Stem/branch infection of blackleg: what to look for
- Lesions/cankers on stems, branches and pods.
- Small lesions at the leaf axil (where the stem of a leaf meets the branch) or long lesions the entire height of the plant.
- Blackened pith inside the plant (slice open to inspect pith and determine infection).
An example of lesions caused by blackleg.
PHOTO: Dr Steve Marcroft
Canola growers across the southern region are advised to monitor their paddocks during stem elongation and look out for signs of blackleg infection on the upper stems and branches of their crops.
Since 2010, when cankers on the upper stems and branches were first observed in a commercial paddock of canola, blackleg infection of stems and branches has been on the rise and, in some instances, has caused severe yield loss.
The more ‘traditional’ blackleg infection causes lesions on the cotyledons and leaves of canola plants before the blackleg grows without symptoms to the crown, resulting in a crown canker at the base of the plant.
Stem and branch infection of blackleg can cause branch death and early pod shatter as branches deteriorate prematurely.
Preliminary data suggests that stem and branch cankers are not always correlated with the presence of traditional crown cankers.
Marcroft Grains Pathology principal Dr Steve Marcroft says cankers on stems and branches were observed in 2011, 2012 and 2013 but were not present at all sites.
“In 2014, the symptoms were widespread and in many cases caused significant yield loss,” he says.
“In 2015, stem and branch cankers were observed at all sites and again caused severe yield losses in some instances.”
The likely cause of the rise in blackleg infections is the advent of no-till and early sowing, which means crops are developing earlier, elongating and, in many cases, flowering in winter – ideal conditions for blackleg infection.
“In the past plants only had leaves during the main infection period in the winter, but now plants may have already begun elongation during the winter, so that the plants have stems and branches to be directly infected,” Dr Marcroft says.
GRDC-funded research undertaken in 2015 by Marcroft Grains Pathology in glasshouses showed that when plants were infected after elongation the upper-stem infections occurred, whereas when they were inoculated during the vegetative stage they developed ‘traditional’ crown canker but escaped the stem/branch infection.
Further research in 2015 as part of the GRDC/CSIRO canola profitability project showed that earlier-sown crops (early April) had a significantly greater level of branch infection than later-sown crops (late April).
Dr Steve Marcroft,
03 5381 2294,
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