Blackleg (Leptosphaeria maculans)

Blackleg: biology

Blackleg is a stubble-borne and seed-borne disease (primarily caused by the fungus Leptosphaeria maculans) that affects canola across all growing areas in western Canada.

According to the Canola Council of Canada, blackleg fungus can overwinter on infected residue for several years, or until the residue breaks down. Estimated average yield losses of less than 10 per cent – or – in extreme situations – 30 to 50 per cent, have been reported in individual fields.

Blackleg is not a new disease and can be found everywhere. For many years, blackleg was effectively managed by resistant varieties and longer rotations.

But over time, researchers have found that blackleg races can change and adapt, resulting in a steady increase in disease incidence over the past 10 years. Researchers are also finding that the timing of blackleg infections can have a major impact on canola.

Early, airborne blackleg infections are mainly responsible for the severe stem infections that occur later in the season. A study published by Dr. Gary Peng at Agriculture and Agri-Food Canada (AAFC) in the Canadian Journal of Plant Pathology in 2020, stated that effective control of early blackleg infections (at the cotyledon stage) can reap the biggest benefits for preventing later infections.

• After infecting the cotyledon, the blackleg fungus moves from the cotyledon to the stem. This restricts the plant’s ability to access moisture and nutrients that are crucial for final yield.
• Initially, white to grey spots speckled with black dots develop on cotyledons and rosette leaves. Similar lesions – elongated and with dark margins – develop on stems.
• Lesions at the stem base may cause cankers (shrunken, pinched-off areas) to form. This leads to premature ripening and severe lodging. High levels of stem cankers and lodging can lead to almost total crop loss.

Scouting

• Prior to seeding: Scout your fields for signs of blackleg – black, speckled fruiting bodies called pseudothecia – on older canola stubble.
• In the spring: Scout during the vegetative stage (from 3 to 6 leaf). Look for greyish-white stem lesions or lesions on the leaf surface with black specks.
• In the fall: Scout immediately after swathing or straight cutting to give yourself the best chance of correctly identifying the disease as blackleg. Cut canola plants at the soil surface and look inside the stem’s diseased tissue. If you do see stem infection, send samples to your local lab to confirm the presence of blackleg.