Blackleg stewardship - taking a proactive & integrated approach

Yield is the key criteria to maximise profitability in canola.

Canola growers and advisors are encouraged to take action early each season and be proactive in developing an integrated approach to managing blackleg in canola.

Promoting these key messages makes good sense and encourages responsible stewardship in managing the risks of blackleg infection and resistance.

Making priority canola hybrid selections regarding major resistance genes and seed dressing fungicides are not enough to support the long term management of the impact of blackleg.

Over reliance on any single such tactic needs to be put into perspective in order to make informed decisions to minimize blackleg risk and maximise yield potential of canola.

We must ALL acknowledge “resistance is a reality”.

Successful people will manage herbicide resistance and likewise blackleg resistance. Growers and advisors do not need bad news or scare tactics.

We all have a choice to add diversity to weed and disease management as resistance is identified and/or evolves. These measures aim to reduce the yield and quality losses associated with the disease but are not likely to eliminate the pathogen or prevent herbicide resistance.

Relying on herbicides alone to manage weeds is not the answer, as research and experience on-farm have clearly shown in Australia and globally.

Regional disease monitoring over many years across Australia, has demonstrated that field populations of L. maculans (blackleg) have high evolutionary potential and rapidly adapt to selection pressure from sowing cultivars with major gene resistance, which can lead to resistance ‘break- down’ within a few years of hybrid release.

Major gene resistance is a gene-for-gene interaction with the pathogen having a virulence gene corresponding to the resistance gene in the host canola plant. Changes in the frequency of L. maculans isolates virulent on cultivars dependent on major gene resistance have led to a ‘boom and bust’ type cycle and resulted in severe yield losses in Australian grown canola.

Breakdowns of blackleg resistance relying on major gene resistance have been reported from commercial field situations in Australia in the past.

ALL Pioneer® brand Y series® and T series® canola hybrids strategically deploy a combination of quantitative (polygenic) and qualitative (major genes) blackleg resistance. Research and experience have proven that quantitative resistance used in an integrated approach with cultural practices and fungicides (seed/foliar) is effective in protecting yield potential while contributing significantly to durable resistance over more years in the field.

Rotation of cultivars every 3 years can influence shifts in avirulent allele frequencies of L. maculans. Validation of the success from rotation of cultivar resistance groups (based on presence of a major gene) over the longer term is inconclusive.

An integrated strategy involving cultural practices and fungicide use should be recommended to help preserve the effectiveness of genetic resistance leading to the durability of the resistance in the long-term.

Responsible stewardship programs for blackleg should include:

1. Know your risks by monitoring the incidence and trends in severity of stem cankering at windrowing the preceding canola crop and scouting early in the season for leaf lesions in newly sown canola;
2. Cultural practices, particularly less intensive rotation of canola, isolation from last year’s canola stubble and agronomic practices such as early sowing in April can help to escape high risk of blackleg infection in field situations in addition to managing other risk factors (e.g. pest damage, losses from heat/soil moisture deficit/late frosts in spring) that may limit yield potential;
3. A higher priority is to separate canola from last year’s stubble particularly in high-risk situations using a farm rotational planner and talking to neighbours;
4. Always use the latest hybrid canola blackleg ratings as the virulence of local blackleg populations may change through crop rotation cycles;
5. Avoid blanket recommendations from advisors to plant only one canola hybrid in a district or rely on the same variety over more than two (2) years;
6. Using fungicides on seed and applied to fertiliser when planting canola particularly in tight rotations is a ‘critical’ best management practice that will reduce fungal infections up to the 3-5 leaf stage;
7. Strategic deployment of foliar fungicides complements other stewardship measures to reduce the severity of stem cankers that in turn helps to protect yield potential;
8. Avoid relying on one tactic only to manage resistance (herbicide, disease) and follow-up with another tactic in an integrated double/multiple knock approach.

In conclusion, the use of race-specific major gene resistance to manage blackleg has been beneficial to improve variety blackleg ratings of new canola hybrids. On-going high priority has been given to the development of new hybrids with novel sources of resistance by all proprietary breeders.

But this is NOT the ‘silver bullet’ solution and should NOT replace industry stewardship programs being proactive in recommending an integrated strategy to manage blackleg and increase yield potential in Australian grown canola crops.

For more information contact your local Pioneer Seeds representative.

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