Differential Reactions Between the Genus Brassica and Aggressive Single Spore Isolates of Leptosphaeria maculans

Based on sirodesmin production and pathogenicity tests with Brassica cotyledons, strains of Leptosphaeria maculans were classified as aggressive (pathotype group A), or non‐aggressive (pathotype group NA). NA strains caused no differential reactions. However, the pathotype group A could be divided into 5 sub‐groups. AO isolates caused non‐sporulating lesions with dark margins while Al isolates sporulated on cotyledons of most Brassica hosts tested. Only the cv. Erfurter Zwerg (B. oleracea var. botrytis) reacted resistant against AO and Al strains. A2 isolates caused resistance reactions on cotyledons of the cvs. Quinta (B. napus var. oleifera) and Runde (B. rapa var. rapa). A3 and A4 isolates were not detectable in our material. Isolates of these pathotype groups, supplied by Dr. P. H. Williams, Madison, USA, caused differential reactions on the oilseed rape cvs. Glacier, Quinta and Jet Neuf. In glasshouse and field experiments strains of pathotype groups Al, A2 and NA were tested on true leaves and hypocotyls of different oilseed rape cultivars. The low aggressiveness of NA isolates was evident under all experimental conditions. A2 strains caused resistance reactions not only on cotyledons but also on true leaves and hypocotyls of Quinta. Moreover, compared with Al, pathotype group A2 was more aggressive on hypocotyls of Jet Neuf. The resistance of this cultivar against Al isolates was clearly visible on hypocotyls and true leaves but not on cotyledons.