Season ripe for Rhizoctonia root rot infestation

Southern Australian growers warned to watch for root disease that can reduce crop yields

Diseases
Grain growers across southern Australia are advised to prepare for higher risks of Rhizoctonia in 2020. The disease can result in patches in barley crops, as shown here. PHOTO Alan McKay

Grain growers across southern Australia are advised to prepare for higher risks of Rhizoctonia in 2020. The disease can result in patches in barley crops, as shown here. PHOTO Alan McKay

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Dry spring and summer conditions in some areas could lead to more Rhizoctonia.

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A dry spring in 2019 across much of southern Australia has set the stage for a high risk of Rhizoctonia root rot infection in 2020 crops.

Researchers have warned the impacts of this soil-borne disease could be severe this season, with near-perfect conditions allowing the pathogen to dominate in free-draining, sandy soils.

PREDICTA® B workshops in Adelaide, Horsham, Wagga Wagga and Perth last year considered the effects, prevalence and control strategies of numerous soil-borne diseases, including root lesion nematodes and Crown rot.

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But many advisers and researchers believe it is the severity of Rhizoctonia that will dominate southern Australia's grain growing regions this season.

In fact, Rhizoctonia prevalence in the 2020 season was ranked as the most threatening soil-borne disease by a majority of participants at all workshops.

Once the effects of the pathogen have taken hold, you'll start to see uneven growth around mid-winter. - SARDI soil biology and molecular diagnostics group leader Dr Alan McKay

Principal Scientist and Leader of Soil Biology and Molecular Diagnostics for Primary Industries and Regions SA's (PIRSA) research division the South Australian Research and Development Institute (SARDI), Dr Alan McKay - who ran the national workshops - says while bare patches in cereal crops are a good indicator of Rhizoctonia, patchy crops are not the only symptom to watch out for.

Dr Alan McKay inspects root systems affected by Rhizoctonia. PHOTO GRDC

Dr Alan McKay inspects root systems affected by Rhizoctonia. PHOTO GRDC

Dr McKay says when crops are sown early and there are no constraints to early root growth, the next opportunity for Rhizoctonia to infect plants is when soil temperature drops to about 10°C - usually in mid-winter - and the crown roots in the top layer of soil are most vulnerable.

"Loss of crown roots reduces the plant's ability to support tillers, resulting in fewer and often shorter tillers per plant," he says.

Dr McKay says Rhizoctonia will continue to develop well into the spring, reducing the size of the root system available to fill grain.

"Most growers have a fair idea if they have Rhizoctonia, based on the presence of bare patches," he says.

"If they have patches in the paddock and they are not sure if it is caused by Rhizoctonia, they can confirm it by digging up affected plants and washing the root system to reveal the characteristic spear-tipping on what is left of the root system."

WA Department of Primary Industry and Regional Development (DPIRD) plant pathologist, Dr Daniel Huberli, says the expression of Rhizoctonia was particularly acute last year in WA, SA, north-western Victoria and southern NSW, given the late start to the season in many parts.

"In 2019, we saw increased evidence of Rhizoctonia activity because we had a dry spring in 2018, followed by a hot, dry summer with a late break to the season," he says.

"This increased inoculum production in 2018 and increased inoculum survival on both the residue of the previous season and the organic matter in the soil."

As a result of the late start, Dr Huberli says, the crops germinated later, when the soil temperatures were cold.

"This meant crop root growth was slowed, which allowed the active pathogen to attack the primary roots of the plants," he says.

Dr Huberli says crops were under stress from limited spring rain.

This, combined with a reduced number of tillers from Rhizoctonia infection, meant significant yield penalties in certain 'hot spots' across grain growing regions.

Perfect conditions

Rhizoctonia enjoys free-draining, sandy soils and thrives after a dry spring and dry summer period. The disease finds it difficult to survive in heavy textured soils and in moist soil over summer.

Rhizoctonia levels are the highest following cereals and grass pastures, and levels are highest in repeat cereal crops.

Barley crops are the most vulnerable because they rely more on tillering to yield.

If the conditions that caused Rhizoctonia to be a problem in 2019 are repeated, we can expect it to be the main root disease again in 2020, Dr Huberli says.

Dr McKay says Rhizoctonia is very difficult to eradicate and if the disease was a problem in a paddock 20 or 30 years ago, it will probably still be active in that paddock.

"Changes to crop agronomy in recent years can lead to a crop that looks good early on in the season, but once the effects of the pathogen have taken hold, you'll start to see uneven growth around mid-winter," he says.

"Once you get to this stage, there isn't much you can do about the crop in that season - and you can really only manage the inoculum levels for the coming year."

Making early decisions is the key to managing Rhizoctonia, Dr McKay says, and growers can consider various strategies to get the jump on the disease this season.

"Firstly, and perhaps most importantly, early sown, well-established crops can better withstand a Rhizoctonia infection in the winter months, since roots will be more established and be deeper in the soil profile at this colder time," he says.

"Once the soil temperature drops below 10°C, plant growth rates slow, allowing soil-borne diseases to take hold. So if the plant has a more mature root system, it can grow despite the presence of Rhizoctonia."

If the season breaks late, there is less time to establish the root system before soil temperatures drop.

Fungicide treatments

Rhizoctonia-infected wheat seedlings (left, centre) and a healthy wheat seedling. PHOTO Alan McKay

Rhizoctonia-infected wheat seedlings (left, centre) and a healthy wheat seedling. PHOTO Alan McKay

Fungicide applications at seeding could provide some protection against the Rhizoctonia pathogen.

Dr Huberli says that according to results from GRDC-invested research trials, a split application of fungicide, one on top of the furrow and one in the furrow, applied behind the press wheel at seeding time offers the best protection later in the season.

"This strategy could be critical if there is another late break to the season and the plant doesn't have the opportunity for early establishment in those warmer soils," he says.

Applying fungicide at three to four centimetres below the seed placement, combined with soil disturbance, will provide a greater opportunity for plants' roots to get down to the subsoil -as Rhizoctonia is generally found in the top few centimetres of the soils.

Other control strategies could include a seed treatment or a coated fertiliser.

With all fungicide treatments, rain is required to spread the fungicide across the root zone. When roots grow outside the fungicide zone, they are not protected.

Dr Huberli says paddocks with a cereal-on-cereal history will be more likely to have higher inoculum levels, and the introduction of legumes and oilseeds will reduce the impact of Rhizoctonia.

If this is not an option and cereals must be planted, he says, consider choosing oats or wheat, instead of barley.

"Barley is the most susceptible to the effects of Rhizoctonia," he says.

Rhizoctonia survives on organic matter, but multiplies on grass roots, so good grass control in break crops is important.

Summer weed control is vital to keep inoculum levels low. Weeds may increase inoculum and, possibly more importantly, they dry the soil, which stops the inoculum from being degraded by other organisms in the soil.

Dr Huberli says planting into a grass-free paddock is critical to get the crop roots off to a good start and help prevent significant yield penalties at the end of the season.

Soil testing

PREDICTA® B soil testing is useful for detecting Rhizoctonia inoculum in paddocks where crops may not be displaying bare patches. The service is available via local accredited agronomists.

GRDC Research Code DAS00125 (DPIRD Fungicide Trials)

More information: Dr Alan McKay, alan.mckay@sa.gov.au; Dr Daniel Huberli, daniel.huberli@dpird.wa.gov.au

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