Dry June causes canola blackleg risks

Blackleg was a significant issue for canola growers last year.
Blackleg was a significant issue for canola growers last year.

A DRY start to winter has contributed to low levels of blackleg in canola throughout the region.

In other areas where the start to the season has been more favourable, white leaf spot is showing up in some canola crops.

Downy mildew is also starting to appear in some canola crops and, if conditions are conducive later in the season, sclerotinia stem rot could make an appearance in spring during flowering.

Marcroft Grains Pathology principal Steve Marcroft said it was difficult for growers to develop a holistic strategy that considered all canola diseases.

However, in considering fungicide control, he said it was important growers weigh up variety choice, seasonal conditions and level of disease risk and seed or in-furrow applied fungicides.

Blackleg was a significant issue for canola growers last year, largely thanks to the above average season.

The disease thrives under continual wet conditions for spore release and infection.

However, seasonal conditions to date this year have not been conducive for blackleg in most areas of the southern region.

Dr Marcroft said fungicide application was recommended in the lead-up to the 2017 season to protect moderately susceptible cultivars, particularly those sown in the medium and high-rainfall zones of the southern region where blackleg risk was higher.

“If a canola crop is already past the vulnerable seedling stage and has no or few leaf lesions, it is likely that plant won’t develop severe crown canker and might not benefit from a foliar fungicide application,” he said.

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

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

Dr Marcroft said foliar fungicide had highest efficacy against blackleg crown canker if applied at the four to six leaf stage, but was 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 before making their decision,” he said.

Because of the favourable conditions and  high yields in 2016, many growers had great success and good economic return from applying a fungicide spray to control blackleg. 

Dr Marcroft said it was too early to tell whether what effect the low levels of blackleg would have on the severity of upper canopy infection later in the season.