Protecting WA crops

Soil amelioration and its impact on plant parasitic nematodes and Rhizoctonia

Mouldboard ploughing at Muresk trial site. Image: Daniel Huberli, DPIRD.

At a glance

• Soil inversion using a mouldboard plough to invert the topsoil and reorder the profile was found in recent trials to be the most effective treatment for reduction of Rhizoctonia solani AG8 (R. solani), root lesion and cereal cyst nematodes (CCN).
• Mixing and loosening tillage treatments only resulted in a short-term reduction of the target organisms and based on these results would not be recommended as worthwhile long-term management strategies for paddocks infested with R. solani, CCN and or root lesion nematode (RLN).
• Rotation with canola in 2021 reduced R. solani population densities more than any of the deep tillage treatments and remains as one of the most cost-effective management strategies for this pathogen. The canola rotation also reduced CCN inoculum densities.
• The Grizzly plough reduced R.solani AG8 levels four-fold and increased grain yield by nearly 0.4 t/ha.

With a large area of deep, coarse textured soils, Western Australia (WA) is leading the way for uptake of strategic deep soil tillage as an amelioration practice. While the multiple benefits of deep tillage for abiotic soil constraints are now recognised, these amelioration practices are also likely to have major impacts on the living components of soil - its ‘biological’ community structure and functions.

A Department of Primary Industries and Regional Development (DPIRD) collaborative project between the Crop Protection group and the ‘Post Amelioration’ project within the Soils group, with co-investment from the Grains Research and Development Corporation (GRDC), has just been completed. The project, ‘Increasing farming system profitability and longevity of benefits following soil amelioration’, investigated the effects that soil amelioration had on the soil biology specifically root lesion nematodes and a soilborne disease.

The project ran for 4 years at 2 trial sites, Yerecoin (yellow sandy earth) and Darkan (duplex sandy loam). These sites were selected because they were impacted by mixed soilborne disease and pest issues, including the fungal pathogen Rhizoctonia solani AG8, root lesion nematodes (RLN) and cereal cyst nematodes (CCN, Yerecoin only). In 2019, 3 tillage methods were applied and compared to minimal tillage: deep ripping for soil loosening, spading for mixing and mouldboard ploughing for soil inversion.

The impact of strategic deep soil tillage on the density and distribution of soilborne pathogens and nematodes over time

Soil inversion was the most effective strategic tillage method for reducing R. solani, RLN and CCN over the length of the project. For 5 seasons post tillage at Yerecoin and 3 seasons at Darkan, R. solani densities remained lower than those in the minimal tillage treatment. RLN levels after inversion were lower for 3 seasons at Darkan but were only lower for the 1st and 5th growing seasons at Yerecoin. Inversion reduced CCN densities consistently across the four seasons, while loosening and mixing treatments did not differ significantly from the minimal tillage treatment.

Soil mixing reduced R. solani at both sites, but RLN densities were only reduced at the Yerecoin site in the first season post tillage.

Soil loosening also reduced R. solani densities for the 1st year post tillage at the Yerecoin site, but RLN densities were not significantly reduced by this treatment.

Further to the trials at Yerecoin and Darkan, a collaborative project between DPIRD, GRDC, the University of Adelaide and CSIRO established a trial site at Muresk in 2021 to compare the effects of inversion and soil loosening on R. solani. This trial had 2 crop types, barley and canola, in the first year of amelioration, followed by all barley in the year after amelioration. The site had a low-medium background of R. solani at the start of the trial.

By the end of 2022, the inversion treatment had significantly lowered R. solani densities in both the 0-10 cm and 10-20 cm soil profile compared to the control, but no difference was observed beyond 20 cm. R.solani densities were significantly lower in canola compared to the barley at both 0-10 cm and 10-20 cm depths.

Crop rotation and variety choice

DPIRD research scientists found that crop and variety choice had a significant impact on R. solani and CCN densities. CCN densities were reduced at Yerecoin when resistant La Trobe barley was sown in 2019 and also reduced when resistant canola was sown in 2021. As expected, R. solani density was reduced under canola at both sites.  

Investigation of soilborne disease and nematode pest plant symptoms following deep soil tillage

For soilborne diseases to occur, a conducive environment and susceptible host plants need to be present along with the pathogen or nematode pests. For soil biology, a conducive environment includes physical and chemical soil characteristics, soil moisture, and temperature. However, strategic tillage changes the soil environment and impacts disease occurrence.

To start the project, a qPCR soil test was used to identify the main soilborne pathogens and pests present at both trial sites and to measure their densities.

At Yerecoin, the main pathogen and pests were R. solani AG8, Pratylenchus neglectus (RLN) and Heterodera australis (CCN, formerly H. avenae).

At Darkan, there was R. solani AG8, Pratylenchus neglectus and P. quasitereoides (RLN).

To determine the impact that strategic tillage had on disease in crops, visual root health assessments of cereals (crown - upper lateral roots and seminal - deeper vertical roots) were recorded each year of the cereal phase.

Wheat seedling at four leaf stage showing both crown and seminal roots. Image: DPIRD.

Root disease symptoms were found for all tillage treatments across the 3 seasons. In the sandy, gravel soils of Darkan, soil inversion reduced both crown root disease and seminal root disease expression in all 3 cereals crops grown. Lower root disease following inversion corresponded with significantly lower R. solani and RLN in the topsoil after inversion in 2019 and 2020.

In the yellow sandy soils at Yerecoin, no deep tillage treatment consistently reduced root disease symptoms. In the first season, soil mixing was the most successful treatment for reducing crown root disease, while in the second season, the inversion treatment resulted in less crown root disease symptoms.

Grizzly plough effect on Rhizoctonia solani AG8

Aerial image of the Grizzly plough trial at Muresk. Image: Veronika Crouch, CFIG.

Following DPIRD’s research at Yerecoin and Darkan, a grower group demonstration trial by DPIRD, GRDC and the Corrigin Farm Improvement Group (CFIG) tested how a Grizzly plough affects R. solani AG8 soil densities in a farmer’s paddock with medium to high levels of background R. solani. In areas of the paddock with high R. solani pathogen levels, the plough reduced these levels fourfold immediately after the treatment, and grain yield increased by nearly 0.4t/ha compared to the adjacent untreated area of the paddock.

DPIRD scientists found that the grizzly plough reduced root disease compared to the control treatments.

DPIRD extends its many thanks to all the growers that allowed their land to be used for these research trials.

Further information

For more information on root lesion nematodes and rhizoctonia refer to:

• DPIRD’s Diagnosing root lesion nematodes in cereals webpage
• DPIRD’s Diagnosing rhizoctonia root rot in cereals webpages
• GRDC’s Rhizoctonia webpage
• GRDC’s Root lesion nematode - Western fact sheet.