Blackleg decision: When does a fungicide pay?

June 6, 2018 – Issue 10

Blackleg infection at or just after the cotyledon stage is the mostly likely to cause any significant yield loss. If fungicide to control blackleg is to provide any economic benefit, it has to be applied very early in the season and the crop has to be at risk of early and fairly widespread infection.

With good management, including a two-year break between canola crops, growing blackleg resistant varieties and rotation of R genes, fungicide is not necessary. In that situation, fungicide to reduce blackleg losses does not usually provide an economic return.

Blackleg lesion on cotyledon.

Fungicide application can provide an economic benefit if:

–The field has a recent history of rising blackleg infection rates, and
–The field is in a tight canola rotation (with a one-in-two-year rotation, a large percentage of blackleg spores on stubble will still be viable), and
–The field is being seeded to the same variety as two years ago (or to a variety without its R gene identified, which means it could have the same major gene resistance as the previous variety), and
–Recent rainfall provides the weather conditions for spore germination and infection (it just takes a few mm of rain and warm temperatures to get pseudothecia to germinate, and they can produce spores for days), and
–The fungicide is effective on blackleg (in a recent blackleg study, Tilt was not as effective as other products on the market), and
–The fungicide is applied early. The earlier the better but follow product labels.

Other helpful decision factors could be:
–Careful scouting shows that a high percentage of stubble pieces from the previous crop have pseudothecia.
–Earliest cotyledons already have small blackleg lesions (which are very difficult to see). Fungicide are protective – they cannot cure existing infections – but these early lesions could be a sign of more to come.
–Plants have entry wounds from hail, flea beetle feeding or other reasons. Blackleg can be taken up by the plant’s stomata, but entry wounds allow for more infection.

Fungicide will be $10 to $15 per acre, depending on the product and rate. If the situation suggests a return on investment is possible, fungicides can be tank mixed with herbicide as long as herbicide is applied early.

Further reading:

Tank mixes of herbicides, insecticides and fungicides
Canola Council of Canada has a new brochure “Understanding Blackleg Resistance”
Canola Encyclopedia blackleg section

Canola Watch