Correct sampling 'a must' to accurately expose disease risk

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Fast fact

Adding cereal stubble to soil samples has improved the accuracy of PreDicta® B testing.

Key point

Improvements to the PreDicta® B sampling protocol are providing growers with a more comprehensive assessment of pest and disease risks in the paddock

Soil-borne diseases are most effectively managed during or before seeding, so identifying disease risk with a PreDicta® B soil test well before planting is essential when growing susceptible crops in rotations where there may be significant carryover inoculum.

Sampling depth, the effect of adding stubble (to the sample) and the reliability of each assay to detect different populations of root lesion nematode (RLN) and crown rot pathogens were all examined by a national molecular diagnostics GRDC investment project led by the South Australian Research and Development Institute (SARDI).

Launched in 1997, the PreDicta® B test uses soil samples comprising multiple cores targeted along the rows of the previous crop to produce reliable estimates of soil-borne pests including cereal cyst nematode (CCN), RLN and the plant pathogenic fungi that cause take-all, crown rot and Rhizoctonia bare patch. In 2013 two separate issues emerged.

Crown rot

In paddocks surveyed in northern NSW by the NSW Department of Primary Industries, the PreDicta® B soil test underestimated the concentration of the Fusarium pathogen responsible for crown rot. Follow-up trials showed Fusarium was also being underestimated in other regions. We suspected samplers were defaulting to a soil nutrition sampling strategy and were collecting cores between previous crop rows and avoiding stubble. The crown rot fungus is known to survive in stubble.

To test this, we enlisted support from the National Variety Trials and National Paddock Survey projects to collect paired samples with and without cereal stubble for all paddocks to be sown to cereals.

When cereal stubble was added to the pre-sowing soil sample, the relationship between PreDicta® B results and the incidence of crown rot in stubble post-harvest (see Figure 1A) matched the pattern we expect when a test and sampling strategy are working.

Reduce the risk of unexpected losses – follow the protocols

  • Select the production zone to sample – usually the one that makes the greatest contribution to paddock profitability. Production zones can be driven by soil type, subsoil constraints, slope, etc. The spectrum and levels of soil and stubble-borne pathogens often vary between production zones.
  • Target cores along the rows of the last cereal crop.
  • Add small pieces of stubble, 5cm long, taken from the base of old cereal or grass weed crowns to PreDicta® B soil samples (see Figure 2).
  • In the summer cropping regions add two pieces of stubble from each of 15 different locations.
  • In winter cropping regions add two pieces of stubble from each of the 15 locations if growing durum, otherwise one piece per location is adequate.
  • Do not avoid cereal stubble: avoiding stubble can lead to significant underestimation of the risk of crown rot.
  • Droughts can increase carryover of inoculum between seasons, both under the rows of previous crops and in stubble that has not broken down, so sample all stubble present.

That is, when PreDicta® B results were low, the incidence of crown rot was also low, and when PreDicta® B results were high, the incidence of crown rot ranged from low to high. Note that low disease incidence when inoculum levels are high typically occurs when either the grower has successfully implemented strategies to reduce infection and/or seasonal conditions were not conducive for infection.

By comparison, when stubble was avoided PreDicta® B was a poor predictor of the incidence of crown rot (see Figure 1B). Too many of these paddocks had low PreDicta® B results at pre-sowing, but high incidence of crown rot infection post-harvest. This confirms that avoiding stubble can lead to a ‘failure to warn’.

This GRDC-funded research led to the recommendation that if cereal or grass stubble is present in the paddock, five-centimetre pieces of stubble from the crown should be added to the soil sample to improve detection of the crown rot fungus (see Figure 2 and panel right).

Root lesion nematode

There was also confusion about the sampling protocol for RLN in the northern grains region. The former Queensland Department of Agriculture, Fisheries and Forestry had a (now discontinued) testing service that recommended taking soil cores up to a metre deep, while PreDicta® B used just the top 10 centimetres of soil.

The NSW DPI conducted field trials from Dubbo in central NSW up into southern Queensland that compared sampling to 15cm deep with sampling to 30cm deep. Results from the two depths were highly correlated across all regions of Australia, allowing the shallower depth to be recommended for the northern region.

Researchers also conducted a national survey to investigate the ability of the assay to detect different RLN and crown rot pathogens. All species isolated from across Australian cropping regions were detected by the tests.

Future research will investigate sampling the pests and diseases of summer crops and winter pulse and oilseed crops, including the potential impact of their stubble on test accuracy.

GRDC Research Codes DAS00137, DAN00175, BWD00025, DAV00128, CAS00002

More information:

Dr Alan McKay
08 8303 9375
alan.mckay@sa.gov.au