Factors affecting potato soft-rot resistance to pectolytic Erwinia species in a tuber-slice assay

Potato-tuber slices and small tubers of 12 cultivars were inoculated with isolates of Erwinia carotovora subsp, atroseptica (Eca), E.c. subsp. carotovora (Ecc) and E. chrysanthemi (Ech). Cultivar resistance was expressed as the mean diameter of the rotted-tissue area. Grouping of the cultivars into classes of relatively high, intermediate or low resistance, was different after incubation in air in comparison to incubation under anaerobic conditions, and depended on the (sub-)species of bacteria used. When compared to the effect of inoculation with different (sub-)species, grouping of the cultivars was less affected by isolates within (sub-)species. Cultivar grouping was hardly affected by sire of inoculation, being either in the medullary tissue or in the cortex. In general, the results of experiments with either held-grown seed tubers or glasshouse-grown small tubers from 2 years were reproducible, except when tuber tissue was inoculated with Ech and incubated aerobically, in which case cultivars only showed relatively small differences for resistance. Correlation coefficients between the results from slice and small-tuber screenings were 0.71 and 0.64 for Eca and Ecc, respectively. This implies some potential for applying the method in the early stages of potato-breeding programmes. The results of this study show that soft-rot resistance of clones to different pectolytic Erwinia spp. can be determined efficiently and reproducibly if the oxygen concentration during incubation is standardized.