Spot the disease

June 27, 2017

Although there is less chance of blackleg disease on canola this season, white leaf spot has been identified on many crops in the southern cropping region.

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.

Canola growers are therefore being advised to take a considered approach to disease management this year.

Oilseeds disease expert Steve Marcroft, who undertakes research through a Grains Research and Development Corporation investment, said seasonal conditions had 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,’’ Dr Marcroft, of Marcroft Grains Pathology, said.

‘‘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 said if crops were already past the vulnerable seeding stage (one to four leaf) and had no or few lesions, these crops most likely would 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,’’ 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.’’

Meanwhile, Dr Marcroft said an increased incidence of canola white leaf spot had occurred in many parts of southern Australia this season.

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