Differentiating A and B groups of Leptosphaeria maculans, causal agent of stem canker (blackleg) of oilseed rape

Stem canker or blackleg of brassicas, caused by Leptosphaeria maculans, is one of the most damaging diseases of winter oilseed rape in the UK. Airborn e ascospores, released in autumn and winter, initiate leaf infections which may lead to colonization of the petiole and, later in the season, formatio n of stem lesions and cankers. Although isolates of the pathogen differ in ability to cause damaging stem cankers, this is not readily apparent from l eaf spotting or stem lesion symptoms. However, several cultural, biochemica l and genetic characteristics appear to be associated with the ability to f orm damaging stem cankers and isolates can be assigned to one of two groups , termed A and B, on the basis of differences in these characteristics. To investigate the relationship between leaf spotting symptoms and subsequent stem canker formation, and to improve understanding of the epidemiology of this pathogen, it is desirable to differentiate between the stem canker for ming A group and the less damaging B group of L. maculans. Characterization of isolate type is also important in seed testing and crop breeding progra ms, particularly in countries such as Canada and Poland where the A type is not ubiquitous. This article reviews methods, including plant assays, asse ssments of growth characteristics in vitro, isozyme analyses, secondary met abolite profiling, serology, and nucleic acid analyses, that can be used to differentiate the A and B groups.