Virulence pattern of aggressive isolates of Leptosphaeria maculans on an extended set of Brassica differentials

Eight hundred and thirty single ascospore lines of Leptosphaeria maculans ( anam. Phoma lingam) were isolated from oilseed rape stubbles of different g eographic regions in Germany. Based on pigment and sirodesmin production, 6 97 isolates (83%) were characterized as aggressive. Of these, 347 were test ed on cotyledons of the B. napus cultivars 'Lirabon','Quinta', 'Glacier' an d 'Jet Neuf' and differentiated into six pathogenicity groups: Al (virulent on call cultivars), A2 (avirulent on 'Quinta'), A3 (avirulent on 'Glacier' ,'Quinta', 'Jet Neuf'), A4 (avirulent on 'Glacier', 'Quinta'), A5 (avirulen t on 'Jet Neuf') and A6 (avirulent on 'Quinta','Jet Neuf'). Fourteen of the aggressive strains were isolated outside Germany. In Germany, mainly A1-is olates (73%), a high proportion of A2-isolates (23%) and no A3- and A4-isol ates were found. Only few A5- and A6-isolates were present. For further dif ferentiation, aggressive isolates were tested on cotyledons of additional 1 2 Brassica genotypes, including genotypes of B. napus, B. rapa, B. oleracea , B. juncea, B. nigra and B. carinata. Using this extended differential set , 233 aggressive isolated could be assigned to 25 subgroups, among them fiv e (A1-3, A1-3, A1-10, A1-11, A2-3) with dominating frequencies. Pathogenici ty groups and subgroups were present with similar frequencies in north-west ern, north-eastern, central and southern Germany. These results show chat t he population of L. maculans is variable in virulence and, thus, could have the potential to adapt to new Phoma-resistant B. napus cultivars.