Crop advisor casebook: Canola shelling out while swathing

A Crop Advisor's Solution from the January 5, 2020 issue of Grainews

There were many unhealthy-looking plants with blackened stems that had developed cankers near the soil surface. The plants were also suffering from premature stalk drydown.
Ty Moffat. photo: Supplied

Jeff is a farmer located near Brandon, Man., who grows canola, wheat and soybeans. He was out swathing a canola field in early September when he decided he should give me a call. The crop, which was at 70 per cent seed colour change, appeared to be shelling out while it was being cut and packed by the swather, and Jeff couldn’t understand why.

It was the same canola hybrid he had planted in previous years — a later-maturing variety that featured blackleg resistance and strong pod integrity to prevent shattering — but this was something he hadn’t seen before.

Jeff said he hadn’t completed swathing and he was hoping I could come by to check things out before he finished cutting the crop. The forecast called for heavy rain, so I said I’d be there in a couple of days to have a look.

When I arrived at the canola field, I observed that Jeff had swathed three headland passes and the rest of the crop was still standing.

Walking through the parts of the field where canola had been swathed, I could see lots of volunteer canola sprouting up, which didn’t surprise me since quite a bit of rain had fallen in the days since Jeff had stopped swathing. Elsewhere, I saw very few canola volunteers, and I also observed the majority of canola plants had intact seed pods.

Looking around though, it was clear something was wrong with the crop. Many of the canola plants had completely dried down, and I could see lots of unhealthy-looking plants with blackened stems that had developed cankers near the soil surface.

At this point, I had a good idea what was going on in Jeff’s canola field, and when we sent some plant stem samples off for testing, the results confirmed my diagnosis.

Crop advisor’s solution: Blackleg to blame for canola shelling out

A closer look at the canola plants revealed the shelling out had occurred because much of the crop had died prematurely. And the reason for that was blackleg, which appeared to be widespread throughout the field. The blackened stems and cankers I observed on sickly-looking plants were all symptoms of the disease.

This was confirmed when the plant stem samples I sent off for testing came back from the lab showing blackleg was present. In addition to blackleg, alternaria black spot and verticillium stripe were present to a lesser degree than blackleg.

Jeff wondered how this could have happened, since the canola variety he’d planted was rated R for blackleg resistance. I explained resistance groups detected in the plant stem samples all belonged in resistance group A, whereas the variety that Jeff used belonged in resistance group C, which is why it was ineffective against the kind of blackleg present in the canola field.

I stressed to Jeff that planting any blackleg-resistant canola hybrid shouldn’t be considered a silver bullet solution to managing blackleg incidence and severity. There are ten resistance groups of blackleg that have been identified in canola and not all of them can be mitigated by using hybrid genetics.

I also reminded Jeff that crop rotation is the most powerful tool farmers have to manage blackleg inoculum loads in the field. Short crop rotations can aggravate disease incidence by creating conditions for new pathogens to develop. Taking at least a two-year break between canola crops in a field can help prevent spore loads from building up in the soil and slow down the evolution of blackleg pathogens.

In addition to lengthening the overall rotation, canola hybrids with different blackleg genetics should also be rotated to ensure the same genetics are not being used over and over again.

Ty Moffat, AIT, works for Richardson Pioneer Ltd. in Brandon, Man.

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