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Think long-term with blackleg management

Current blackleg infection rates will not have most canola growers shaking in their boots. While blackleg was found in 65 per cent of Manitoba canola fields last year, average incidence within those fields was relatively low at 7.1 per cent of plants infected (Debbie McLaren, AAFC Brandon). That’s up only slightly from 61 per cent of fields and 6.1 per cent of plants in 2006.

Here is a typical blackleg stem lesion. Notice the black rim around the lesion, the lighter grey colour inside the lesion and the black picnidia — spots — that do not rub off easily. The best time to assess blackleg is the week of or the week before swathing. While it is possible to look for infected residue from previous canola crops at seeding, or look for leaf lesions from the cotyledon stage onward, assessment of the whole plant and particularly the basal stem a few days before swathing gives the best indication of the incidence and/or severity of the disease.

In Saskatchewan, the percentage of fields infected went from 38 per cent in 2006 to 44 per cent in 2007, but the incidence dropped from 5.2 per cent of plants infected in 2006 to only 3.0 per cent in 2007 (Penny Pearse, Saskatchewan Ministry of Agriculture). With a resistant variety, most canola growers are seeing no major increase in blackleg — in the short term.

However, some individual fields did have significant blackleg infestations last year, driven in most cases by a combination of ideal weather conditions for the disease and less than ideal management. At the very least, it’s time to think again about your blackleg control strategies.

3 critical steps for blackleg management

1. Variety selection.

Most of our current varieties are resistant (R) or moderately resistant (MR) to blackleg. Before any new varieties are recommended for registration they are tested for their blackleg tolerance at various locations across the Prairies, and rated based on their performance relative to the check varieties. These ratings represent a variety’s blackleg tolerance relative to the highly susceptible variety Westar, but only to the spectrum of blackleg strains present at the field trial locations. Even within the group of R rated varieties, there is a wide range of performance. As the blackleg population shifts to other pathogenicity groups (PG), some varieties that were once R may drop to MR and possibly susceptible depending on their tolerance to the new races.

Although there has not been a thorough examination of the sources and types of resistance used in current varieties, research indicates that the resistance carried by some varieties may depend on relatively few genes. If the blackleg pathogen population has not adapted to these specific resistance genes in the variety grown, then that variety will have a high level of resistance to the disease. A recent study suggests that the pathogen population has adapted to some of the resistance genes that were introduced to Argentine canola in the past. But the pathogen has not adapted to other resistance genes, which should be good sources of resistance. Unfortunately, some of the best genes for resistance are in other brassica crops — not Argentine canola, which means it will take some breeding efforts to utilize these new resistance sources.

Ideally, what we’d like is a list that indicates the type of resistance and identifies the specific resistance gene(s) in each canola variety, and then a record of the blackleg races present at various locations across Western Canada. With this information farmers would be able to know that the canola varieties they’re using have the appropriate resistance for the blackleg races in their fields. However, that will require a significant amount of research and intensive field surveys to characterize both the resistance mechanisms and the fungal population.

2. Crop rotation.

Blackleg survives in infected stubble pieces, sending out airborne ascospores to infect more crops in subsequent years. If you plant canola on canola, the amount of infected residue increases and the ascospores don’t have far to travel to infect your canola. If you wait three years, or more, the infected residue will break down, dramatically reducing the blackleg spore production within that field.

While this is the recommended practice, farmers are tightening canola rotations and so far, are not seeing a huge rise in blackleg. That will continue as long as they continue to utilize R rated varieties and the PG status of the field does not change. Over time, however, this selection pressure will force the blackleg population to shift to more virulent strains in order to survive.

3. Field scouting.

If you’re not spraying for blackleg (and hardly anyone does anymore), at least check the field just prior to or during swathing to see how much blackleg infection you’ve got. Ideally, pick 20 plants at five locations randomly throughout the field to get a reasonably accurate assessment. Like most diseases, blackleg can vary over a field, so one corner or side of the field may be worst than other parts of the field. Record the number of plants infected, being sure to accurately identify the cause of any symptoms. Also record the field location, variety, fertility and yield. Tracking this information over time can help spot any trends of increasing disease levels, signaling the need for a change in management practices.

Over years, the information will give you a better idea of what level of blackleg you have, how particular varieties are holding up, and the effectiveness of your management practices to control blackleg.

Identification of the disease may still be an issue. Blackleg is sometimes confused with greystem. But greystem comes later in the season, flourishes (or at least is more noticeable) after swathing/harvest, and is more abundant on Polish than Argentine canola. Generally it is not believed to have a significant affect on yield. So a little time spent doing a survey should help you learn to recognize blackleg — and other disease as well.

Surveying just prior to swathing is also a good time to assess many other diseases, including sclerotinia stem rot, alternaria black spot, fusarium wilt, clubroot et cetera.

Foliar spray

This is not a common practice, and rightfully so. It generally does not pay to spray for blackleg. AAFC research in Saskatchewan found little to no difference in blackleg severity in sprayed versus unsprayed plots, especially in the resistant varieties. Even in the susceptible cultivar, differences rarely produced economical yield advantages.

— Derwyn Hammond is a Canola Council of Canada agronomy specialist based in Brandon, Man. Randy Kutcher is a research scientist with Agriculture and Agri-Food Canada in Melfort, Sask.

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