STUDIES IN FOREST PATHOLOGY: XV. ROOTLETS, MYCORRHIZA, AND SOIL TEMPERATURES IN RELATION TO BIRCH DIEBACK

Cylindrosporium sp., isolated from dead yellow birch roots, inhibited the growth of another fungus isolated from mycorrhiza and dead roots. Growth rate of the former increased while that of the latter decreased with increased temperature above the optimum for rootlet, development. Twenty-four species of fungi were isolated from mycorrhiza. Seedlings grown aseptically in soil and in agar were inoculated with fungi isolated from dead and dying rootlets and from mycorrhiza, but none of the organisms displayed sufficient virulence to cause birch dieback. The optimum temperature for development of potted seedlings was about 18.5 °C. Increases or decreases of 2 °C. or more from the optimum for yellow birch development resulted in increased rootlet mortality, decreased frequency of mycorrhiza, and a thinner mantle of hyphae in mycorrhiza, followed by reduction in foliage vigor. Normal rootlet mortality of 6% in a 55-year-old yellow birch stand was increased to 19 and 60% by raising soil temperature 1° and 2 °C, respectively, above the normal during one summer. Soil moisture was not lowered a statistically significant amount by a 5 °C. increase until the sixth day following rain. Mycorrhiza existed on residual living roots in about the same proportion as on undisturbed root systems. The possibility that birch dieback may be the result of soil temperature increases is discussed. It is suggested that cutting practices in hardwood stands should remove all yellow birch trees or prevent increased temperatures in soil occupied by residual birch.