DISCHARGE AND DISSEMINATION OF ASCOSPORES BY VENTURIA-INAEQUALIS DURING DEW

Abundant airborne ascospores of the apple scab pathogen (Venturia inae qualis) have never before been observed during periods of dew. We stud ied ascospore release in V. inaequalis in two orchards in southeastern Norway using Burkard 7-day volumetric spore traps. At Angstrom s in 1 990, 1992, and 1997, and at Svelvik in 1992, a total of 14.8, 1.4, 0.2 7, and 26.9%, respectively, of the season's total spore release was tr apped during periods of dew. Dew followed by spore release was observe d 22 days at the two locations. During one night with dew at Angstrom s in 1990 and two nights with dew at Svelvik in 1992, approximately 13 and 20%, respectively, of the season's total spore numbers were obser ved. High numbers of spores were trapped prior to sunrise, and on an a verage, 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 fo r 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 i n previous studies under laboratory conditions when protracted periods favorable for ascospore maturity occur without opportunity for ascosp ore discharge. This is the first confirmed report of relatively large (>10% of the season's total inoculum) numbers of airborne ascospores i n orchards during dew. The sequential occurrence of specific weather c onditions, for example (i) fair-weather days, (ii) cool nights with ab undant dew formation, (iii) significant release and dispersal of airbo rne ascospores, and (iv) poor drying conditions or additional hours of leaf wetness due to fog or rain, would be required for dew-released a scospores to constitute a threat of infection. Absent the foregoing, r elease during dew is more likely to deplete the ascospore supply with no consequent increase in the overall risk of disease.