Blackleg of canola: scouting and variety rotation

Symptoms and where to look

Blackleg is a complex of two fungi. L. biglobosa is weakly virulent, infects late and is minor; L. maculans infects from the cotyledon stage onward and causes the loss. Infected residue releases airborne ascospores after rain (peak release several hours after more than 2 mm / 0.08 in), and Saskatchewan notes release can begin as early as May.

Leaves (June). Dirty white to greyish spots, round to irregular, dotted with tiny black pycnidia visible with a hand lens. Pycnidia ooze pycnidiospores that rain-splash onto neighbouring plants. Lower leaves of seedlings are the ones that matter: the fungus grows from the leaf down the petiole into the stem, and infection before the two-leaf stage is the kind associated with serious yield loss.

Stems (July to August). Lesions at the stem base or at leaf scars, white or grey with a dark border, often carrying pycnidia. Severe infection becomes a dry, sunken canker that girdles the stem near the soil line, causing premature ripening by mid-July and lodging. Cut the crown: virulent infection shows black or brown discolouration inside.

Look-alikes

Grey lesions higher on the stem are usually the weak L. biglobosa or secondary infection after hail; Saskatchewan notes upper lesions are far less damaging than basal cankers. Sclerotinia also bleaches stems and ripens plants early, but its stem is papery, often shredded, and splits to reveal black sclerotia with no pycnidia. Clubroot patches ripen early too; look for root galls. Fusarium wilt and drought leave the crown cross-section clean.

Conditions that favour it

Warm, humid weather with frequent showers during the seedling stage, infected canola stubble in or beside the field, and canola grown more often than one year in three. The Canola Council reports ascospores released from residue for at least three years and pycnidiospores for three to five, with ascospores travelling at least 10 km (6 mi) on the wind. Flea beetle wounds and hail give the fungus entry points. Seed is a minor source (under 1 per cent in Canadian seed).

Scouting and the stem clip survey

The Canola Council sets four scouting windows:

1. Before seeding. Check old canola residue for pseudothecia and pycnidia.
2. Cotyledon to two-leaf. Look for the first leaf lesions, especially near canola stubble.
3. Three- to six-leaf. Walk a W pattern and check at least 50 plants; lesions on more than 10 per cent of plants signal a risk of significant disease.
4. Swathing. The stem clip survey below.

How to do the stem clip survey. Timing is 60 per cent seed colour change, around swath timing, while plants are still green. Pull at least 50 plants across the field, clip each just below the crown, and rate the cross-section:

Record incidence (plants scoring 1 or more) and average severity. The Canola Council cites University of Alberta research of about 17 per cent seed yield loss per unit increase in average severity, with a newer model showing yield dropping sharply at severity 2 and above. A field averaging 0.3 is under control; one averaging 1.5 with a cluster of 3s and 4s is telling you the variety's major gene no longer matches the local races.

Resistance genes and rotating resistance groups

Canola varieties sold in Western Canada can carry a blackleg major-gene resistance group label alongside the R/MR rating. The groups, as listed by the Canola Council of Canada:

A major gene works only when the local blackleg population carries the matching avirulence (Avr) gene. Growing the same cultivar, or the same group, repeatedly on one field selects for races that have lost that Avr gene, and the Canola Council counts at least 16 virulent races on the Prairies. Rotating cultivars across resistance groups slows that selection. Two hybrids with different names can carry the same group, so switching brands without reading the label may change nothing.

When the clip survey shows resistance slipping, send stems for a race identification test (labs in all three provinces offer it). The report lists the Avr genes present; pick a variety whose major gene matches the most frequent race, and where many races show up, lean on strong quantitative (adult-plant) resistance. The R/MR rating still matters: R means severity up to 30 per cent of the susceptible check Westar, MR 30–49 per cent.

Rotation, stubble and the fungicide question

Rotation. The Canola Council's baseline is a minimum two-year break (canola no more than one year in three), up to three years where infected residue is slow to break down, and new canola at least 50–100 m (160–330 ft) from last year's canola stubble. Saskatchewan advises against seeding canola into or beside stubble that carried infected plants in the previous three years; Manitoba Agriculture recommends a four-year rotation once blackleg is detected. Control volunteer canola and wild mustard in the non-canola years, since both carry the fungus between crops.

Stubble. The residue is the inoculum, so faster decomposition helps, but tillage is not required. Saskatchewan reports that zero tillage does not increase blackleg risk when a cereal–pulse rotation is kept, and straw choppers that cut and spread residue aid breakdown; the Canola Council's Melfort trials found neither tillage nor burning significantly reduced severity. Seed treatments protect the cotyledon to early true-leaf window against seed-borne and some early airborne infection, but most do not cover later ascospore showers.

Foliar fungicide. Registered actives in Canada include azoxystrobin and pyraclostrobin (FRAC Group 11), propiconazole and mefentrifluconazole (Group 3) and fluxapyroxad (Group 7), several as premixes. They are protectant, not curative, so the Canola Council's rule is cotyledon to four-leaf, before lesions are visible, and only when the crop is susceptible, incidence and severity are high (usually a short-rotation field) and conditions are wet. On R-rated cultivars the Council's trials showed little or no reduction in severity or yield gain; on susceptible cultivars incidence typically fell about 20 per cent and severity about one point, which often did not cover the cost. Leave an untreated strip and clip stems in both at swathing. Rates, application limits and pre-harvest intervals are on the label registered in your province; a product registered by the US EPA is not automatically registered in Canada, so confirm the PCP number in the PMRA label search.

When management pays and when it does not

Blackleg costs yield when early infection meets a variety the local races can defeat. The payoff from longer rotation, a resistance-group change and clean seed arrives over several seasons as a stem clip average that stays under 1. A fungicide pays only in the high-risk case above, and even then early timing is everything. If you see basal cankers in late July, the money is already lost for this crop; spend it on the clip survey and the race test, and use the answer to pick next year's seed.