Twenty-two R gene-free cultivars, introduced between 1900 and 1954, were field-tested for their level of partial resistance to a complex race of Phytophthora infestans. Disease assessments, expressed as areas under the disease progress curve, appeared closely correlated to resistance ratings given between 1929 and 1954. This, and the stability in time since 1929 of the ratings in the Dutch Descriptive List of Varieties of Field Crops, suggest that the resistance concerned is durable. Lesion growth rate was found to be a very important component of resistance in these cultivars and also in more recently introduced ones, whereas latent period varied little between the cultivars. The most resistant culfivars were Robijn, Populair, Pimpernel, Libertas and Surprise, which were also among the latest maturing in the material. These five cultivars are closely related and may have the same resistance genes.Abbreviations: ADPC --area under the disease progress curve; LGR = lesion growth rate; LP = latent period.

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 site of inoculation, being either in the medullary tissue or in the cortex. In general, the results of experiments with either field‐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.

In t991 and 1992, 12 potato cultivars were screened at two locations for resistance to blackleg, after vacuum infiltration of the seed with Erwinia carotovora subsp, atroseptica or E. chrysanthemi. Cultivar differences for resistance to E.c. subsp, atroseptica and E. chrysanthemi were found which were consistent over locations and years. Seed tubers of the same cultivars were also screened for resistance to both Erwinia spp. by using a tuber slice inoculation method. Correlation coefficients for comparisons between resistance to blackleg in the field and tuber tissue resistance under aerobic or anaerobic conditions were not significant. This could partly be explained by drastic changes in relative tuber tissue resistance of the cultivars within a 5 weeks period after planting in the field. Presprouting of seed tubers in diffuse daylight had a less pronounced effect on relative tuber tissue resistance than planting in the field. Monitoring the process of mother tuber decay during the growing season of 1993 after vacuum infiltration with E.c. subsp, atroseptica and E. chrysanthemi revealed that cultivars differed in the extent to which these bacteria enhanced the process of mother tuber decay. These differences partly explained the cultivar differences for resistance to blackleg in the field.Abbreviations: Eca = Erwinia carotovora subsp, atroseptica; Ech = Erwinia chrysanthemi; NOP = Noordoostpolder; Wag --Wageningen

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.

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