While swathing napus canola last year, some Prairie farmers reported an odd phenomenon: blackish-green dust was covering their swathers. The cause, says Clint Jurke, a Canola Council of Canada agronomy specialist, was more than likely alternaria black spot, and more of a curiosity than cause for concern.
“The windows of their swathers were turning blackish-green. As far as I know, it didn’t cause any significant yield loss in those situations. (Alternaria black spot) comes in after the yield has already been set,” says Jurke. “It’s typically not a disease we’re all that concerned about anymore.”
This was not the case two decades ago in Western Canada when Brassica rapa species (Polish canola) made up roughly half the canola acreage and B. napus (Argentine canola) the other half. The thick, waxy coating covering the leaves and stems of B. napus varieties protects plants from alternaria spores penetrating their tissues. This waxy layer is much thinner on B. rapa and B. juncea species, which presently make up less than one per cent of the total canola acreage in the West, increasing their susceptibility to the disease. If the disease sets in early enough, significant yield loss of rapa and juncea varieties is possible.
For napus varieties, alternaria black spot — which is caused by the fungi Alternaria brassicae, A. alternate and A. raphani — usually moves into a crop after the waxy layer starts to wear off near the end of the season as plants are maturing, drying down and filling the seed. The pathogen can then colonize the plant tissue.
Colonizing fungi can turn swaths black and sooty-looking under humid conditions, but they still aren’t going to cause much of a yield loss, says Jurke. “For most canola acres, alternaria is not much of a production risk,” he says.
However, if the canola lies in the swath for a long period of time, alternaria can weaken the pods’ ability to stay together. Strong winds, heavy rains or hail may shatter pods even further. “If a grower has a huge canola acreage and he knows his canola is going to lay in the swath for six weeks or so, in circumstances like that there’s more opportunity for shatter to start occurring. Then you might want to think about taking additional action on controlling alternaria in those fields,” says Jurke.
Under most conditions, when growing napus canola applying crop protection products to control alternaria doesn’t add up. “It’s such a random event that it’s pretty hard to plan around, and to spend money on a fungicide to control the disease… you’re looking at $25 to $28 per acre for something that’s very unlikely to cause yield loss. The likelihood of getting a positive return on it is pretty small,” he says.
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Early onset of alternaria black spot has occurred in napus canola under exceptional conditions. Removal of the waxy layer on the leaves and stems as a result of hail or sand blasting can result in increased levels of alternaria early in the season. “In those cases, it might be better to leave your canola standing so it’s not laying in swath and shattering out more in the swath. When the plants stand they can remain a little bit drier and hopefully can be straight cut at the right time,” says Jurke. “Or if a grower swaths it, then combine it first and do not wait.”
While alternaria black spot may be one less thing napus canola growers have to worry about, the disease does remain a real threat to juncea growers in southern Saskatchewan and southern Alberta and rapa growers in Alberta’s foothills and Peace River region. Control of the disease through fungicide application or four-year rotations is more of a priority for these growers in order to reduce the amount of alternaria in the field.
“It’s sort of like blackleg — it comes from old canola residue. As long as that residue persists, there’s always alternaria around,” says Jurke.
He suggests growers spraying for sclerotinia also control alternaria with a second application of fungicide, especially in years of increased precipitation. “The wetter the year, the greater the chance of alternaria infection in those juncea and rapa crops,” he says.
Infection occurs around flowering, halting the processes that produce yield and interfering with photosynthesis, nutrient flow and pod and seed filling, says Jurke. “If you don’t control the disease at the appropriate time for those two species, then they’re going to lose yield.”
Although it’s impossible to tell this early in the season if conditions will favour alternaria infection and spread in napus canola crops at swathing, blackish-green dust covering farmers’ swathers this fall should be less a concern and more of a conversation piece.