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.

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 cultivars 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.

Eleven clones obtained from a cross between cv. Katahdin and fusion products betweenSolanum tuberosum and the non tuber-bearing speciesS. brevidens, were backcrossed again withS. tuberosum (clone AR80-127-5). Small tubers harvested from 583 seedlings of these second backcross (BC2) populations were screened for tuber tissue resistance toErwinia carotovora subsp.atroseptica under aerobic conditions. After multiplication in the field. BC2 clones again were screened for soft rot resistance under aerobic as well as anaerobic conditions. In general, the resistance of the BC2 populations was reduced in comparison with the BC1. Variation for resistance was found within the BC2 populations, but there was no correlation between the results of the three tests performed. Four BC2 populations were also screenned for resistance to blackleg in the field. Significant differences were found between populations for mean percentage of diseased plants, but these differences could not be explained by the resistance of the parental clones.

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.

Eleven clones obtained from a cross between cv. Katahdin and fusion products between Solnum tuberosum and the non tuber-bearing species S. brevidens, were backcrossed again with S. tuberosum (clone AR80-127-5). Small tubers harvested from 583 seedlings of these second backcross (BC2) populations were screened for tuber tissue resistance to Erwinia carotovora subsp. atroseptica under aerobic conditions. After multiplication in the field, BC2 clones again were screened for soft rot resistance under aerobic as well as anaerobic conditions. In general, the resistance of the BC2 populations was reduced in comparison with the BC1. Variation for resistance was found within the BC2 populations, but there was no correlation between the results of the three tests performed. Four BC2 populations were also screened for resistance to blackleg in the field. Significant differences were found between populations for mean percentage of diseased plants, but these differences could not be explained by the resistance of the parental clones.

Uit de proeven blijkt, dat de Nederlandse aaltjespopulatie op de MecklenburgseS. nigrum zijn volledige levenscyclus kan volbrengen, echter niet op de NederlandseS. nigrum. Bij nader onderzoek bleken deS. nigrum-herkomsten naast pathologische ook morfologische en kiemings-fysiologische verschillen te bezitten; de variëteitsnamen zijn niet met zekerheid vastgesteld. Op grond van deze waarnemingen is het dus waarschijnlijk, dat de tegenstrijdige gegevens uit de literatuur, in elk geval ten dele, een gevolg zijn van het gebruik van verschillende variëteiten vanS. nigrum.