Early warning in the bag

New sampling strategies offer better ways to estimate potential soilborne disease risks, reports SARDI researcher Dr John Heap

The process: first, take a good sample - it is critical to represent the whole paddock; then test it; and then analyse the results, as Dr John Heap and Dr Alan McKay are doing above.

Soilborne diseases can cause unexpected and sometimes large yield losses, so any information about potential disease risk before sowing can obviously be valuable when planning a cropping program.

The opportunity to do this now exists with the development of kits that can determine the level of inoculum of many soilborne diseases, including cereal cyst nematode (CCN), takeall, Rhizoctonia bare patch (AG8), crown rot, Pratylenchus neglectus, Pratylenchus thornei, and blackspot of peas.

The kits, called PreDicta B™, are marketed by C-Qentec Diagnostics, a subsidiary of Bayer CropScience, and the idea is to get soil samples tested before sowing. PreDicta B™ was developed by the South Australian Research and Development Institute (SARDI) and CSIRO and remains the only product of its type worldwide.

The key to using PreDicta B™ to assess potential disease risk is to collect a 500 gram soil sample representative of the paddock - a task which can become quite a challenge as paddock sizes increase.

Soilborne diseases often occur in patches throughout a paddock, so the sampling strategy needs to deal with this variation if it is to reliably identify potential significant disease risks.

With colleague Dr Alan McKay, we have been working on a SARDI project funded by the GRDC, aimed at developing a sampling strategy or protocol to help in the accurate detection of soilborne diseases. Our work has shown that to reliably estimate risk from soilborne diseases, the soil sample needs to be made up of many individual cores.

The optimum number of core samples is 45, comprising three cores from 15 locations; a sample size which stabilises the variability of most soilborne pathogens. How the sample is collected is also important.

We found that a small 10 millimetre diameter x 100mm core was better for collecting samples by hand. This delivers the required 500gm sample without the need to sub-sample. Larger diameter cores can be used but care should be taken to thoroughly mix the sample before taking off the 500gm subsample.

Not doing this properly can increase sampling error by up to 30 percent. Cores should be collected, whenever possible, from the stubble rows of the previous cereal crop. If cereal rows are not visible, then cores should be collected at random.

Most disease inoculum is concentrated around the roots and crowns of the previous cereal crop, and our research confirms that you are more likely to identify potential disease risks by targeting these rows.

There are two main sampling strategies available:

Sampling the whole paddock using the following five-point protocol will usually identify the main disease risks:

If a grower has access to maps of the production zones within the paddock, they can be used to select 15 locations that better represent the main production areas before entering the paddock.

Analysis of PA production zones in a paddock often reveals that one zone is more likely to produce higher yields than others. Soilborne diseases in this zone therefore pose a greater risk to paddock profitability.

Sampling by production zones will improve assessment of disease risk. Our research has shown that the spectrum, and/or levels, of soilborne diseases often segregate by production zones. This is not totally unexpected, as soil changes and related factors that drive variation in crop performance between zones also affect soilborne diseases.

Bulking cores from different zones can result in loss of potentially valuable information on disease risk in specific production zones. Growers who do not wish to test each zone for disease inoculum may decide instead to test only the higher-yielding zones to reduce risk of unexpected large yield losses in these areas.

Specialised soil sampling equipment, AccuCore™, has been developed by SARDI, GRDC and Spurr Soil Probes to collect a representative 500gm soil sample without the need to sub-sample from a bucket in the field.

Two versions of AccuCore™ are available, the standard 10mm x 100mm core for collecting three cores from 15 locations and a 10mm x 150mm corer for deeper sampling.

The latter is designed to collect 30 cores, two from each of 15 locations. A sample collection stand is also available. The stand can hold two sample bags, one for PreDicta B™ and the other for nutrition testing.

AccuCore™ was released last season, and the experience has been used to make minor changes to the design of AccuCore™ and major changes to the collection stand. Both are available from Spurr Soil Probes.

Current research is examining the impact of soilborne diseases on yield in different production zones. Once this is understood, researchers plan to investigate the use of precision and variable rate technology to provide more flexible and reliable strategies to minimise losses from soilborne diseases.

These strategies include:

Modern farm machinery increasingly has the capability to deliver inputs variably, using on-board computers linked to GPS, to known points within the paddock. These systems will one day be used to reduce the costs and risk of yield losses from soilborne diseases.

GRDC Research Code: DAS00035.

For more information:
Dr John Heap, 08 8303 9444, heap.john@saugov.sa.gov.au
PreDicta B™ kits: Greg Skinner, 0419 418 402
Spurr Soil Probes, 08 8296 4138