After initial results concerning an induced resistance effect of the plant extract of Hedera helix against fire blight on the ornamental Cotoneaster salicifolius further investigations were undertaken with the highly susceptible apple rootstock M26 under controlled greenhouse conditions. Additionally, in physiological studies, total phenol content and enzymatic activities of peroxidase (PO) and polyphenoloxidase (PPO), as markers of resistance, were evaluated. After application of the plant extract (PE) to the foliage of the host plant, using a 2 day induction interval, a reduction of disease severity of up to 31 % was observed. Correspondingly the multiplication of E. amylovora was reduced by up to 43 % during the course of the experimental period. In physiological studies, significant changes in the phenol metabolism of the host plant, as an indication of a resistance reaction, was observed. A marked increase of the total phenol content corresponding to significantly higher activities of peroxidase (PO) and polyphenoloxidase (PPO) were found after plant extract treatment in inoculated and uninoculated shoots particularly, 2-4 days after application. It was concluded that there was an induced resistance effect of the plant extract from H. helix against fire blight.
