Change canola cultivars regularly to reduce blackleg legacy
Canola growers can reduce potential yield losses and the probability of blackleg disease resistance breakdown occurring by changing cultivars every three years.
That’s according to blackleg authority Steve Marcroft who says sowing the same cultivar every year is likely to break the cultivar’s resistance to blackleg – the most severe disease of canola in Australia.
Speaking at recent Grains Research and Development Corporation (GRDC) grains research Updates throughout the southern cropping region, Dr Marcroft has told growers and advisers that every year a cultivar is sown from the same resistance group, the number of virulent isolates that can attack that particular cultivar increases.
“The fastest cases of resistance breakdown have occurred over three years, but in most regions it will take longer than three years,” said Dr Marcroft, of Marcroft Grains Pathology, based in Horsham (Victoria).
“Therefore, the best policy is to monitor the level of blackleg in your cultivar on your farm. If you observe the level of disease increasing, then switch to a different group. If blackleg severity is not increasing, you can continue with your current cultivar.”
Blackleg disease causes yield loss and in some circumstances total crop failure.
“It is managed by breeding disease resistance into canola cultivars and by crop management practices,” Dr Marcroft said. “However, the blackleg fungus is adept at overcoming cultivar resistance, leaving many crops vulnerable to significant yield loss.”
Cultivar resistance has been overcome in many regions around Australia, the most recent being in Hyola®50 which went from a rating of resistant to susceptible on the Eyre Peninsula in South Australia last year. Fortunately, according to Dr Marcroft, Hyola®50 is still resistant in most other growing regions.
Dr Marcroft advised that if growers detected a blackleg problem and were growing a susceptible cultivar, their first action should be to change to a cultivar with higher levels of resistance.
Dr Marcroft said growers and advisers should refer to the Blackleg Management Guide that is updated in March each year and can be accessed via the GRDC webpage www.grdc.com.au/BlacklegResistanceRatings and the National Variety Trials website www.nvtonline.com.au. The guide includes current blackleg ratings and blackleg resistance groups.
From 2011 onwards, all cultivars and NVT lines have been classified for their type of blackleg resistance. Dr Marcroft said there were two types of resistance to blackleg – seedling and adult plant resistance.
“We use individual blackleg isolates to identify the seedling resistance and we use stubble that is releasing blackleg sexual spores to screen plants for stem canker adult plant resistance. Using the combination of the seedling and adult data we classify all cultivars into seven different groups.”
Dr Marcroft recommends the following method for determining whether a cultivar is in danger of having its resistance overcome:
- Monitoring your crop should not take any more than 30 minutes.
- Observe your crop at or just prior to windrowing.
- Look for cankered and dead plants, if you see cankered plants yield loss is occurring, pull up dead plants to confirm that they have died from stem canker.
- Walk approximately 200 metres in a W-shaped transect, randomly select (pull out of ground) 15 plants along each leg of the W.
- With a pair of secateurs cut the roots off at the crown (point where the roots join to the stem).
- Score each plant for stem canker internal infection, look at the cross section of the cut stem and estimate the area that is discoloured from blackleg infection. Stems should be scored as 0,5,10,20,30,40,50,60,70,80,90 or 100 % infection. Score any dead or fallen plants as 100%. Work out the average infection score and keep this score as a comparison for future years.
- You can repeat this process for different parts of the paddock. For instance, you may wish to determine disease levels at the edge of the paddock which was near to last year’s canola stubble and then in the middle of the paddock which was 500m away from last year’s stubble.
- As a rule of thumb, plants with 50% or more internal infection will have significant yield loss. If you have plants with more than 50% infection, consider changing your current blackleg management.
To assist canola growers with management of blackleg and other crop diseases, the GRDC has also just released a new Diseases of Canola Back Pocket Guide. It is available for viewing and downloading via www.grdc.com.au/GRDC-BPG-CanolaDiseases.
Caption: Dr Steve Marcroft says sowing the same canola cultivar every year is likely to break the cultivar’s resistance to blackleg.
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