Take the guess-work out of disease management with PreDicta B

Author: Rebecca Barr | Date: 27 Mar 2015

Alan McKay standing in a greenhouse

Dr Alan McKay (pictured) suggests growers use PredictaB is they have a history of soil-borne diseases or paddocks previously sown to durum.

SOURCE: Emma Leonard

Changes to the recommended sampling strategy for PreDictaB in recent years have greatly improved identification of paddocks that have medium to high risk of crown rot before growers sow their crop.

SARDI soil biology and molecular diagnostics leader Alan McKay said in 2013 the NSW DPI found that PreDicta B was underestimating the risk of crown rot risk in 22 percent of cases.  This led to research to improve the sampling strategy, with the subsequent changes virtually eliminating the underestimation of medium to high crown rot risk in 2014.

“The key change has been the inclusion of stubble in the sample.  Many diseases, especially crown rot, survive on stubble so it’s critical to include stubble in the testing sample,” he said. 

“If there is stubble in the paddock then there must be stubble in the PreDicta B sample to reliably assess the risk of crown rot.  Stubble is typically avoided when collecting soil samples for soil nutrient testing, however avoiding stubble when collecting PreDicta B samples will result in significant underestimation of risk of crown rot and even take-all.”

The new sampling protocol for PreDictaB involves adding one piece of stubble about 5cm long from the base of a previous cereal plant or grassy weed from each of 15 locations within the paddock where the soil cores are collected.  If sowing durum, two pieces of stubble should be collected from each location.

Results are returned within one to two weeks, and provide risk categories which an agronomist trained in PreDictaB testing can use to suggest a plan for the season.

“A common feature of most soil borne diseases is that most management decisions to reduce the risk of disease need to be implemented before the crop is sown.  If the PreDictaB results indicate there is a risk of a certain disease, the grower and their agronomist can decide what they will do to manage the risk,” Dr McKay said.

“Agronomists accredited to deliver PreDicta B have been through a course on managing soilborne diseases, and the resource manual they use is updated regularly”.

Correct sampling strategy

  • Collect three small cores (1cm diameter and 10cm deep, if using a larger diameter core, take fewer cores per location) from each of 15 different locations within the target paddock or production zone.
  • Take the soil cores from along/in the rows of previous cereal crop, if these are still visible and retain any stubble collected by the core (most soilborne pathogens are concentrated under the rows of the last cereal).
  • If the rows can’t be seen, take the cores at random.
  • Taking the soil sample in the inter-row, where pathogens concentrations are lowest is only recommended if a susceptible crop is to be sown between the rows and a grower wants to know if inoculum levels are low enough to take the risk.
  • Add one piece of cereal stubble (if present) to the sample bag at each of the 15 sampling locations – this improves the detection of crown rot and other stubble borne pathogens. Each piece should be from the base of the plant and include the crown to the first node (discard material from above the first node).
  • The maximum sample weight should not exceed 500g.

For instance if there’s a risk of rhizoctonia, then crops can be sown early, fungicides can be considered or a break crop sown to reduce the risk.  If there is a risk of take-all, sowing later in the seeding program while maintaining the paddock weed free is preferable to allow more time for the inoculum to breakdown. If crown rot is present, growers would likely avoid growing durum. All these decisions need to be made before the start of the season, so where growers think there might be some disease in their soils, using PreDictaB can take the guess-work out of disease management.

“PreDicta B is most useful for assessing disease loads in paddocks with high frequency of cereals in the rotation, especially those with a history of disease that the grower is trying to manage, and paddocks previously sown to durum.  It’s worth spending a bit of extra time to make sure the soil and stubble in the paddock is sampled properly.  At $235 per paddock, PreDictaB is a small price to pay for peace of mind,” Dr McKay said.

Disease risks in 2015

The dry spring in late 2014 favoured rhizoctonia and crown rot, which Dr McKay said has been confirmed by early trends from PreDicta B tests. 

“Take-all risk this year looks like being the highest we have seen it since the early 2000s with medium to high take-all risk being detected across south-east Australia, despite the dry spring last year. Growers contemplating cereal on cereals may need to be careful.

“If take-all risk is low to medium there are fungicides that can help, while if levels are high non-host crops should be considered. As with many diseases, environmental conditions play a large role in whether yield loss occurs and for take all, the greatest losses will be if winter rainfall is above average followed by moisture stress around flowering.”

“In general growers have successfully reduced cereal cyst nematode (CCN) to very low levels in most paddocks but we have detected a few paddocks in South Australian and Victoria with high risk of CCN. Disease levels will build if growers over-use susceptible varieties and in these situations, PreDicta B can be used to monitor CCN levels and warn the grower of emerging risk before crop yields are affected,” Dr McKay said.

GRDC research

The GRDC-funded National Molecular Diagnostic Project, a collaboration between SARDI, NSW DPI, DAFWA, VicDEDJTR (Victorian Department of Economic Development, Jobs, Transport and Resources) and the University of Southern Queensland, is continuing through to 2018. Dr McKay said upcoming developments to the service include a website showing risk trends in different areas.

“The website will allow growers to look at their region and compare results with previous years to assess emerging trends for each pathogen monitored,” he said.

“We’re also continuously adding tests for more pathogens, currently we are working on common root rot, eyespot, yellow spot, more root lesion nematode species and phytophthora root rot of chickpeas. The project is also enhancing the training of consultants; courses are run annually in November and December.”

More Information

 Dr Alan McKay, 08 8303 9375, alan.mckay@sa.gov.au or Shawn Rowe shawn.rowe@sa.gov.au

GRDC Project Code DAS00137, DAN00175, DAV00128

Region South, North