Co-infection and induction of systemic acquired resistance by weakly and highly virulent isolates of Leptosphaeria maculans in oilseed rape

The interaction between highly virulent (HV) and weakly virulent (WV) isolates of Leptosphaeria maculans on oilseed rape leaves was examined. In the growth room, leaves of the susceptible cultivar Westar were inoculated with HV and WV isolates together, a HV isolate followed, up to 6 days later, by a WV isolate, or a WV isolate followed, up to 6 days later, by a HV isolate. Both the HV and WV types were detected by PCR in lesions produced by all of these co-inoculations. Isolates from these lesions also yielded colonies of both types. The size of lesions produced by the HV type was reduced if the WV type was introduced at the same time as the HV type. Inoculation with the WV type less than 48 h after the HV type also significantly reduced lesion diameter caused by the HV type, but no reduction in lesion size occurred if the WV type was added 64 h or later after the HV type. Lesions caused by inoculation with the WV type followed by the HV type were also significantly smaller than those caused by the HV type alone. Maximum reduction in the size of the lesion occurred when the HV type was inoculated 24 h or later after the WV type. Inoculation of the second true leaf with the WV type significantly reduced the size of lesions produced by the HV type the elsewhere on the same leaf, on neighbouring leaves, and in the stem. We conclude that the WV type can coinfect and coexist with the HV type within a single lesion in leaves and inoculation of the WV type can induce systemic acquired resistance (SAR) in oilseed rape to the HV type. This is the first report of SAR in oilseed rape. (C) 1996 Academic Press Limited