Effect of fall application of fungal antagonists on spring ascospore production of the apple scab pathogen, Venturia inaequalis

The influences of Microsphaeropsis sp., M. arundinis, Ophiostoma sp., Diplo dia sp., and Trichoderma sp., all antagonists of Venturia inaequalis, on as cospore production were evaluated under natural conditions and compared wit h urea and Athelia bombacina, a known antagonist. In the autumn, the fungi were applied to leaf disks artificially inoculated with V. inaequalis and t o scabbed apple (Malus domestica) leaves incubated under controlled and nat ural conditions. In addition, large-scale trials were conducted with Micros phaeropsis sp. applied either as a foliar postharvest spray or as a ground application at 90% leaf fall. All fungal isolates, except Ophiostoma sp., w ere recovered from the leaf material that overwintered in the orchard. All treatments, except those with Ophiostoma sp., resulted in a significant red uction in V. inaequalis ascospore production on the lear disks incubated un der controlled conditions or in the orchard. In 1997, leaves with apple sca b lesions treated with urea or Microsphaeropsis sp. produced significantly fewer ascospores of V. inaequalis than did nontreated leaves, with a reduct ion of 73.0 and 76.3%, respectively. In 1998, leaves treated with Microspha eropsis 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. Ba sed on all tests combined, the most efficient isolate was Microsphaeropsis sp. Postharvest applications of Microsphaeropsis sp. reduced the total amou nt of airborne ascospores trapped by 70.7 and 79.8% as compared with the no ntreated plots in 1997 and 1998, respectively. Microsphaeropsis sp. provide d a significant and consistent reduction in ascospore production in all tes ts.