Characterization of epiphytic yeasts of apple as potential biocontrol agents against apple scab (Venturia inaequalis)

Twenty-two indigenous yeasts isolated from leaves and fruits of German apple orchards were tentatively allocated by a polyphasic approach to ascomycetous and basidomycetous genera. In the laboratory screening, the isolates displayed only weak inhibitory activity when tested for their antagonistic activity against Venturia inaequalis (Cooke) Winter amend. Aderhold, the causal agent of apple scab. However, in greenhouse trials, three strains suppressed scab severity on seedlings of Malus domestica (Borkh.) cv. 'Golden Delicious' as efficient as fungicide controls. Colonization of the surface of apple leaves by the isolate H25 and its antagonistic activity towards conidia of the apple scab pathogen was demonstrated by scanning electron microscopy. Application of a concentration of 1.5 x 107 yeasts per ml on 9-year-old apple trees cv. 'Golden Delicious' in an experimental orchard in Dossenheim, Germany, once in May 1998 led to significant scab reduction by strain H 10 whereas the strains H15, and H25 were less effective. The low performance of the antagonists observed in the season 1999 is not due to inefficient propagation since their establishment under field conditions and even spreading to other trees in the immediate neighbourhood of application was demonstrated using a culture method.