Dr Alan McKay (left) and Dr Paul Bogacki
from SARDI examine Rhizoctonia’s
effects on root growth at trials near
Karoonda in the South Australian Mallee
PHOTO: Deanna Lush
Trials in the southern region have found the type of seeder soil openers and seed treatments used can influence Rhizoctonia inoculum soil levels and position in the soil profile.
Dry conditions in spring and summer favour high Rhizoctonia levels and grain growers in areas of southern Australia that are susceptible to the disease are being warned to consider strategies to reduce the risk for the 2013 crop.
Dr Alan McKay from the South Australian Research and Development Institute (SARDI), Dr Jack Desbiolles from the University of South Australia (UniSA) and Dr Gupta Vadakattu from CSIRO have been working on trials through the GRDC and the South Australian Grains Industry Trust (SAGIT) to assess the impact of different types of seeder soil openers on Rhizoctonia levels during seedling establishment.
Trials were conducted at Karoonda, Port Julia and Geranium in South Australia with knife points, rippled coulter discs and flat discs, and inoculum levels were assessed five weeks after seeding.
In undisturbed soil, about 70 per cent of the inoculum was in the top 2.5 centimetres of soil, 20 per cent in the 2.5 to 5cm layer, six per cent was in the 5 to 7.5cm layer and three per cent was in the 7.5 to 10cm layer.
“The knife points, set to 10cm deep, lift uninfected soil from deeper in the profile and throw the most infected soil at the surface into the middle of the row. The knife points are also shattering the soil and helping to break up the hyphal network,” Dr McKay says.
“The rippled coulter set to cut 10cm deep has provided some reduction in the top 2.5cm, but instead of lifting soil up, it’s pushing it, and Rhizoctonia, down the profile. There’s also a risk of pushing stubble down the soil profile.”
The trials found that flat discs set to 6cm deep more than doubled inoculum levels in the top 5cm of soil.
“With a flat disc, there is little shattering of the hyphal network between the rows so Rhizoctonia can recolonise quickly. And because the discs have only cut the soil to 6cm, the seedling roots have had more difficulty in getting down the profile quickly, away from the disease.
“If you are going to use a flat disc, make sure you cut to 10cm, sow early, deep band nitrogen and consider the new Rhizoctonia fungicides to help reduce infection.”
Other trial work by Dr Paul Bogacki from SARDI and Dr Daniel Hüberli from the Department of Agriculture and Food, Western Australia (DAFWA), is examining management options for Rhizoctonia involving use of new seed treatments Vibrance® and EverGol® Prime, registered this year.
“In our trials, the seed treatments average about a five per cent yield increase in Rhizoctonia-affected paddocks when compared with untreated trials,” Dr McKay says.
He says there has been a lot of interest in trials where fungicides were being banded above and below the seed. “The idea of putting the fungicide above the seed is to get rain to move it into the surface soil to protect the crown roots. Banding below the seed is to protect the seminal roots,” he says.
Dr McKay says SARDI, UniSA and DAFWA, with support from the GRDC and SAGIT, are working with Syngenta and Bayer CropScience to get enough data to support a label registration.
He says growers in Rhizoctonia-affected areas should consider options to reduce the risk of damage this coming season.
“The soil profile is very dry. Hopefully we get a good opening rain so crops can be sown into warm soil to get seedlings up and away quickly and allow roots the opportunity to grow away from Rhizoctonia inoculum near the soil surface,” he says.
“If the rain doesn’t wet the profile, the crop roots may have difficulty escaping the Rhizoctonia.”
Dr McKay says management options to consider include ensuring adequate nutrition, deep banding nitrogen to stimulate deep root growth, considering new seed treatments to provide early protection, disrupting compaction layers and being aware of herbicide residues that may have carried over from the dry 2012 spring.
- Rhizoctonia is a network of hyphae fungi in the soil, mostly in the top five centimetres, but it can extend down to 10cm during the growing season.
- The fungus is adapted to dry conditions and occurs mainly in the low to medium- rainfall regions of southern Australia.
- The main hosts are cereals and grasses, but it attacks a wide range of plants and survives on residues.
- Damage is greatest when root growth is slow due to subsoil constraints or cold soil temperatures (less than about 10°C).
- When seedlings (seminal) roots are damaged, bare patches in crops can result. When crops are sown early and establish quickly, the seminal roots develop quickly and the crop will appear healthy until the soil temperature drops in early to mid-winter and Rhizoctonia begins to attack the crown roots. This causes uneven crop growth associated with reduced tiller production.
- Inoculum levels increase during the growing season, especially in spring, and reach maximum levels as the crop dries off. They remain high during summer if there is low or infrequent summer/autumn rainfall.
- Rhizoctonia levels are reduced by repeated significant summer rain and good weed control, which prolongs moist soil.
- The disease has traditionally been thought of as attacking seedlings and being unaffected by rotation, but research by the GRDC has found that this is not true.
- Grass-free canola, mustards, field peas, chickpeas, medic pastures and fallow help reduce inoculum, increasing yield in cereals by between nine and 47 per cent.
A fact sheet on Rhizoctonia is available at: www.grdc.com.au/GRDC-FS-RhizoctoniaSW
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