Disease survey found pathogens were common on faba bean
A three year survey found that foliar disease incidence on faba bean was more than 75% in Alberta and Saskatchewan reaching 100% in Alberta in 2019. The major fungal genera identified were Alternaria, Botrytis, Fusarium and Stemphylium. Botrytis fabae, which primarily causes chocolate spot disease, was isolated from 60 of 68 samples, while eight isolates were B. cinerea.
Faba bean acreage continues to grow on the Canadian Prairies as better adapted and smaller seeded varieties become available. A foliar disease survey was conducted from 2017 to 2019 in Saskatchewan and Alberta. This survey had three objectives: a) determine the incidence and severity of all foliar diseases on faba bean in Alberta and Saskatchewan, b) identify the pathogens associated with foliar diseases and c) determine the pathogenicity of fungal isolates associated with diseases.
The foliar disease survey over the three years covered a total of 103 faba bean fields in Alberta and Saskatchewan. The fields were surveyed at mid-pod stage from late July to mid-August. Disease incidence and severity were calculated. Disease severity ratings were on a scale of 1 to 5 with 1 having no symptoms and 5 having extensive lesions on leaves, stems and pods covering >10% of leaf surface with severe defoliation, heavy sporulation and blackening of leaf tissue.
Diseased leaf tissue was cultured, and single spore isolations were performed. A total of 4,057 fungal isolates were recovered and fungal genera were identified. DNA extraction was also conducted from cultures and used to identify the pathogenicity of the fungal isolates.
Disease incidence was high but with low severity
In 2019, cooler temperatures, high humidity and high rainfall occurred in both Alberta and Saskatchewan compared to 2017 and 2018. This resulted in higher disease incidence and severity in 2019 in Alberta. In 2019, disease incidence in Alberta was 100% and was 73.1% in Saskatchewan.
Disease incidence in 2017 was 75.2% in Alberta and 87.2% in Saskatchewan. In 2018, Alberta disease incidence was 85.8% and 81.4% in Saskatchewan.
Disease severity was generally low across the fields and years. In Alberta, disease severity was 1.9 in 2017, 1.7 in 2018, and 3.2 in 2019. In Saskatchewan, severity was 1.9 in 2017 and 2018, and 2.0 in 2019. To put that severity into perspective, a disease rating of 2 means the leaves have small, discrete lesions (2-3 mm), covering 1 to 2% of leaf surface with no defoliation.
Average disease severity was always lower in the upper canopy, and higher in the lower canopy.
Fungal species identified
Of the 4,057 fungal isolated, the major fungal genera identified were Alternaria, Botrytis, Fusarium and Stemphylium. Alternaria was most commonly found with 43.3% of isolates recovered from faba bean leaves. This was followed by Stemphylium at 24.4%, Botrytis spp. at 11.1% and Fusarium at 6.5%. Other species identified, representing 14.6% of isolates included Chaetomium, Cladosporium, Epicoccum, Phaeosphaeria, Sphaerellopsis, Microdochium and Mortierella, and a few Colletotrichum sp. and Sclerotinia spp.
The frequency of the identified species differed between years and province. The highest Botrytis spp. frequency was at 29% in Alberta in 2019 but only 3.2% in 2018. Another example of this variation was that Stemphylium spp. frequency was highest in Saskatchewan in 2019, but none were found in isolates in Alberta in 2018.
Pathogenicity testing found that 60 of the 68 Botrytis isolates were B. fabae representing 88.2% of the isolates and eight isolates were B. cinerea. Botrytis cinerea and B. fabae were identified as aggressive pathogens, with B. fabae, which is specific to faba bean, primarily causing chocolate spot disease.
Stemphylium spp. were rated as moderately aggressive on faba bean leaves. Stemphylium spp. symptoms were difficult to distinguish from chocolate spot. Alternaria spp. produced typical typical bull’s eye lesions, with low to moderate severity.
The researchers concluded that Botrytis and Stemphylium are commonly present and can cause moderate to severe disease on faba bean foliage. The common presence of Stemphylium spp. requires further research to determine if the species found on faba bean are common in other pulse crops, which would have implications for pulse crop rotations.
Funding was provided by the Western Grains Research Foundation, the Saskatchewan Ministry of Agriculture and the Alberta and Saskatchewan Pulse Growers Boards.
Kaur, S., Kader, K. A., Banniza, S., Davidson, R., & Chatterton, S. (2025). Survey of foliar diseases in faba bean (Vicia faba L.), and pathogenicity of fungi associated with diseases, on the Canadian Prairies. Canadian Journal of Plant Pathology, 47(3), 275–286. https://doi.org/10.1080/07060661.2025.2456872
Photo by Syama Chatterton
