The reliability of ®eld, greenhouse and cut-shoot screening procedures for the assessment of the susceptibility of Scots pine (Pinus sylvestris) genotypes to Melampsora pinitorqua, the causal agent of twisting rust, was evaluated. Fourteen genotypes evaluated on half-sib progenies after natural infection of 2-year-old plants showed comparable rank for rust susceptibility with those evaluated after controlled inoculation of 1-year-old seedlings in a greenhouse. The dynamic of pycnia production was assessed at 2-day intervals in cut-shoot assays under controlled conditions, giving reliable rankings of Scots pine genotypes between years and being signi®cantly correlated with rust susceptibility assessed after natural infection of 2-year-old progenies. These results underlined the importance of some critical factors such as inoculum load and host phenology in the reliability of susceptibility evaluations assessed under different experimental conditions. Arti®cial inoculation of 1-year-old seedlings in greenhouse experiments could provide a useful early test for the management of Scots pine breeding programmes and study of inheritance of twisting rust susceptibility. However, in speci®c investigations the cut-shoot assay would constitute a reliable laboratory test for studying host± pathogen interactions and the variability in pathogenicity of Melampsora pinitorqua populations.

The genetic variability of Scots pine (Pinus sylvestris L.) for twist rust susceptibility and the consequences for genetic improvement of the species were evaluated after arti®cial inoculation of 1-year-old seedlings in greenhouse tests. Wind-pollinated progenies, factorial and incomplete diallel mating designs were used to compare two natural populations of Scots pine (Haguenau, France, and Taborz, Poland) and their hybrids (Haguenau´Taborz). Families from Taborz were signi®cantly (P < 0.001) less susceptible to twist rust than those from Haguenau. Inter-population hybrids were as susceptible as the Haguenau population. No clear relationship between severity of infection, shoot length and phenological stage at time of inoculation was observed in intra-and interpopulation crosses. Dierences observed between the two natural populations for their response to twist rust are suggested to be the consequence of local adaptation simultaneously for climatic conditions and pathogen pressure. Within each of the two populations, the eects of general combining abilities (GCA) for rust susceptibility were predominant, although estimation of individual heritabilities remained at a moderate level (0.30±0.54). In interpopulation crosses, GCA eects of Haguenau and Taborz populations were still predominant. Moreover, hybrid performance could be predicted with con®dence based on the intrapopulation GCA values of parents. Genetic gain on twist rust resistance could be achieved quickly through intrapopulation mass or backward selection based on windpollinated progeny tests.

The genetic variability of Scots pine (Pinus sylvestris L.) for twist rust susceptibility and the consequences for genetic improvement of the species were evaluated after artificial inoculation of 1‐year‐old seedlings in greenhouse tests. Wind‐pollinated progenies, factorial and incomplete diallel mating designs were used to compare two natural populations of Scots pine (Haguenau, France, and Taborz, Poland) and their hybrids (Haguenau × Taborz). Families from Taborz were significantly (P < 0.001) less susceptible to twist rust than those from Haguenau. Inter‐population hybrids were as susceptible as the Haguenau population. No clear relationship between severity of infection, shoot length and phenological stage at time of inoculation was observed in intra‐ and interpopulation crosses. Differences observed between the two natural populations for their response to twist rust are suggested to be the consequence of local adaptation simultaneously for climatic conditions and pathogen pressure. Within each of the two populations, the effects of general combining abilities (GCA) for rust susceptibility were predominant, although estimation of individual heritabilities remained at a moderate level (0.30–0.54). In interpopulation crosses, GCA effects of Haguenau and Taborz populations were still predominant. Moreover, hybrid performance could be predicted with confidence based on the intrapopulation GCA values of parents. Genetic gain on twist rust resistance could be achieved quickly through intrapopulation mass or backward selection based on wind‐pollinated progeny tests.