Inhibition of Pseudothecial Development ofVenturia inaequalisby the BasidiomyceteAthelia bombacinain Apple Leaf Litter

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Abstract
An immunocytochemical stain was developed to detect the basidomycete Athelia bombacina in apple leaf litter. The polyclonal antibodies for A. bombacina were sufficiently specific that only hyphae of this fungus were detected in immunocytochemically treated sections of dead leaves inoculated with A. bombacina. Apple leaves naturally infected with Venturia inaequalis were inoculated with A. bombacina, incubated outside from November 1986 to May 1987, and sampled monthly. Sections stained immunocytochemically showed that A. bombacina grew endophytically and epiphytically. The antagonist prevented neither growth of hyphae of V. inaequalis into the interior of leaves, nor initiation of pseudothecia. There was no particular spatial association between hyphae of the two fungi, nor any sign of direct parasitism of hyphae or pseudothecia of V. inaequalis. Pseudothecia in leaves with the antagonist did not mature further than the stage of producing pseudoparaphyses, reaching an average of 84 .mu.m in length. Pseudothecia in leaves without A. bombacina developed asci normally and were 108 .mu.m long by 1 May. These data were confirmed by results from an abbreviated sampling scheme of McIntosh apple leaves during 1987-1988 and by observations of pseudothecial inhibition in crab apple leaves following delayed application of the antagonist.