ECOLOGY OF SURVIVAL AND RECOVERY FROM BLIGHT IN AMERICAN CHESTNUT TREES (CASTANEA-DENTATA (MARSH) BORKH) IN MICHIGAN

Between 1986-1987, 259 locations with American chestnut trees were examined in Michigan's lower peninsula. One hundred forty-four locations had trees with chestnut blight disease, while at 115 locations blight was not . All locations combined had 1492 (75%) diseased trees and 493 (25%) disease-free trees greater than or equal to 10 cm dbh. At 52 locations with blight 9813 chestnut trees less than 10 cm dbh were located and examined, while at 22 blight-free locations 4161 chestnut trees were found. At forty-two locations in Michigan, American chestnut trees had abnormal cankers and were recovering from the effects of chestnut blight disease. Quantitative measures were established at these locations to determine the extent of hypovirulence. At these locations there were 989 chestnut trees greater than or equal to 10 cm dbh and 3233 smaller chestnuts. It appears that initially all locations with abnormal cankers had normal virulent blight. Only after a lag of 15-25 years did abnormal cankers appear. The percentages of abnormal cankers and live branches above those cankers were determined for all 42 locations and compared to soil texture. Between 13% and 100% of the cankers at each location were abnormal. The percentage of live branches above abnormal cankers ranged from 41% to 100% per location. Trees on sites with sandy soils had more abnormal cankers and more surviving stems above them than those on heavier textured soils. Possible ecological factors which may explain the differential success of hypovirulence on different soil types include: (1) a better competitive advantage of the American chestnut on well drained sandy soils, (2) the origin of hypovirulence from sandy textured soils, and (3) more dispersing agents of hypovirulent strains on sandy textured soils.