surprise and quickly taken a hold in cotton fields. However if we think back to Stephen’s words, and examine Brendon’s situation, it was probably inevitable.

By turning to a rotation system, Brendan has turned the tables on soil health and yield loss.

“The primary disease of concern for us is verticillium wilt,” Brendon told CRDC's Spotlight.

“We used to only grow cotton, so our soils built up a lot of Verticillium inoculum.

“Disease severity increased until poor performing fields were yielding 40 percent less than our highest, dragging our average down.

“Research indicated that one way to combat Vert was through crop rotation so we began growing corn in the summer of 2014-15.

“We now grow a cotton/wheat/corn/faba bean rotation and have seen a good improvement in yields.

“The variation between top and bottom yielding fields in the 2017-18 season was only 15 percent, so our average yield should be more consistent in the future and hopefully not too badly affected if there is a cool wet season favourable to Vert infection.

“We are happy with our progress but do see plenty more gains to be made yet.”

A wide range of diseases inflict an expensive burden on the growers in our industry and Brendon reminds us that “It costs the same to grow a low yielding, disease-affected crop as it does to grow a high yielding disease-free crop”.

“The difference in profit can be many thousands of dollars per hectare and a very large figure for the industry as a whole.

“Diseases are constantly evolving, with their impact spreading and becoming more severe.

“We need a focused, coordinated effort with a ‘continuous improvement’ mindset.

“We are able to combat disease by targeting research towards the pathogen, the host and the environment and great work has already been done with plenty of opportunity to achieve more.”

Brendan says there are still pressing issues such as the spread of diseases. With support from CRDC he attended the 10th Australasian Soilborne Diseases Symposium in Adelaide last year.

“One of the most pressing issues is to determine if Vert is being moved around the industry in planting seed as one researcher at the symposium hypothesised,” he said.

“One of the other challenges is extension, as changing practices in the industry can be difficult.

“Many growers can identify the cost of disease to their business, but may be unsure what path to take to combat the issue.

“I believe the best way to encourage practice change is through hands on experimentation.

“Demonstration sites and master classes could get the ball rolling disseminating new knowledge.

“But,” he says “we also need to provide growers with tools to measure and assess changes in disease burden over time as they implement change”.

And so we come back to the words of Stephen.

“One of the most powerful changes my increased understanding of disease has given me is a different way of thinking about disease,” Brendon said.

“I no longer think of disease as something imposed on us from outside but something we have created for our self through unsustainable practices.

“I think of disease now a message guiding us on a path towards finding ways to work with natural systems rather than against them.

“This mindset helps to make the issue less stressful and indicates that with the right approach diseases can be overcome and that the future looks bright.”

Brendon said the topic of disease suppressive soils, as he is creating on his farm, was discussed at the soils symposium by a visiting US researcher who suggested that there was potential in Verticillium to recombine and form new strains – that biology will find a way around you – “which was a ‘whoa’ moment for many attendees”.

“The symposium discussed the elements of disease suppressive soils, two major factors in this system being the use of fallow and rotation crops.

“It highlighted the importance of understanding how diseases and management interact and looking at a systems perspective not just one disease and one management style of it.”

This article appears in the Autumn 2019 edition of CRDC's Spotlight magazine.