<i>Aureobasidium</i> <i>pullulans</i> from the Fire Blight Biocontrol Product, Blossom Protect, Induces Host Resistance in Apple Flowers

Fire blight, caused by <i>Erwinia amylovora</i>, is a devastating disease of apple. Blossom Protect, a product that contains <i>Aureobasidium pullulans</i> as the active ingredient, is one of the most effective biological controls of fire blight. It has been postulated that the mode of action of <i>A. pullulans</i> is to compete against and antagonize epiphytic growth of <i>E. amylovora</i> on flowers, but recent studies have found that flowers treated with Blossom Protect harbored similar to or only slightly reduced <i>E. amylovora</i> populations compared with nontreated flowers. In this study, we tested the hypothesis that <i>A. pullulans</i>-mediated biocontrol of fire blight is the result of induced host resistance. We found that <i>PR</i> genes in the systemic acquired resistance pathway, but not genes in the induced systemic resistance pathway, were induced in hypanthial tissue of apple flowers after the Blossom Protect treatment. Additionally, the induction of <i>PR</i> gene expression was coupled with an increase of plant-derived salicylic acid in this tissue. After inoculation with <i>E. amylovora, PR</i> gene expression was suppressed in nontreated flowers, but in flowers pretreated with Blossom Protect, the heightened <i>PR</i> expression offset the immune repression caused by <i>E. amylovora</i>, and prevented infection. Temporal and spatial analysis of <i>PR</i> gene induction showed that induction of <i>PR</i> genes occurred 2 days after the Blossom Protect treatment, and required direct flower-yeast contact. Finally, we observed deterioration of the epidermal layer of the hypanthium in some of the Blossom Protect-treated flowers, suggesting that <i>PR</i> gene induction in flowers may be a result of pathogenesis by <i>A. pullulans</i>.