Seedlings of Betula alba inoculated with Botryosphaeria dothidea were exposed to controlled water stress, freezing stress, and defoliation stress.  Susceptibility to canker formation, and to colonization of wood and bark by the pathogen, increased with decreasing water potentials, beginning at approximately the –12 bar level as measured by the pressure bomb method.  Disease susceptibility induced by water stress was reversible; the rate of canker expansion declined and callus tissue formed at canker margins after seedling turgidity was restored by watering.  Canker formation and bark colonization occurred following exposure of partially cold-hardened seedlings to a rapid drop in ambient air temperature from 5 C to –30 C.  The extent of colonization of wood increased proportionally with exposure to minimum temperatures of –10 C, –20 C, and –30 C, respectively.  Susceptibility of seedlings in a more advanced stage of cold hardiness was not affected by freezing in these tests.  Canker formation, and colonization of bark and wood, occurred following 4 weeks of exposure of inoculated seedlings to defoliation stress, and increased with length of exposure.  Wounding was found to be a prerequisite for invasion of stems by the pathogen in seedlings exposed to defoliation stress.

Additional key words: predisposition, water stress, freezing stress, defoliation stress, canker.