Genetic variation within and between populations of Pinus sylvestris L. (Scots pine) for susceptibility to Melampsora pinitorqua Rostr. (pine twist rust)

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 gree nhouse tests. Wind-pollinated progenies, factorial and incomplete diallel m ating designs were used to compare two natural populations of Scots pine (H aguenau, France, and Taborz, Poland) and their hybrids (Haguenau x Taborz). Families from Taborz were significantly (P < 0.001) less susceptible to tw ist rust than those from Haguenau. Inter-population hybrids were as suscept ible as the Haguenau population. No clear relationship between severity of infection, shoot length and phenological stage at time of inoculation was o bserved 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 cond itions and pathogen pressure. Within each of the two populations, the effec ts of general combining abilities (GCA) for rust susceptibility were predom inant, although estimation of individual heritabilities remained at a moder ate 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 quickl y through intrapopulation mass or backward selection based on wind-pollinat ed progeny tests.