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.

Nineteen accessions of the tuber-bearing species Solanum berthaultii, S . chacoense, S . leptophyes, S. microdontum, S. sparsipilum, S . sucrense, S. venturii, S. vernei and S. verrucosum were tested for their resistance to late blight in two years of field experiments . Plants were artificially inoculated with zoospores of race 1 .2 .3 .4.5 .7 .10 .11 and the development of the disease was followed . Resistance ratings, calculated as the areas under the disease progress curves (ADPC), demonstrated a high resistance in all accessions except in S . sparsipilum, S. leptophyes and their interspecific hybrid . Segregations suggest that major genes for resistance are present in S. sucrense and S . venturii, and may also play a role in S. verrucosum . It is not yet certain wether the resistance of the other accessions is comparable to the partial and durable resistance of S . tuberosum cultivars like Pimpernel, as inheritance and mechanism have yet to be established . However, segregations suggesting the presence of single major genes with complete dominance were not found in these other accessions . Tuber initiation in the field occurred in only one accession, S . tuberosum ssp . andigena, and maturity of the clones was not related to their resistance . In the other accessions maturity types could not be assessed, as the clones require short day conditions for tuber initiation .

Solanum nigrum and S. villosum, and their sexual hybrids with S. tuberosum and S. demissum respectively, were inoculated with a complex race of Phytophthora infestans. No visible reaction was seen on S. viUosum and one genotype of S. nigrum. Another genotype of S. nigrum and the hybrids showed a hypersensitive response on most inoculated leaves. In one experiment, some sporulation was observed on detached leaves of a hybrid derived from S. nigrum. Microscopical examination of infections in S. nigrum and in a hybrid from S. nigrum, showed that penetration of epidermal cells and subsequent intercellular growth of the pathogen into the spongy mesophyll occurred, but without the formation of haustoria, and that invaded and neighbouring cells became necrotic. Callose appositions were found in epidermis and mesophyll cells of all inoculated genotypes, and also in epidermal cells of the unrelated nonhost species Brassica campestris.

The effects of plant age, leaf age, and leaf position on race-nonspecific resistance against Phytophthora infestans were investigated in a series of field and controlled environment experiments with five different potato (Solanum tuberosum) cultivars. Leaf position proved to be the most significant factor; apical leaves were far more resistant to late blight than basal leaves. Plant age and leaf age had only minor effects; therefore, the resistance of a specific leaf remained about the same during its entire lifetime. The gradual increase in late blight resistance from basal leaves to apical leaves appeared to be a general effect, irrespective of cultivar, growing conditions, or resistance test. Therefore, it is important to consider leaf position in tests for late blight resistance, because contrasts in resistance may be ascribed erroneously to differences between genotypes or treatments, whereas they are actually caused by differences in leaf position.

Ten institutions in nine countries joined together to test the stability of resistance of 14 potato genotypes to the oomycete pathogen Phytophthora infestans in three separate trials. Seven of the genotypes were tested in one trial involving seven locations, and all 14 were tested in two subsequent trials, each involving eight locations. Stability of resistance was tested with nonparametric tests and with an additive main effects and multiplicative interaction (AMMI) model. Overall, resistance to P. infestans was robust; resistant genotypes were consistently resistant in all locations and trials. The nonparametric analysis indicated that specific genotypes were basically stable across sites for resistance. In trial 3, the Z statistic for overall stability was significant at 0·05%, indicating a significant level of interaction across the trial, but there were no significant interactions for specific genotypes in this trial. The genotype by environment (G × E) effect of the AMMI model was highly significant in both trials, but the mean square of G × E was less than 10% of the genotype effect in each trial. The first two principal components (PCA1 and PCA2) of the AMMI analyses together explained 75 and 80% of the interaction effects in trials 2 and 3, respectively. Based on both nonparametric and AMMI analyses, Ecuador and Argentina were locations of relatively high interaction effects for both trials 2 and 3, although in Ecuador this interaction was not associated with any particular potato genotype. Other locations also had high interaction effects, but these occurred in only one trial. The genotypes Chata Blanca and, to a lesser extent, Torridon were relatively unstable in trials 2 and 3, but in the case of Torridon, resistant, this did not represent a significant loss of resistance.