Comparison of the seasonal pattern of airborne Venturia inaequalis ascospores with the release potential of V-inaequalis ascospores from a source

Release of Venturia inaequalis ascospores was monitored during four ascospore release seasons by two independent methods. In the first method, airborne ascospores were monitored in the field with a Burkard volumetric spore sampler located in the center of the source area and operated continuously during each season. In the second method, the potential release of ascospores was determined every 3 to 5 days during each season by measuring spore release from scabbed apple leaves freshly collected from the field and tested under standard conditions in spore release towers in the laboratory. In addition, the number of scabbed leaves per unit of ground area in the source area was determined periodically during each season. The date when half the season's total ascospores was released in the towers lagged behind the Burkard sampler by 7, 18, 11, and 14 days (mean = 12.3 days) in 1992, 1993, 1994, and 1995, respectively. This lag suggests a late-season decrease in the dispersal potential of V. inaequalis ascospores. Much of the lag between ascospores released in the towers and those airborne in the field can be explained by a natural decline in the number of source leaves per unit of ground area as the season progressed.