Susceptibility of Pinus sylvestris to the stem rusts of Pinus contorta in Western Canada

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.