One hundred Pectobacterium carotovorum, isolates, recovered from soft rot symptoms on potato tubers in Algeria and previously characterised taxonomically, were assessed in a half-tuber test for differences in ability to cause tuber rotting on two cultivars (Désirée and Bintje) respectively considered to be moderately and very susceptible to soft rot. A first trial at 20°C, involving the complete collection of isolates at two inoculum concentrations (2.105 and 2.107 cel. ml-1), showed significant effects of inoculum dose, host cultivar and biochemical and molecular groups on pathogenicity. A significant interaction between pathogen groups and cultivars was also apparent. P. carotovorum. subsp. atrosepticum (Pca) was more pathogenic on cv. Bintje than on cv. Désirée, while the susceptibility of these two cultivars to P. carotovorum. subsp carotovorum (Pcc) was the opposite. Some Pcc isolates were non-pathogenic to both cultivars, and others were pathogenic on cv. Bintje but not on cv. Désirée. A second trial, conducted at 20 and 25°C with a high inoculum concentration (2.107 cel. ml-1) of forty isolates representative from the collection, confirmed the previous findings, and showed a significant effect of temperature on pathogenicity. Pca isolates were more aggressive than Pcc isolates at both temperatures, but the difference was greater at 20°C. Our data therefore suggest that cultivar resistance rankings depend on the Pectobacterium subspecies considered, and should therefore be assessed separately for the various Pectobacterium subspecies.
Key words: Sensitivity, soft rot, aggressiveness, cultivars, resistance.
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