ProCrop

Managing Blackleg in Canola

Blackleg is present in most canola areas in North Dakota, and is most common in the northern part of the state. The blackleg fungus survives on infected canola crop residue. Canola residue can be present for three years following canola production in North Dakota conditions. Wild mustard is also an important host. The blackleg fungus may also occur at low levels in the seed. Seed infection is seldom more than 1 or 2%, but this amount can introduce the fungus into new areas.

There are different strains of the blackleg fungus known as pathogroups (PGs). These PGs are characterized by their ability to cause disease (aggressiveness). In North Dakota, PG1 (less aggressive) and PG2 (highly aggressive) are common. In the 2003 growing season, PG3 strains were identified in Manitoba, Canada. The PG3 strains have the ability to cause disease on some canola varieties that are resistant to PG1 and PG2 strains. Variety resistance varies depending on the group(s) present in a production area. The aggressive strains produce deep girdling cankers near the soil line. These cankers reduce plant vigor and may cause lodging and severe field losses. Highly aggressive strains are present in Saskatchewan, Manitoba, and in some areas of North Dakota.

The blackleg fungus is spread by rain-splashed spores, windborne spores, and by infected seed. Long distance spread to new production areas is usually by infected seed. The best way to prevent introduction of aggressive strains of blackleg into new production areas is to purchase certified seed with a phytosanitary clearance and have it treated with a fungicide. Treatment is highly effective and will eliminate or nearly eliminate seed borne blackleg. See the North Dakota fungicide guide at http://www.ext.nodak.edu/extpubs/plantsci/pests/pp622w.htm

Even if seed does test blackleg-negative, there could be a few infected seeds in the total seed lot. A fungicide seed treatment will decrease the chances of blackleg surviving on the seed; however 100% control is not always achieved. Do not treat below recommended rates. A fungicide treatment minimizes the risk of blackleg contamination if blackleg-free seed was accidentally mixed with infected seed during cleaning operations. Recommended treatments may also provide protection against other soil-borne diseases and flea beetles.

There is always a possibility that blackleg could be introduced by wind-borne spores. Longer rotations will increase the probability that low levels of disease infection will disappear when the infected stubble has rotted. Practice proper weed control, particularly of volunteer canola and wild mustard. If volunteer canola and mustards are not controlled, a long crop rotation will be ineffective. In areas where blackleg is prevalent, seed treatment with fungicide for blackleg may be of limited value.

Do not grow canola in a field more frequently than once every three or four years, especially in areas where the presence of aggressive blackleg has been confirmed . In one research study in the mid 1980's in Canada, it was shown that in all the fields which had canola previously, most of the infection appeared to come from old infective residue within the field itself and not from adjacent fields. Longer rotation reduces or eliminates the amount of early infections in the rosette stage that cause the most yield loss. Planting canola every other year is not recommended for many reasons including disease concerns. In areas where aggressive blackleg has been confirmed, use a blackleg resistant variety when planting canola in a three year rotation or less.

Bury canola in the top 3-6 inches of soil in the field. Alternatively, where soil erosion is a problem or moisture conservation is needed, incorporate the canola stubble just before planting. This practice speeds stubble decomposition and reduces the disease infection potential in the field. In the following three seasons, use shallow tillage or direct seeding to avoid bringing infected canola residue to the surface. Additionally, plant non-host crops such as small grains, grasses, alfalfa, clover and pulses.

Keep infested land free of volunteer canola and any wild mustards. Volunteer canola that is allowed to grow in the spring can be a significant early source of inoculum that can cause destructive early (3-4 leaf) infections in nearby commercial fields

Do not plant canola seed from infested land.

While difficult in northern North Dakota, if at all possible, avoid planting canola adjacent to infested land for two to three years. A separation of � mile will reduce field edge infections. In areas where aggressive strains are present, a fungicide application postemergence to canola at the field edge may be effective in reducing infections. This practice has not been evaluated in North Dakota.

Where severe blackleg has become established, growers could consider crop spraying with a fungicide. See the ND fungicide guide for fungicides labeled for foliar control of blackleg. Very little work has been done in North Dakota to evaluate the economics or effectiveness of fungicide for blackleg management.

When considering which blackleg tolerant variety to plant, recognize that "resistant" or "tolerant" canola can still have infected stems. In areas where aggressive blackleg strains are prevalent, choose varieties with a R or MR rating. In areas where aggressive blackleg strains are at low levels or has not been found, all registered varieties are suitable. A variety with a rating of R is not immune to blackleg and can still have some level of disease. The current blackleg resistance rating system is based on a variety�s level of disease in comparison to the variety Westar, which serves as a susceptible check. A resistant variety must have shown disease levels below 29% of the disease level of Westar (see below). Disease severity levels are measured on a 0 (no disease) to 5 (dead plant) scale. For example, if Westar had an average rating of a 4 in the Canadian blackleg nurseries, and variety A had an average rating of a 2, then variety A would be considered MS because it had 50% of the disease that Westar did (2 � 4 = 50%) (see below).

A combination of management practices including, proper rotation intervals, good variety selection for disease tolerance, residue management, seed treatment and perhaps foliar fungicide when appropriate, will enable growers to manage blackleg disease and produce a profitable crop.

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