Canola Watch: Blackleg | Swath timing

‘Tis the season for disease sampling! If you’re wondering how to submit verticillium stripe or blackleg samples, click on the three-minute video above as CCC agronomy specialist Breanna Miller prepares a sample for PSI labs. 

Why are CCC agronomy specialists so concerned about blackleg this year?

A wet spring and hot, dry finish to the season are ideal conditions for blackleg development. Many growers are reporting significant infection; many others likely don’t yet realize how much the disease is cutting into their yield (or even that their field is infected). Whereas a disease like aster yellows is highly visible but generally has relatively low yield impact, blackleg can be largely hidden but have high yield impact. Growers who have not yet committed to enhanced blackleg management (race testing, cultivar selection, seed treatments) could be among those who experience up to 60% yield loss. Blackleg’s characteristic black specks (fungal fruiting structures) on old canola stubble/residue can be scouted in all crops and in all seasons of the year. We have tools for this disease! The blackleg pathogen takes years to disappear from a field, so submit stem or stubble samples for testing, then watch this How to interpret blackleg race tests video to help you choose resistant cultivars for next year. Growers who choose a cultivar without a major resistance gene should at the very least opt in on a blackleg seed treatment (Saltro or Lumiscend). (Blackleg loss calculator) (Six basics of blackleg management) (Canola Encyclopedia: blackleg)
 

Is it still too early to swath?

A CCC survey in 2020 showed that nearly half of Canadian canola growers swath too early. Patience will be rewarded: correct swath timing, which means waiting until the crop reaches a minimum of 60% seed colour (SCC) change, has been proven to be a major contributor to seed yield, seed weight and seed quality. To assess SCC, divide the main stem (or highest yielding portion of the plant) into thirds, then look at the seeds from each third. At 60% SCC, seeds from the top third of the main stem will still be green but firm to roll. Seeds from the middle third will be mostly brown and have some speckling and mottling. Seeds from the bottom third will be the most mature and will have completely turned brown/black in colour. (Swath later for higher yield) (Canola timing, quality and yield)
 

What late-season pests to watch for this week?

Flea beetles have been reported in relatively high numbers in some fields. They are unlikely to cause economic damage at this time in the season. Only consider a pesticide application if flea beetle feeding is shattering pods and/or damaging pod integrity. Isolated pockets of bertha armyworm pressure remain. Use this guide to effectively monitor bertha armyworm adults and larvae. Crickets are causing some crop damage in several southern locations, but keep in mind that some crickets offer benefits too: fall field cricket adults eat grasshopper eggs. Regardless of pest type, only apply an insecticide if the population exceeds economic threshold, and only if there remains an adequate pre-harvest interval. (Insect scouting guide) (Field guide)
 

Why are so many canola plants suddenly brown and dead?

Unhealthy and odd-coloured plants may have been in the crop for some time, but they become much more visible at this time of year when they die ahead of a still-green crop. Many diseases can contribute to root rotting, blackened stems, and premature wilting, browning and dying. Proper disease assessment requires hands-on scouting and a pair of clippers. Pull plants and look for:

• Blackleg: cut cross-sections of the stem at the soil surface, looking for black tissue on the inside and brown woody exterior cankers. 
• Sclerotinia: look for white stem lesions. Infected stems eventually bleach and often shred and shatter very easily, revealing white moldy growth and small (less than 2 cm), hard, cylindrical sclerotia.
• Clubroot: look for root galls, which limit nutrient and water uptake and can cause premature ripening, wilting, yellowing and plant death.
• Verticillium stripe: look for stunting and premature aging. Infected stems will show faint vertical striping, though usually not until around harvest. Peel back the outer skin of the stem to look for blackening on the inside of the stem. Verticillium stripe attacks from the soil and appears as a grey starburst pattern or discolouration. May be in same plants as blackleg. 

Found disease? Send dry samples to a lab for disease identification and, in the case of blackleg, race confirmation. (Tips on how to disease scout) (Verticillium stripe or blackleg?) (Canola disease scouting guide)