Season 2017 is producing mixed fortunes in terms of canola diseases for growers in the southern cropping region.
While the risk of blackleg disease has reduced with the lack of consistent rainfall in many parts of the region, there has been an increased incidence of canola white leaf spot.
Growers of canola are therefore being advised to take a considered approach to disease management this year.
Oilseeds disease expert Dr Steve Marcroft, who undertakes research through a Grains Research and Development Corporation (GRDC) investment, says seasonal conditions have so far not been conducive to the severe development of the blackleg fungus.
“Blackleg likes continual wet conditions for spore release and germination, which is why blackleg severity on seedlings was so high in 2016,” says Dr Marcroft, of Marcroft Grains Pathology.
“In contrast, a large area of southern Australia has received rainfall in only a couple of major rain events, and conditions have remained dry between these events. Consequently, blackleg lesions are only starting to occur now.”
Dr Marcroft advises that if crops are already past the vulnerable seeding stage (1-4 leaf) and have no or few lesions, these crops most likely will not develop severe crown canker and therefore may not benefit from a foliar fungicide application.
“However, if a crop was sown later, has a moderately susceptible (MS) or lower blackleg rating and is currently still in the vulnerable seedling stage, it may develop severe crown canker and therefore benefit from a foliar fungicide application,” Dr Marcroft says.
“Growers should monitor crops for blackleg lesions on the first four leaves, estimate the potential crop yield and decide if it economical to protect the crop.
“Foliar fungicide has the highest efficacy against blackleg crown canker if applied at the 4-6 leaf stage, but is still very effective up to the 8-9 leaf stage.
“If growers are unsure about the blackleg severity on their crop and the potential yield, they can wait until the 8-9 leaf growth stage and then make a disease management decision.”
Meanwhile, Dr Marcroft says an increased incidence of canola white leaf spot has occurred in many parts of southern Australia this season.
“The disease is distributed worldwide, but in Australia it is not usually severe enough to cause yield loss. However, if environmental conditions are favourable, it can result in significant defoliation, causing reduced plant vigour and subsequent yield loss.
“White leaf spot is not usually severe enough to warrant fungicide control.”
White leaf spot normally only occurs on oldest leaves near the soil surface. Occasionally it is observed moving up the canopy, infecting younger leaves and reducing the leaf area significantly – this is when yield loss may occur. It is not currently well understood why it can occasionally become more severe in individual regions and seasons.
Nutrient-deficient canola crops can be more severely affected by the disease.
The symptoms of white leaf spot are leaf and stem lesions that are greyish white to light brown. Leaf lesions can be up to one centimetre in diameter and coalesce to form large irregular-shaped lesions.
Mature lesions often have a brown margin. White leaf spot lesions do not contain pycnidia (black dots) which are characteristic of blackleg.
Beyond this growing season, Dr Marcroft encourages an integrated approach to managing white leaf spot. This involves controlling cruciferous weeds and volunteer canola, employing strategic crop rotations and reducing infection from wind-borne spores by not sowing near the previous year’s canola stubble (both the white leaf spot and blackleg fungus survive on canola stubble).
The disease is not usually seed-borne, but it can spread by infected seeds or infected debris with the seed.
Comprehensive information on management of white leaf spot, blackleg and other canola diseases can be found in the GRDC’s Diseases of Canola and Their Management: The Back Pocket Guide, which can be viewed and downloaded via https://grdc.com.au/GRDC-BPG-CanolaDiseases.
Further information specific to blackleg disease can be found in the GRDC’s Blackleg Management Guide, https://grdc.com.au/GRDC-FS-BlacklegManagementGuide.
Image S Marcroft