ABSTRACT The influences of Microsphaeropsis sp., M. arundinis, Ophiostoma sp., Diplodia sp., and Trichoderma sp., all antagonists of Venturia inaequalis, on ascospore production were evaluated under natural conditions and compared with urea and Athelia bombacina, a known antagonist. In the autumn, the fungi were applied to leaf disks artificially inoculated with V. inaequalis and to scabbed apple (Malus domestica) leaves incubated under controlled and natural conditions. In addition, large-scale trials were conducted with Microsphaeropsis sp. applied either as a foliar postharvest spray or as a ground application at 90% leaf fall. All fungal isolates, except Ophiostoma sp., were recovered from the leaf material that overwintered in the orchard. All treatments, except those with Ophiostoma sp., resulted in a significant reduction in V. inaequalis ascospore production on the leaf disks incubated under controlled conditions or in the orchard. In 1997, leaves with apple scab lesions treated with urea or Microsphaeropsis sp. produced significantly fewer ascospores of V. inaequalis than did nontreated leaves, with a reduction of 73.0 and 76.3%, respectively. In 1998, leaves treated with Microsphaeropsis sp., urea, Trichoderma sp., A. bombacina, and M. arundinis reduced ascospore production by 84.3, 96.6, 75.2, 96.6, and 52.2%, respectively. Based on all tests combined, the most efficient isolate was Microsphaeropsis sp. Postharvest applications of Microsphaeropsis sp. reduced the total amount of airborne ascospores trapped by 70.7 and 79.8% as compared with the nontreated plots in 1997 and 1998, respectively. Microsphaeropsis sp. provided a significant and consistent reduction in ascospore production in all tests.