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The importance of site type and tree species on disease incidence of Gremmeniella abietina was investigated. Blocks of four conventional regenerations of Pinus contorta and Pinus sylvestris, situated on sites formerly supporting pure stands of P. sylvestris and Picea abies respectively, were examined for the occurrence of G. abietina. Trees planted on spruce sites were found to be more severely diseased by G. abietina than trees planted on pine sites. Pinus contorta was more severely attacked by G. abietina than P. sylvestris on both site types. The results show that plantations of P. contorta and P. sylvestris in areas originally covered with pure spruce forests run a high risk of damage by G. abietina.

Survival, height, and health status of Pinus sylvestris and Pinus contorta were studied in a randomized factorial experiment in northern Sweden. Treatments studied per species were no scarification (control), patch scarification, and mounding. The experiment was evaluated 18 years after planting. For both species, survival was higher in mounded plots than in control plots. Pinus contorta was 11% higher than P. sylvestris in the treatment with highest survival, but this difference was not significant. Ninety‐three per cent of the P. contorta trees were infected and 22% killed by Gremmeniella abietina compared with 21% and 5% for P. sylvestris. Eighty‐three per cent of the P. sylvestris trees were infected and 24% killed by Phacidium infestans compared with 8% and 2% for P. contorta. Leaning P. contorta trees were more severely damaged by G. abietina than those not leaning. Mounding reduced the mortality caused by G. abietina in P. contorta and by Ph. infestans in P. sylvestris. Due to the increasing mortal...

The susceptibility of Pinus sylvestris to the rusts of P. contorta—Endocronartium harknessii, Cronartium coleosporioides, and C. comandrae—was determined by an assessment of these diseases in 1993 in five seven‐year‐old experimental plantations in British Columbia and the Yukon in western Canada. After seven years, infection by E. harknessii was much higher on P. contorta than on P. sylvestris (27.3% resp. 2.5%). The average number of galls per infected tree was, however, higher on P. sylvestris than on P. contorta (5.99 resp. 3.61). C. coleosporioides was uncommon but occurred on P. contorta at all the five sites, while on P. sylvestris it occurred only at the two southern sites. 28.2% of the P. contorta trees were infected by C. comandrae at one site, but it was recorded only sporadically at the four other sites. P. sylvestris was slightly affected by C. comandrae at two of the sites.