Comparison of scales for rating severity of blackleg in canola

Abstract

Plants of Brassica napus were evaluated for severity of blackleg using four rating scales based on one or more aspects of the symptoms. The association among ratings was determined based on exterior symptoms, girdling of the hypocotyl, stem penetration, and area of discoloured (infected) tissue in the cross-section of hypocotyl and tap root. The X2-values indicated that the ratings for all scales were associated. The rating scales differed for number of symptomless plants and for disease index. Fewer plants were classified as symptomless when ratings were based on area of discoloured tissue and stem penetration than when ratings were based on exterior symptoms and girdling. The disease index was highest for ratings using area of discoloured tissue. When rating large numbers of plants, differences among rating scales may be unimportant. When rating small numbers of plants, differences among rating scales can be important and a rating scale based on area of diseased tissue is recommended. © Taylor & Francis Group, LLC.