Blackleg An Increasing Risk, Even With ‘Resistant’ Varieties

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The word is resistant, not immune. That s the word from plant pathologists and the Canola Council of Canada when it comes to managing blackleg. Resistant varieties help, but the disease still requires management.

Blackleg was a real problem that Alberta had ample warning about as pathologists and breeders saw it coming across the Prairies from the east in the 1980s, says Alberta Agriculture plant pathologist Ron Howard. Breeders incorporated resistance into their varieties and it seems we became complacent about it.

The Chinese woke us up when they flagged it as an issue in canola seed they imported, because they hadn t had the disease in their crops. Disease issues can make or break an export market, so we ve been looking for blackleg more and we see it could become a real problem again, Howard said.

Two things could allow blackleg to become a major disease problem again. Scientists thought there were four or five strains of blackleg, now they believe there are 16 strains and more could be evolving. Each strain may overcome resistance genes that have protected crops against other strains of the fungus. The other issue is that the price of canola is so attractive some farmers are growing canola in very tight rotations. Any fungus that overcomes the crop s resistance can survive to infect future crops.

Growing resistant varieties doesn t entirely protect you from blackleg. To be rated as R, or resistant to the disease, it needs only to reduce the severity of the disease by 70 per cent. Rotation is the main tool to control blackleg. The longer the break between canola crops, the greater the opportunities for soil bacteria and other organisms to consume the blackleg fungus.

Change it up

If you must grow canola on canola or after only a one-year break between canola crops, change varieties, even change herbicide-tolerant systems, advises Troy Prosofsky, Canola Council agronomist. Also, monitor all your canola crops for signs of blackleg so you can adjust your crop management and avoid a serious outbreak. He advises watching for blackleg-affected plants when you re scouting for other problems in canola, starting in early spring, blackleg can infect canola as early as the cotyledon stage.

Watch for spindly unhealthy plants early in the year, or premature ripening later in the summer, he says. Pull those plants and look for light-coloured cankers at the base of the stem.

Blackleg blackens part of the stem. Around the canker little black, slightly raised spots, like whiskers, are the pycnidia, or fruiting bodies. In damp weather these can ooze pink spore masses that can spread from rain splashes to other parts of the plant and to others nearby. As the plant grows, the canker lesion can move through the plant to form a stem canker, usually at ground level. The fungus can fill the stem and cut off moisture and nutrients passing between the roots and above-ground parts of the plant and may cause it to topple over.

Infected stems and roots, which are woody and resist decay, can allow the fungus to survive in soil for some years. Fungus in old crop residue can reproduce sexually (the source of genetic exchange that leads to new strains of fungus) to produce spores that are carried on wind currents sometimes as much as five km.

Some fungicides can protect crops against future infection with blackleg, but Prosofsky doesn t advise it. It s already too late then, he says. We need to manage blackleg by using different resistance genes. It s tricky because the sources of resistance are the seed companies proprietary information. So we just advise changing varieties, and if you can, longer rotations.

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TheChinesewokeusup whentheyflaggeditas anissueincanolaseed theyimported.

Ron Howard

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