Correct sample strategy estimates crown rot risk
ADVISERS may need to make some changes to their approach to sampling for the PreDicta B soil test to ensure an accurate assessment of crown rot.
SARDI’s Dr Alan McKay says there have been several occasions where a PreDicta B test result taken before sowing has not adequately reflected the risk of crown rot in some paddocks.
“We are working closely with the national crown rot program led by Dr Steven Simpfendorfer plus collaborators in each state to make PreDicta B more reliable,” he said. “The early results show this is a sampling issue, and that adding short pieces of stubble from the base of the plant significantly improves the detection of the Fusarium species that cause crown rot.”
Work is underway to check that all Fusarium species known to cause crown rot are being detected by the current PreDicta B assays.
“So far we have sequenced and tested several hundred isolates from around Australia and all Fusarium pseudograminearum and F culmorum isolates have been detected by the PreDicta B assays,” Dr McKay said. “However the survey has uncovered several other species isolated from crown rot affected plants. Further work is continuing to determine if these actually cause crown rot or are simply secondary invaders.”
Dr McKay says getting the sampling strategy right underpins the results of all the tests.
“Based on the results obtained to date, we are recommending the following sample strategy to improve the value of PreDicta B as a management tool.”
Correct sampling strategy involves:
- Collect three cores at 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 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 wanted 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. 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.
“Some agronomists take one sample and split it in half for soil nutrition and PreDicta B tests,” Dr McKay said. “While this is not recommended because soil nutrition samples are generally collected between the rows, if that is the only option then adding stubble to the PreDicta B sample will improve crown rot detection.”
Adding stubble to the PreDicta B sample also means there is greater opportunity to detect other stubble borne pathogens.
“There is a lot of activity to improve the value of PreDicta B as a management tool for growers. In addition to developing more robust sampling strategies, our national GRDC-funded molecular diagnostic project is collaborating with other GRDC-funded projects to develop new assays for the pathogens associated with white grain, yellow leaf spot, and eyespot.”
Pratylenchus yield loss risk categories are also being developed in collaboration with the national nematology program led by Dr Grant Hollaway.
More information: www.sardi.sa.gov.au/diagnostic_services/predicta_b
GRDC Project Code