Author:

Keywords:

Biological control agents, Erwinia amylovora, Pome fruits, Bumble bees

Abstract:

During bloom apple and pear are susceptible to fire blight infection which occurs when Erwinia amylovora bacteria colonize the stigma and subsequently reach, due to rain or dew droplets, the flower hypanthium where they penetrate the openings of the nectary or nectarthodes. First the flowers necrotize then the peduncles, the shoots and finally the complete stem. While the disease gradually spreads through the tissues, ooze droplets are formed that contain high concentrations of bacteria. In Belgium the infection risk increases with increasing temperature making the secondary bloom later in the season more vulnerable than the primary bloom in early spring when night temperatures are still low and frost occurs. Similarly, trees that are planted late in the season and flower in summer as well as young fruits and shoots damaged by hail in the summer can become rapidly infected by E. amylovora. This infection is worsened if ooze droplets are present in the orchard. The current strategies for control focus on preventive treatments such as sprayings with plant defense enhancer molecules like fosethyl aluminium (Aliette®) and laminarin (Vacciplant®) as well as with heavy metals including copper and manganese that help to reduce the inoculum in the orchard during the season. To avoid phytotoxicity the Aliette® treatments are done before and/or after bloom making the use of biocontrol organisms (BCOs) during bloom an attractive approach to protect the flowers. Although the antagonistic yeast Aureobasidium pullulans (Blossom ProtectTM) that can block flower colonization by E. amylovora is registered in Belgium, its use is limited today. As flowers gradually open, the BCO should be sprayed at least two to three times during bloom to protect all flowers, which is time consuming and expensive. In the research project that is presented here, we aim at a continuous application of the BCO at the sites of flower infection (i.e. stigma and hypanthium) by bumble bee (Bombus terrestris) vectoring. Besides A. pullulans, also existing BCO strains of Bacillus subtilis and Pantoea agglomerans as well as a new strain of Paenibacillus polymyxa are tested. A search for new candidate BCOs naturally present in apple and pear flower microbiomes is also ongoing. Initial results show that (i) under optimal infection conditions there is a risk of E. amylovora spread by bumble bees after visiting infected flowers (ii) BCOs are acquired to a variable degree by bumble bees, (iii) in 2015 primary bloom flower visitation in the orchard was absent in pear and limited in apple, whereas bumble bees visited secondary flowers of both better and (iv) tested BCOs show potential for fire blight control in the greenhouse.