Discharge and Dissemination of Ascospores by Venturia inaequalis During Dew

Abundant airborne ascospores of the apple scab pathogen (Venturia inaequalis) have never before been observed during periods of dew. We studied ascospore release in V. inaequalis in two orchards in southeastern Norway using Burkard 7-day volumetric spore traps. At Ås in 1990, 1992, and 1997, and at Svelvik in 1992, a total of 14.8, 1.4, 0.27, and 26.9%, respectively, of the season's total spore release was trapped during periods of dew. Dew followed by spore release was observed 22 days at the two locations. During one night with dew at Ås in 1990 and two nights with dew at Svelvik in 1992, approximately 13 and 20%, respectively, of the season's total spore numbers were observed. High numbers of spores were trapped prior to sunrise, and on an average, 48.4% of the spores were trapped prior to 0400 in the morning. Episodes in which more than 1% of the season's inoculum was released during dew occurred around bloom of apple, which is the peak period for ascospore discharge, and followed more than 2 days of fair weather (clear, warm days and cool, humid nights). The ordinary suppression of ascospore release in V. inaequalis during darkness has been overcome in previous studies under laboratory conditions when protracted periods favorable for ascospore maturity occur without opportunity for ascospore discharge. This is the first confirmed report of relatively large (>10% of the season's total inoculum) numbers of airborne ascospores in orchards during dew. The sequential occurrence of specific weather conditions, for example (i) fair-weather days, (ii) cool nights with abundant dew formation, (iii) significant release and dispersal of airborne ascospores, and (iv) poor drying conditions or additional hours of leaf wetness due to fog or rain, would be required for dew-released ascospores to constitute a threat of infection. Absent the foregoing, release during dew is more likely to deplete the ascospore supply with no consequent increase in the overall risk of disease.