Agronomy Update Features Agronomy
Agronomy update: Blackleg plus Verticillium stripe increases yield loss

The story of blackleg is pretty well known. The disease, caused by Leptosphaeria maculans, was first detected in northeast Saskatchewan in 1975. Yield losses of up to 50 per cent were observed in those days before resistant varieties were developed in the early 1990s. However, more recently, there has been some breakdown in resistance, and blackleg symptoms can be seen on resistant hybrids.

The new kid on the block is Verticillium stripe, caused by Verticillium longisporum. It was first identified on the Prairies in 2014, and the Western Committee on Plant Diseases’ Oilseed Diseases Report for 2022 indicated the Verticillium stripe was found in 38 per cent of Manitoba fields surveyed with 23 per cent of the plants showing symptoms in those fields. Preliminary results from Saskatchewan indicate it is no longer a rare disease in eastern Saskatchewan, and in Alberta, the prevalence of the disease was 2.4 per cent with an average incidence of 0.1 per cent.

Because these two diseases can occur in the same fields, it begs the question, ‘What happens if both diseases infect a canola crop?’ To delve into that possibility, a research study was conducted by the University of Alberta to examine possible yield interactions between blackleg and Verticillium stripe, and to also establish the relationship between blackleg severity and the yield of blackleg-resistant canola hybrids in small plots and commercial fields. It was led by Sheau-Fang Hwang with Stephen Strelkov and Yixiao Wang. 

The research had several components. The first was a small plot field trial at the Crop Diversification Centre-North at Edmonton conducted over two years in 2019 and 2020. This trial evaluated the relationship between blackleg severity and yield in plots inoculated with isolates of L. maculans.  Two blackleg-resistant canola hybrids, 45H31 and CS2000, were grown. 

Researchers also evaluated blackleg yield loss in nine commercial canola fields in the County of Wetaskiwin and three fields in the County of Lacombe, Alta., in 2019. Plants within a one-square-metre area at five locations in each field were sampled, assessed for blackleg severity and yield. All varieties were rated blackleg resistant.

Blackleg/Verticillium stripe interactions were evaluated at CDC-North in small plots in 2020 and 2021. Canola hybrids 45H31 and CS2000 were grown, and the plots were inoculated with both L. maculans and V. longisporum in various ratios of the pathogens. Plots were rated for disease severity, and yield was calculated. Similar protocols were used in greenhouse experiments.

Generally, blackleg severity in the field plots was low, and never exceeded a mean rating of 2.4 for either hybrid. This is likely the result of both varieties being rated resistant to blackleg. Yields decreased dramatically as disease severity increased to two or higher. Above a disease rating of two, yield loss increased by approximately 20 per cent for each one point increase in severity. For example, at blackleg severity of two, the yield loss per plant was approximately 20 per cent. Yield projections found that at a severity rating of three, it was approximately 40 per cent, and was around 60 per cent at a severity rating of four.

In the commercial fields, the two varieties reacted to blackleg disease a bit differently. The hybrid DKTF 94CR had a slightly higher blackleg rating of 2.9, while 75-42CR had ratings of 2.7. While the ratio of blackleg rating to yield differed between hybrids, at higher blackleg severity ratings above two, both had similar yield losses as disease severity increased and had a similar yield loss trend as in the small field plots.

Looking at the interaction between blackleg and Verticillium strip, at the two commercial sites over two years, when plots were inoculated with both L. maculans and V. longisporum, blackleg severity increased. For example, the most severe blackleg ratings of 1.3 to 1.6 was on 45H31 at site one in 2020 in treatments inoculated with V. longisporum and L. maculans.

The interaction between pathogens was also seen in Verticillium stripe severity. For example, the most severe Verticillium stripe severity was seen in three statistically similar treatments on CS2000 in 2021. These were when V. longisporum inoculant was applied alone with a severity rating of 1.6; in the treatment of a 1:3 mix of L. maculans and V. longisporum with a severity rating of 1.2; and in a treatment of a 1:1 mix of the pathogens with a 0.8 rating. 

Yield was not significantly different between treatments, indicating that the disease severity caused by the various treatments was not severe enough to impact yields overall. However, in some cases, Verticillium stripe caused greater yield losses than blackleg.

The researchers concluded that the interaction between L. maculans and V. longisporum may cause more severe losses in canola, highlighting the need for proactive disease management strategies. The story will continue to unfold. 

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.