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Severe damage by Scleroderris canker, caused by the fungus Gremmeniella abietina , was recorded in provenance and progeny tests as well as in conventional plantings of Pinus contorta in areas with a harsh climate in northern Sweden in 1987. Damage was related to a period with extreme weather conditions. The Swedish Forestry Act placed restrictions on the planting programme of P. contorta in 1987, 1989 and 1991. The disease situation in northern Sweden in the mid 1990s is discussed as well as the risks associated with the introduction of exotics in relation to the historical background. Attention is also drawn to parasitic fungi, which are infecting P. contorta in its natural range in western Canada, and to the potential threat they represent to the indigenous Pinus sylvestris in Sweden.

The North American tree Pinus contorta var, latifolia was experimentally introduced in Sweden already in the 1920s, and has been used in Swedish forestry on a large scale since the 1970s. These pla ...

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.

Tree height and number of western gall rust (caused by Endocronartiumharknessii (J.P. Moore) Hirat.) bole and branch infections per living tree were recorded in three experimental 7-year-old lodgepole pine (Pinuscontorta Dougl. ex Loud.) plantations located near Fort St. James, Mackenzie, and Fort St. John, B.C. An extra set of data for trees originating from north of 60°N was also taken at Fort St. John. Each plantation consisted of 40 types of genetic material (i.e., different parentage and latitude of origin), while the extra set at Fort St. John consisted of 17 types of genetic material. The 4357 trees examined had an average of 1.81 gall rust infections per tree, and 49.1% of all trees were infected. Ninety percent of all infections originated in a single year (1990). The number of galls per unit target area declined with increasing height above ground. The percentage of galls located on the bole was 3.10, 10.53, 8.03, and 9.88% for the four populations, respectively. The number of western gall rust infections per tree was linearly related to tree height. The number of infections on boles did not vary with tree height, but the proportion of all infections located on the bole was linearly related to the reciprocal of tree height. The proportion of infections on the bole did not vary significantly with infection severity. Finally, the proportion of galls on the bole did not differ significantly between types of genetic material.

From 1987–1991, conditions in 110 conventional plantings of introduced lodgepole pine (Pinuscontorta Dougl. ex Loud.) were investigated in northern Sweden. Severe damage by Scleroderris canker, caused by the fungus Gremmeniellaabietina (Lagerb.) Morelet, was recorded at high elevations in 1987. Damage was related to a period with extreme weather conditions. The disease was initially severe in low lying areas, from where it quickly spread throughout the plantations during 1988. Temperatures above average during 1988–1990 favoured tree vigour, which in turn slowed the spread of the disease. The frequency of stem cankers increased in seriously infected areas during 1989, and new severe damage was recorded locally in 1990 and in 1992. A strong correlation was found between disease severity and the temperature sum at the site. Lodgepole pine planted on spruce sites was often severely affected. A negative correlation was found between the frequency of G. abietina and the abundance of birch thicket. In large areas in northern Sweden with a more favourable climate, lodgepole pine plantations were healthy and productive up to 1991. However, instability, leading to increased susceptibility to disease, has become a problem of great concern in some of these areas during the 1990s.