Ascospore release by Venturia inaequalis during periods of extended daylight and low temperature at Nordic latitudes

Darkness suppresses ascospore release in Venturia inaequalis, but the impact of light levels during the extended twilight and dusk that typify Nordic spring conditions is poorly understood. Volumetric spore traps were operated at two different locations in Norway over several years. During the season of asocspore release (approximately 1 April to 30 June), on 25 occasions when rain started during night (after 23:00 h and before 04:00 h) and leaves remained wet until at least midnight the following day, the cumulative percentage of spores trapped at sunrise did not exceed 1%, irrespective of temperature. Three hours after sunrise, cumulative ascospore release reached 0.8%, 3.0%, and 8.1% at temperatures of 0 to 5°C, 5 to 10°C, and >10°C, respectively, and 50% release occurred at 11, 9, and 8 h after sunrise. Additional field and laboratory studies indicated that the protracted dawn and dusk of Nordic latitudes, either alone or in combination with low temperatures, does not substantially alter previously reported patterns of ascospore release.