INCREASED canola density in WA, and therefore canola stubble, will inevitably lead to increased blackleg inoculum (spore density), however it's unlikely to have severe forms of both versions of blackleg - crown canker and upper canopy infection (UCI) - in the same growing season.
That was the key message from Marcroft Grains Pathology principal Steve Marcroft during his presentation at the virtual Grains Research and Development Corporation (GRDC) Grains Research Updates in Perth recently.
As the more well-known version of blackleg, crown canker years occur from late sowings which result in plants remaining as seedlings during the winter infection period.
UCI on the other hand is a more recent issue and comes as a result of sowing and flowering times moving 20 to 30 days earlier than in the past, meaning plants commence flowering in late July or early August.
Early flowering results in increased infection and will provide the fungus with more time to cause damage prior to harvest.
In saying that, if crops commence flowering later in the growing season, UCI can still occur, but will not cause any yield loss regardless of disease severity.
During 2021, Dr Marcroft and his team compared disease severity in low or high quantities of one-year-old stubble and two-year-old stubble that was either lying or standing.
For each stubble treatment, two times of sowing allowed for crown canker development (late sown) or UCI.
Dr Marcroft said as expected, the earlier sown plants had significantly higher upper canopy infection severity compared to the later sown plants.
"Very low levels of upper canopy infection were detected in the late sown plants, with no significant differences between any treatments," Dr Marcroft said.
"As with crown canker severity, stubble load had a significant effect on the severity of upper canopy blackleg infection in early sown plants across all stubble treatments.
"The impact of stubble orientation caused some significant differences, but only at the low stubble density."
Standing one-year-old stubble caused significantly larger stem lesions (34.5 centimetres) than lying one-year-old stubble (8.3cm) at low density, however no differences were detected in branch infection.
Similarly, the standing two-year-old stubble caused significantly larger stem lesions (20.4cm) than the lying two-year-old stubble (9.6cm).
Again, no differences in branch infection were observed.
The data support previous findings that earlier sowing times lead to significantly higher levels of upper canopy infection than later sowing times.
Furthermore, stubble load had a significant impact on disease compared to stubble age or orientation.
"In our research high stubble quantity generated significantly higher disease severity for both crown canker and upper canopy infection, so it's important to avoid one-year-old stubble by keeping new crops 500 metres from the previous year's crop," Dr Marcroft said.
"Two-year-old stubble still produced inoculum and disease but the difference between treatments were minor, making the management of two-year-old stubble probably not warranted.
"The largest effect on disease severity was time of sowing - early sown crops get UCI, while late sown crops remain safe, however late sown crops are vulnerable to crown canker while early sown crops remain safe."
Research over the past few years has indicated most years will be defined as either a crown canker or UCI year and it's rare to have severe forms of both blackleg diseases in the same season.
In most regions, 2021 was a crown canker year.
The relative risk of each disease is determined by the timing of the opening rains each year, the subsequent germination date and then the first flower date.
It will be rare to warrant applying fungicides to control both crown canker and upper canopy infection in the same year.
Dr Marcroft said UCI occured when the plants become reproductive early in the growing season, typically when crops commence flowering in late July to early August.
"This can coincide with cool, moist conditions conducive to infection and allows enough time for the pathogen to cause tissue necrosis prior to harvest," he said.
"UCI flower and branch infection can occur at any time but only results in yield loss when it occurs early in the season because the pathogen must have enough time to grow from the infection point to within the vascular tissue to cause necrosis and yield loss.
"In 2021, crops that commenced flowering in September in many cases developed UCI infection but the infection did not progress to the vascular tissue, so no yield losses occurred."
Fungicide control of UCI can result in very variable yield returns, so it's important to understand the disease risk before applying a fungicide.
If UCI occurs and causes yield loss, fungicides applied at 30 per cent bloom will provide economic responses.
However, the skill in predicting yield returns from fungicide application is determining if UCI is likely to cause yield loss.
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