Components of partial resistance to potato blackleg caused by pectolytic Erwinia carotovora subsp. atroseptica and E. chrysanthemi

The various phases of the infection process in potato blackleg, caused by pectolytic Erwinia spp., were interpreted as components of partial resistance and studied in detail. Mother tubers of glasshouse-grown plants of two potato cultivars were inoculated with antibiotic resistant marker strains of Erwinia carotovora subsp. atroseptica (Eca) or E. chrysanthemi (Ech). Stem tissue samples of these plants were collected 3, 7, 14 and 21 days after inoculation and subsequently analysed for the presence of the bacteria. As soon as 3 days after inoculation, Erwinia bacteria were detected in the stem tissue of the plants. However, blackleg developed only when the rotting mother tuber tissue reached the sites where the stems were attached to the mother tuber. An inoculation method was developed for determining cultivar differences in the incidence and rate at which rot in tuber cylinders proceeds to sprouts on top of such cylinders. A set of 12 cultivars was screened for this putative component of resistance which was called 'stem base resistance'. Significant differences for stem base resistance were found among the cultivars. These differences were clearer when inoculation was carried out with Ech in contrast to Eca. Glasshouse-grown plants of the same set of cultivars were also screened for resistance of the above ground part of the stem. Cultivars differed significantly for this so-called 'stem tissue resistance', but results of some cultivars were not consistent among years of testing. Multiple regression analysis revealed that stem base resistance, stem tissue resistance, tuber tissue resistance and resistance of mother tubers in the field account for 63% (Eca) and 75% (Ech) of the variance found after screening for resistance to blackleg in the field. It was concluded that if results of this study are extrapolated to any set of potato clones, selection for resistance in breeding programmes under laboratory or glasshouse conditions, would be most efficient when directed to clonal differences for stem base resistance.