Rule of Thumb: Avoid Blackleg: five golden rules

Beating Blackleg in 1999, Agriculture Western Australia

Australian canola growers, buoyed by good prices in 1998-99 but wary of the effect blackleg could have on 1999 plantings, need to abide by four golden rules:

  • select the variety with the highest resistance for the district
  • avoid paddocks with canola residues, especially 1-2 years old
  • reduce existing canola residues by grazing, burying, raking or burning
  • consider the risk level and whether or not to use the fungicide Impact®.

And the fifth rule, play safe with a 'one-in-four rotation' for canola on any given paddock.

Blackleg is spread by wind, with the heaviest spore fallout occurring within one kilometre of any canola residue.

Each year, canola residue continues to produce spores at a diminishing rate until the stubble has completely broken down. "In WA this breakdown could take up to four years, hence we recommend a one-in-four rotation," said Paul Carmody, Principal Oilseed Agronomist with Agriculture WA.

Caused by the fungus Leptosphaeria maculans, blackleg is the disease canola growers most dread.

According to Mr Carmody, each farm has its own unique environment which determines how the disease develops "The more canola residue and the closer its proximity to a new crop, the greater the risk of severe infection in the emerging crop." Seasonal conditions also influence disease levels.

Blackleg can attack a canola crop at any stage, but early infections are most critical in terms of canker development and yield loss. All varieties of canola seedlings are susceptible to blackleg until they achieve a 'degree' of adult plant resistance, usually at about the six-leaf stage.

Mr Carmody said canola in higher-rainfall areas (greater than 450 mm) was at greatest risk of blackleg.

For WA growers in the higher-rainfall zone he advised: "sow only varieties with WA adult disease scores of six or more, such as Dunkeld, Scoop, Rainbow and Grouse".

Program 2.5.2 Contact: Mr Paul Carmody 08 9690 2173