Potential and limits of acylcyclohexanediones for the control of blossom blight in apple and pear caused by Erwinia amylovora

Growth-regulating acylcyclohexanediones such as prohexadione-calcium and trinexapac-ethyl have been shown to be effective in controlling fire blight infections on shoots. Since blossoms represent the primary site of infection for the fire blight pathogen, Erwinia amylovora, trinexapac-ethyl and prohexadione-calcium were evaluated for their ability to reduce fire blight infection on apple and pear flowers. Field experiments and experiments under controlled conditions were conducted on apple flowers for 4 years. A reduction of up to 50% of blossom blight was observed in treated plants. In addition, treatment with trinexapac-ethyl reduced up to the 77% the percentage of fireblight-affected flowers from which disease progressed into shoots. On pear, numbers of flower infections were reduced by a quarter and flower infections leading to diseased shoots was reduced by up to 50%. Mechanisms underlying diseased reduction following treatment with the two acylcyclohexanediones was studied using a confocal laser scanning microscope combined with a gpf-labelled strain of E. amylovora. These non-invasive techniques demonstrated bacterial migration was reduced by up to 60 and 66% in apple and pear xylem, respectively.