TEMPERATURE AND AGING OF HOST TISSUE AFFECT THE INTERACTIONS BETWEEN DIFFERENT OILSEED RAPE CULTIVARS AND PATHOTYPE GROUPS OF LEPTOSPHAERIA-MACULANS

The population of Leptosphaeria maculans is composed of different pathotype groups (PG). Among these groups are non-aggressive (PG NA) and aggressive ones (PG A). The latter can be divided into at least 5 subgroups (PG A0 to PG A4). On cotyledons of all rape cultivars tested (Quinta, Lirasol, Jet Neuf) NA isolates caused small necrotic lesions, when the plants were grown at 18-degrees-C. At 27-degrees-C these lesions were enlarged but remained necrotic and incompatible. On the contrary, the necrotic defense reaction of Quinta cotyledons to A2 isolates was only detectable at 18-degrees-C. This interaction reacted completely compatible at 27-degrees-C. Generally, canker symptoms were promoted by higher temperatures. However, the low aggressiveness of NA isolates, the resistance of Jet Neuf and the differential resistance of Quinta to A2 isolates remained visible on hypocotyls at 27-degrees-C. When the interactions of Quinta and Jet Neuf with NA, A1 and A2 isolates were studied on detached leaves or excised leaf petiole and hypocotyl segments all resistance reactions were broken or at least strongly weakened. On ageing host tissue even non-aggressive isolates caused severe symptoms with heavy sporulation.