Effectiveness of bacteria and yeasts from apple orchards as biological control agents of apple scab

A total of 931 bacteria and yeasts were isolated from soil and different apple tissues collected in abandoned and commercial orchards. Ninety-two isolates that exhibited in vitro antibiosis and 268 that did not (total 360) were tested for their ability to suppress scab (caused by Venturia inaequalis) on apple seedlings in the greenhouse. Thirty-two isolates completely inhibited germination of conidia of V. inaequalis in vitro and 106 significantly reduced the amount of apple scab that formed on apple seedlings when they were applied 24 h before inoculation with V. inaequalis. No correlation existed between in vitro antibiosis and ability to suppress scab on seedlings; however, one isolate that was identified as an isolate of Pseudomonas syringae (isolate 508) prevented conidial germination and effectively suppressed scab to a level comparable to the fungicide captan. When compared to pathogenic isolates of P. syringae that were isolated from various crops, 508 suppressed scab to a greater extent except in one case. (C) 1996 Academic Press, Inc.