Electrophoretic analysis of natural populations of Leptosphaeria maculans directly from leaf lesions

On oilseed rape, 207 leaf lesions attributed to Leptosphaeria maculans were classified as typical or atypical. Starch gel electrophoresis of glucose phosphate isomerase (GPI) performed on extracts of 229 leaf lesions comprising the 207 with L. maculans symptoms and 22 with Pseudocercosporella capsellae symptoms, yielded four different electrophoretic patterns of alloenzymes designated ET1 to ET4. In addition to ET1 and ET2, characteristic respectively of A- (highly virulent) and B- (weakly virulent) group isolates of L. maculans, the previously undescribed ET3 allozyme was recovered from a few typical and atypical L. maculans leaf lesions. The fastest ET4 allozyme was specific to P. capsellae. All but two typical leaf lesions produced the ET1 allozyme, whereas atypical lesions produced one of the three L. maculans allozymes. Occasionally a mixture of two allozymes was recovered from a same-leaf lesion. GPI electrophoresis performed directly on leaf lesions proved a useful and reliable method to identify L. maculans, and to differentiate between L. maculans and P. capsellae. This method of discrimination enabled deductions, from 377 leaf lesions analysed, about the structure of L. maculans populations on different oilseed rape varieties.