Rhizoctonia risk remains high heading into the 2020 growing season, with growers in the low to medium-rainfall zones of the southern region encouraged to conduct PreDicta® B testing to ensure they understand their root disease burden.

Back-to-back dry seasons in many areas, an increased area sown to cereals and a dry spring have combined to increase the risk of Rhizoctonia root rot, which can cause yield losses of between 20 per cent and 50 per cent.

Effects on cereal crops

SARDI senior reseach officer Blake Gontar. Photo: SAGIT

Soil biology and molecular diagnostics leader at the South Australian Research and Development Institute (SARDI), Dr Alan McKay, says Rhizoctonia has affected many cereal crops in low and medium-rainfall areas of the southern region in 2019.

"Proactive disease control is required as yield losses due to soil-borne diseases can be greater than 20 per cent," Dr McKay says.

"If growers are not sure which paddocks are at risk, they can use PreDicta® B testing well ahead of sowing in 2020 to develop and implement plans to manage disease risks in the season ahead."

Management options

SARDI senior research officer Blake Gontar is working with Dr McKay on a number of GRDC investments related to soil-borne diseases.

Mr Gontar says there is no silver bullet when it comes to Rhizoctonia control, but basics such as good summer weed control and crop choice will help reduce the risk and effect of Rhizoctonia next season.

"Where possible, growers should consider growing wheat instead of barley and increase seeding rates to compensate for loss of tillers," he says.

The combination of more paddocks sown to cereals in 2019, due to a tough 2018 growing season, and low spring rainfall means that inoculum levels will be increasing.

"Wheat is more tolerant of Rhizoctonia than barley, with yield losses in wheat typically two-thirds of those in barley.

"Other options include ensuring adequate nutrition at seeding to encourage vigorous early root growth and, if the break is early, consider grass-free break crops or pastures to reduce Rhizoctonia inoculum for 2021 crops.

"The combination of more paddocks sown to cereals in 2019, due to a tough 2018 growing season, and low spring rainfall means that inoculum levels will be increasing.

"Unless there is substantial summer rain, Rhizoctonia will be worse next year."

Changing nature

Earlier this year, GRDC commissioned SARDI to run a series of six grower workshops across the southern region - in conjunction with AgCommunicators, Birchip Cropping Group, Mallee Sustainable Farming and the Eyre Peninsula Agricultural Research Foundation - to showcase Rhizoctonia symptoms in early sown crops and discuss current management options.

Held in-paddock, the workshops provided a summary of new knowledge on Rhizoctonia developed over the past 10 years, including:

• the various symptoms
• identification
• changes to the expression of the disease in modern farming systems
• seasonal factors affecting yield loss
• sowing strategies
• nutrition
• management.

Regional agronomists attended each workshop so participants could gain a local perspective and ideas for ongoing management considering their nutrition and disease management.

Economics

Dr McKay says while progress has been made to reduce the impact of Rhizoctonia, it is still costing the industry significant money in low to medium-rainfall areas with lighter-textured soils.

"While there's still scope for improvements in the management of Rhizoctonia, there are steps growers can take both pre-sowing and within season to reduce the impact," Dr McKay says.

"The first step is to make sure growers know which crops are still being affected, so we are keen to showcase what to look for and discuss current management options to help reduce yield losses."

Traditionally, Rhizoctonia causes crop damage by pruning newly emerged roots - known as spear-tipped roots - which can occur from emergence to crop maturity.

This infection results in water and nutrient stress to the plant, as the roots have been compromised in their ability to translocate both moisture and nutrients. With later sowing, this has typically caused bare patches to show up in emerging crops.

However, with an increasing trend towards earlier sowing, Dr McKay says crop damage from Rhizoctonia is not showing up until later in the season.

"It is possible that Rhizoctonia may cause bare patches in early sown crops if early seminal or primary root growth is restricted by a range of other constraints including compaction layers, herbicide residues, low moisture, low soil temperatures and nutrient deficiencies," Dr McKay says.

"But in early sown crops, bare patches are not the most common symptom of Rhizoctonia.

"When crops are sown early into paddocks with Rhizoctonia present, they usually grow well until mid-July before canopy growth becomes uneven, particularly in barley.

"Essentially, up until this point, crop growth may look healthy, but the key message is this is not a good indicator of root health."

Dr McKay says by late winter or early spring, early sown crops affected by Rhizoctonia may only have short spear tips near the crown as the only remnants of crown roots.

"If spring rainfall is low, the fungus may grow down the soil profile to infect seminal roots," he says.

"This can lead to good above-ground biomass, with disturbingly little root system to support grain fill."

In early sown crops, bare patches are not the most common symptom of Rhizoctonia.

In seasons with adequate spring rainfall and good soil moisture, crops with poor root systems may still finish well.

However, in a tough spring with little soil moisture and roots affected by Rhizoctonia, yield penalties could be significant.

"It is important growers and advisers are aware of what these symptoms look like so they can plan for following crops," Dr McKay says.

"This also highlights the importance of PreDicta® B testing in determining inoculum levels and therefore the level of risk to following crops."

Suppression of disease

Aside from summer weed control, early sowing and good early crop nutrition, there are fungicides available to help with the suppression of Rhizoctonia.

"Our research has shown that at this stage, liquid streaming fungicide above the seed at sowing is the only treatment to provide useful protection of crown roots," Dr McKay says.

However, Mr Gontar adds that this process is expensive and can be complicated, hence low uptake of the technology so far.

"More work is required to investigate ways to optimise liquid streaming, or develop novel application techniques, and improve grower confidence," he says.

Dr McKay says PreDicta® B testing enables growers to identify pathogens such as Rhizoctonia and monitor the effect of changed farming practices and seasons on disease populations and make better informed variety, rotation and paddock management decisions.

More information: Dr Alan McKay, SARDI, 08 8303 9375, alan.mckay@sa.gov.au; Blake Gontar, SARDI, blake.gontar@sa.gov.au, GRDC Tips and Tactics - Rhizoctonia