STUDIES IN FOREST PATHOLOGY: XXIV. POLYPORUS TOMENTOSUS FR. AS A MAJOR FACTOR IN STAND-OPENING DISEASE OF WHITE SPRUCE

Stand-opening disease in Saskatchewan spruce forests is characterized by the occurrence of patches, often of an acre or more, of declining and dead trees. Polyporus tomentosus Fr. is a major cause of the disease, which develops slowly, with 20 to 30 years elapsing between initiation of attack and death of the trees. Extensive root decay develops before the above-ground symptoms become apparent. Inoculation experiments have shown that P. tomentosus grows about 1.5 in. per year, and that the fungus is parasitic on white spruce.Polyporus tomentosus may enter healthy roots at a point of contact with infected roots; no evidence was found that it entered through root tips or directly from the soil. The fungus remains viable in infected roots for at least 16 years. The disease is commonly associated with injury caused by root-tunnelling weevils in the genus Hylobius. Host vigor did not appear to influence infection or disease development. In culture, the optimum temperature for P. tomentosus was 20 °C and the optimum pH was 4.5. At lower temperatures P. tomentosus appeared to have a competitive advantage over fungi isolated from dying rootlets. Small seedlings grown in agar with P. tomentosus were killed in 8 months. Under these conditions P. tomentosus was much less virulent than Rhizoctonia, solani Kühn and Phytophthora cactorum L. and C. Schroet.