Agronomy Update Features Agronomy Canola
Foliar fungicide not recommended for blackleg-resistant canola

With increased blackleg (Leptosphaeria maculans) incidence and shorter crop rotations in many parts of the Canadian Prairies, there was a growing interest in foliar fungicide treatment to reduced disease risk and impact on yield. This interest was also fuelled by a shift in blackleg races, which meant that the original resistant cultivars were no longer as effective in minimizing the impact of blackleg.

With funding from the Canola Agronomic Research Program, research led by Gary Peng at Agriculture and Agri-Food Canada investigated the impact of foliar fungicides on blackleg disease incidence, severity and yield at early versus late application, and one versus two applications at different growth stages on the susceptible canola cultivar, Westar. This was to represent the worst-case scenario of cultivar resistance erosion. 

The research was carried out at Vegreville, Alta., Melfort and Scott, Sask., and Carman and Brandon, Man. The plots at each trial site were seeded adjacent to a previous field of the blackleg susceptible Westar. Four fungicide products were selected for in-season application: pyraclostrobin (Headline), azoxystrobin (Quadris), propiconazole (Tilt), and azoxystrobin + propiconazole (Quilt). 

Each fungicide was applied at the two to four leaf stage of Westar to target early infection of cotyledons and the lower true leaves of canola. To compare efficacy between early and late treatments, Headline was also applied at early bolting. Two-fungicide treatments were also sequentially applied to Westar; Headline or Tilt at the two to four leaf stage, followed by Tilt (to Headline-treated plots) or Headline (to Tilt-treated plots) at early bolting, to compare with the early treatments. 

Headline was also applied to the resistant cultivars 43E01 (MR) and 45H29 (R) at the two- to four-leaf stage to assess the impact of cultivar resistance on fungicide efficacy. These resistant cultivars both carry the specific R genes Rlm1/LepR3 and Rlm3.

Disease severity was assessed at growth stage 5.3 to 5.4, when canola seed in the lower pods started turning from mottled green-brown to brown. Mean Disease Severity (MDI) and Disease Severity Index (DSI) ratings were used for the assessment.

Blackleg levels varied substantially on non-treated Westar with MDI ranging from 28 per cent to 96 per cent (average 67.9 per cent) and DSI from eight per cent to 69 per cent (average 31.8 per cent). 

Applying Headline, Quadris or Quilt at the two- to four-leaf stage, or Headline and Tilt (or vice versa) at the two- to four-leaf and early bolting stages to Westar reduced the disease incidence and severity significantly compared to the non-treated control. However, the two-application treatments applied at the two to four leaf and bolting stages was no better than the early application of Headline alone. 

On the moderately resistant cultivar, 43E01, the disease incidence was high at 65 per cent with moderate severity at 32.9 per cent on the untreated control. The application of Headline reduced the incidence to 48.6 per cent and severity to 19 per cent, respectively.

For the resistant cultivar 45H29, the disease incidence was still fairly high at 53.9 per cent but severity lower at 21.2 per cent. The application of Headline significantly reduced the incidence to 42.3 per cent and severity to 13.7 per cent.  

The treatments that reduced MDI and DSI on Westar also limited the yield loss caused by blackleg in a range of 16.5 to 26.9 per cent, relative to the untreated control. Untreated Westar yielded 26 bu/ac (1,441 kg/ha), while the treatments that reduced blackleg showed higher yield in the 31 to 33 bu/ac (1,720 to 1,829 kg/ha) range.

On the moderately resistant and resistant cultivars, however, even though the application of Headline at the two- to four-leaf stage reduced MDI and DSI significantly, there was no yield benefit. 

The results of the research clearly show that in-season fungicide treatment will provide little benefit when disease pressure is low (MDI <30 per cent) in Western Canada, especially on resistant canola cultivars. 

Growers and agronomists are encouraged to scout for blackleg at swathing to assess if a cultivar is starting to show higher blackleg incidence and severity – a sign that the blackleg race may have shifted in the field. They can also submit samples to several labs in western Canada to determine the blackleg race present in the field. This information can help guide cultivar selection based on resistant gene labeling of cultivars. The Canola Council of Canada has a list of labs that conduct blackleg race identification at blackleg.ca. 

However, growers shouldn’t just rely on cultivar resistance to manage blackleg. For every resistant gene available to plant breeders, there is a virulent race that can overcome it on the Prairies. Other management practices such as extending crop rotations, the use of Certified seed, and controlling Brassica weeds and volunteer canola are additional ways to reduced the amount of blackleg inoculum in the field.  

Bruce Barker divides his time between CanadianAgronomist.ca and as Western Field Editor for Top Crop Manager. CanadianAgronomist.ca translates research into agronomic knowledge that agronomists and farmers can use to grow better crops. Read the full Research Insight at CanadianAgronomist.ca.