Stems of oilseed rape (Brassica napus) bearing pseudothecia of Leptosphaeria maculans were collected in different parts of West Germany. From single ascospores of this material a total of 352 fungal lines were derived, tested on rape cotyledons and devided into 124 aggressive and 228 nonaggressive pathotypes. The two groups could be distinguished by certain characteristics. On nutrient agar, aggressive strains grew more slowly. In liquid cultures, non-aggressive types produced a yellowbrown pigment whereas the culture filtrates of aggressive strains remained unpigmented. The culture filtrates of aggressive lines contained phytotoxic compounds which were identified as sirodesmins. These compounds were absent in culture filtrates of non-aggressive pathorypes. The very close correlation between aggressiviness as a pathogen and the phytotoxin production indicates that phytotoxins of Leptosphaeria maculans could be involved in the disease development.

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.

Relação entre o número de escleródios no refugo e a incidência de S. sclerotiorum na respectiva amostra de sementes de feijão.

Rafaela Zázera e Vitória Macedo pela convivência, apoio e trocas de experiências durante o doutorado. A todos que contribuíram para a realização deste trabalho, com palavras, com gestos ou com um simples sorriso.... a minha gratidão.