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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.

Canadian potato germplasm (Solanum tuberosum L.) is unique in its geographic and climatic ranges of adaptation, but little is known about the genetic diversity of the improved Solanum gene pool established over the past century. Simple sequence repeat (SSR) markers were applied to assess the genetic diversity of 114 Canadian and 55 exotic potato accessions. Thirty-six SSR primer pairs were applied and 232 polymorphic bands were scored for each accession. The frequencies of polymorphic bands ranged from 0.01 to 0.98 and averaged 0.35. The proportion of total SSR variation occurring between Canadian and exotic germplasm was 0.6%; among the Canadian cultivars of four major breeding periods 2.7%; among heirloom varieties, modern cultivars and elite breeding lines 4%; and between tetraploid and diploid lines 3.7%. Slightly more diversity was found for exotic, than the Canadian, germplasm. The modern cultivars displayed slightly more diversity than the heirloom varieties and the early cultivars revealed slightly more variation than the recent ones. Clustering 169 accessions revealed more than ten groups, but the groups were not distantly separated. Both the genetically most distinct accessions and the possible genetically duplicated accessions were identified. These findings not only demonstrate the narrow genetic base of the Canadian potato germplasm, but also are useful for managing the existing potato collection and for selecting genetically distinct potato materials to widen the genetic background of the potato gene pool.Resumen El germoplasma de de papa (Solanum tuberosum L.) canadiense es único en su adaptación geográfica y a variedad de climas, pero se conoce poco acerca de la diversidad genética del acervo genético mejorado de Solanum establecido durante el siglo pasado. Para evaluar la diversidad de 114 accesiones accesiones de papa canadiense y 55 foráneas se utilizaron marcadores de secuencia simple repetida (SSR). Se aplicaron 36 pares de iniciadores SSR y se evaluaron 232 bandas polimórficas para cada accesión. Las frecuencias de bandas polimórficas tuvieron un rango de 0.01 a 0.98 con un promedio de 0.35. La proporción de la variación total de SSR que ocurrió entre el germoplasma canadiense y foráneo fue de 0.6%; entre los cultivares canadienses de los cuatro principales periodos de mejoramiento 2.7%; entre las variedades ancestrales, los cultivares modernos, las líneas élite de mejoramiento 4%; y entre las líneas tetraploides y diploides 3.7%. Se ha encontrado una diversidad ligeramente mayor para el germoplasma foráneo que para el canadiense. Los cultivares modernos mostraron una diversidad ligeramente mayor que las variedades ancestrales y los cultivares antiguos revelaron una ligera mayor variación que los recientes. La agrupación de 169 accesiones reveló más de 10 grupos, pero éstos no estaban separados distantemente. Tanto las accesiones genéticamente más distintas como las posiblemente duplicadas fueron identificadas. Estos hallazgos no sólo demostraron la estrecha base genética del germoplasma can...

Tuberosum x Tuberosum families and Tuberosum x long-day adapted Andigena families are compared with each other and with their parents using multivariate analyses. The Tuberosum x Tuberosum families, representative of those used in present-day breeding programs, have lower mean tuber weights, marketable yields and total yields than their parents, and a trend towards the Andigena clones which represent their putative ancestors. The Tuberosum x Andigena families are superior to the Tuberosum x Tuberosum families in total yield and tuber number, but have reduced tuber size, later maturity and more persistent stolons. The characteristics of the Andigena parents dominate the inter-Group hybrids. The multivariate analyses facilitate the identification of superior hybrid families and superior parents. The results are discussed in the context of further Andigena selection, cytoplasmic male sterility, and reciprocal differences in inter-Group hybrid families. Future breeding strategies are proposed.