April - May 2025 Vol 46 No 2

Strategic cover cropping to control cotton root disease

Growing alternate crops is a long-term campaign that provides an effective defence strategy for the sustainable management of root diseases in cotton. Cover cropping (or nurse cropping) is simply growing those alternate crops, individually or as mixed species, but not taking them to harvest.

Rob Long, Crown Analytical Services, says strategic use of certain cover crops can suppress root disease inoculum to levels that do not inhibit yield.

“We know that 16 bales per hectare is achievable when an irrigated crop has no root disease constraints,” he says. “In irrigated systems, we can monitor the soil inoculum levels for diseases such as Verticillium wilt and black root rot, before and after a cotton crop, and slot in a cover crop that is known to suppress the disease if the inoculum levels are high.”

This defensive strategy has multiple benefits in cotton systems, providing opportunities to manage disease pressure, cycle nutrients, smother weeds, boost stored soil moisture, build organic matter, promote biological diversity and protect the soil from erosion.

Black root rot and verticillium wilt inoculum levels fluctuate in the soil in response to the crops grown. Cotton is a favoured host, so inoculum levels tend to rise during the cotton season.

“Where cotton follows cotton, inoculum levels trend upwards until they reach a level that begins to impact yield,” says Rob. “Therefore, once the (unknown) inoculum threshold is reached, strategies that reduce inoculum levels will increase cotton yield per hectare and per megalitre applied.

“Our current knowledge suggests that growing certain crop species is the most effective strategy to reduce disease inoculum levels. The mechanisms for this effect are not fully understood, but are likely to be a combination of ‘starving’ the inoculum by growing a non-host species, and the allelopathic effect of certain crops.”

The ‘starvation’ effect can be measured when soil is left bare for a short period, and the inoculum levels drop somewhat, but not always sufficiently, to protect the following crop from yield loss due to infection.

In addition to the direct impact on soil pathogens, cover crops also support other aspects of soil health that promote robust and healthy cotton crops.

Rob says that increasing scrutiny of manufactured nitrogen fertiliser use in agriculture adds to the value of including legume crops in multi-species cover crops to boost atmospheric nitrogen fixation. When cover crops are terminated at peak biomass and not harvested for grain, the high biomass inputs to the soil fuel microbial activity and organic carbon storage.

“In recent drought years, growers who planted cover crops saw the value of surface cover for keeping soil moisture closer to the surface and allowing planting opportunities on smaller rainfall events,” says Rob. “Surface cover increases fallow efficiency – the amount of soil moisture stored during a fallow period – through reduced evaporation and increased infiltration.”

The improved infiltration, particularly on hardsetting and sodic soils, and water use efficiency extend to the cotton production phase, potentially increasing yield per megalitre applied or reducing the cotton crop’s irrigation requirement.

Cover cropping provides the continuity of living plant roots needed to maintain AMF (formerly known as VAM) and avoid long-fallow disorder. Maintaining robust AMF populations ensures the crops have improved access to stored soil nutrients such as phosphorus.

“Diversity in crops nurtures interactions between soil microbial populations, with different microbial species flourishing under different crops,” says Rob. “Although there are many things we don’t fully understand about how microbes interact in the soil biota, it seems clear that the more diversity, the better.”

Cover crops also provide a physical barrier to raindrop impact, surface water runoff and wind erosion. There are tangible savings to the farming operation when erosion damage in fields is minimal and there’s less silt to be dragged out of tail drains and retention ponds.

The choice is yours – cover crop options

About 18 crops have been used to varying degrees as cover crops in cotton regions. These major known species fall into three main categories – cereal, legume and brassica – with significant differences between species within each broad group. There are also options for summer and winter cover cropping.

“We know more about the benefits and disadvantages of some and less about others,” says Rob. “Growers can identify potential cover crop candidates to suit a particular field or scenario.

“This might include crafting multi-species mixes to expand the benefits of a single cover cropping season, responding to field conditions, and favouring disease suppression over nutrition or vice versa.”

From a disease management perspective, trials and field analysis supported by the Richard Williams Disease Action Research Initiative have helped fill some knowledge gaps by comparing disease inoculum levels before and after growing various cover crop species and mixtures.

Over time, it is necessary to gather data to fill these knowledge gaps and support more informed decision-making and rotational planning in the future.

Rob says that once more data is available about the impact of the cover crop tactic with the cotton production system and species choice on the following cotton crop yield, growers will be able to be more proactive in managing root disease inoculum.

To sum up 

Rob recommends growers gain an understanding of the current disease baseline in their fields and consider the multiple benefits of cover cropping, including disease management.

“Try out crops and mixes that offer the most significant potential impact, using the least water,” he says. “Measure the results in terms of kg lint/ha, kg lint/ML and $/ha.”

Ongoing soil pathology research may uncover the answer to the biggest unknown – how do cover crops influence root disease inoculum levels and the yield of subsequent
cotton crops?

Reining in verticillium wilt at ‘Ellematta’, Bellata

Phil Christie owns and operates Orlando Farms, growing cotton and grain in the Bellata district in the Gwydir Valley, NSW. Phil has taken a keen interest in disease management over the years and has used crop rotation and cover cropping to improve his irrigated and dryland farming systems.

Cotton is king when it comes to Orlando Farms’ profitability. When Verticillium wilt inoculum levels in a couple of paddocks rose significantly after the 2021 cotton season, Phil was determined to restore equilibrium so that they could achieve the high yield potential for these paddocks.

“Crown Analytical measured inoculum levels in several paddocks across the farm in mid-2022,” says Phil. “In Fields 4 and 5, the disease levels were high at 526 and 770 pgDNA/g soil, respectively. We were concerned about disease risk but decided to plant cotton and try a range of in-season interventions to offset the risk.”

Phil tried three methods to compensate for high inoculum levels:

• Nutrition tactics, including applying excess potassium and testing the effect of high and low rates of N;
• Planting 50/50 conventional and Bollgard varieties as conventional varieties are considered less susceptible to Verticillium; and,
• Adjusting their irrigation management with shorter run times and not irrigating if cool or rainy weather was forecast.

At the end of the 2022 cotton crop, the conventional cotton yielded 9.5 bales per hectare, the Bollgard 606 yielded 11.5 bales per hectare, and Bollgard 606 planted on the pigeon pea refuge area yielded over 16 bales per hectare.

“This demonstrated that the disease pressure was costing us five bales per hectare,” says Phil. “This is more than the cost of two bales per hectare that would be attributed to the late watering and less than what we might have lost if we had applied more water.”

None of the in-season tactics helped reduce the inoculum levels. In fact, the 2022 cotton crop caused inoculum levels to increase by 370 and 105 per cent, respectively.

Inoculum readings of 2480 pgDNA/g soil and 1582 pgDNA/g soil ruled out cotton for the following season. Phil knew he needed to implement a rotational crop strategy to return these fields to cotton production.

Phil left the fields bare over winter in 2023, which reduced the inoculum levels by 67 and 49 per cent, respectively. He then planted cover crops of irrigated corn in field 4, which reduced the inoculum by a further 65 per cent, and partly irrigated sorghum in field 5, which reduced the inoculum by a similar amount. By the time these crops were terminated in March, inoculum levels were down to 285 and 243 pgDNA/g soil, respectively. Phil wanted to reduce the inoculum further, so he planted canola in both fields. Ten weeks after planting the canola, the inoculum had dropped another 34 and 33 per cent, respectively, and at the end of the canola crop, the inoculum levels were down to 131 pgDNA/g soil and 120 pgDNA/g soil.

When inoculum levels are high at planting time, growing cotton can be expected to increase inoculum in the soil (e.g. Fields 4 and 5 in the graph). When initial inoculum levels are low (e.g. Field 2 in the graph), the increase in inoculum is moderate and returns more rapidly to acceptable levels in the soil. 

“Our experience suggests that measuring 100 units or less of Verticillium inoculum is a reliable trigger for us to plant cotton,” says Phil. “If we can avoid escalating inoculum levels, we can expect to produce 15 bales per hectare.”

Phil has noticed that most of the disease suppression effect seems to occur in the first few weeks of growing a suitable cover crop. He plans to test this theory by planting sorghum first and then planting cotton after the sorghum has established. After a few weeks of the cotton and sorghum growing together, he would spray out the sorghum.

Rob says that another grower has tested this concept using white French millet, which reduced inoculum levels by 28 per cent in 12 weeks.

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