A mixed bag

Season 2017 is producing mixed fortunes in terms of canola diseases for growers in Australia’s southern cropping region.

While the risk of blackleg disease has reduced with the lack of consistent rain in many parts of the region, there has been an increased incidence of canola white-leaf spot.

Growers of canola are therefore getting advice to take a considered approach to disease management this year.

Oilseeds disease expert Dr Steve Marcroft of Marcroft Grains Pathology, undertaking research through Grains Research and Development Corporation investment, said seasonal conditions had so far not been conducive to 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,” he said.

“In contrast, a large area of southern Australia has received rain in only a couple of major weather events, and conditions have remained dry between these events.

“Consequently, blackleg lesions are only starting to occur now.”

Dr Marcroft advised that if crops were already past the vulnerable seeding stage, one to four leaf, and had no or few lesions, these crops were unlikely to develop severe crown canker and therefore might not benefit from a foliar fungicide application.

“However, if a crop was sown later, has a moderately susceptible or lower blackleg rating and is currently still in the vulnerable seedling stage, it might develop severe crown canker and therefore benefit from a foliar fungicide application,” he said.

“Growers should monitor crops for blackleg lesions on the first four leaves, estimate the potential crop yield and decide if it is economical to protect the crop.

“Foliar fungicide has the highest efficacy against blackleg crown canker if applied at the four to six-leaf stage, but is still very effective up to the eight to nine-leaf stage.

“If growers are unsure about the blackleg severity on their crop and the potential yield, they can wait until the eight to nine-leaf growth stage and then make a disease management decision.”

Dr Marcroft said an increased incidence of canola white leaf spot had 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,” he said.

“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.”

Dr Marcroft said white leaf spot normally only occurred on oldest leaves near the soil surface.

Beyond this growing season, Dr Marcroft encouraged an integrated approach to managing white leaf spot, including controlling cruciferous weeds and volunteer canola, employing strategic crop rotations and reducing infection from windborne spores by not sowing near the previous year’s canola stubble, because both the white leaf spot and blackleg fungus survived on canola stubble.