Protection of apple and pear flowers against fire blight infections using biocontrol organisms applied via bumble bees

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, 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 have better visited secondary flowers and (iv) tested
BCOs show potential for fire blight control in the greenhouse