Management of Monilinia laxa, the causal agent of brown rot blossom blight in almond (Prunus dulcis), relies heavily on the use of chemical fungicides during bloom. However, chemical fungicides can have nontarget effects on beneficial arthropods, including pollinators, and select for resistance in the pathogen of concern. Almond yield is heavily reliant on successful pollination by healthy honey bees (Apis mellifera); thus, identifying sustainable, effective, and pollinator-friendly control methods for blossom blight during bloom is desirable. Flower-inhabiting microbes could provide a natural, sustainable form of biocontrol for M. laxa, while potentially minimizing costly nontarget effects on almond pollinators and the services they provide. As pollinators are sensitive to floral microbes and their associated taste and scent cues, assessing effects of prospective biocontrol species on pollinator attraction is also necessary. Here, our objective was to isolate and identify potential biocontrol microbes from an array of agricultural and natural flowering hosts and test their efficacy in suppressing M. laxa growth in culture. Out of an initial 287 bacterial and fungal isolates identified, 56 were screened using a dual culture plate assay. Most strains reduced M. laxa growth in vitro. Ten particularly effective candidate microbes were further screened for their effect on honey bee feeding. Of the 10, nine were found to both strongly suppress M. laxa growth in culture and not reduce honey bee feeding. These promising results suggest a number of strong candidates for augmentative microbial biocontrol of brown rot blossom blight in almond with potentially minimal effects on honey bee pollination.
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A Bacterial Symbiont Protects Honey Bees from Fungal Disease
ABSTRACT Fungal pathogens, among other stressors, negatively impact the productivity and population size of honey bees, one of our most important pollinators (1, 2), in particular their brood (larvae and pupae) (3, 4). Understanding the factors that influence disease incidence and prevalence in brood may help us improve colony health and productivity. Here, we examined the capacity of a honey bee-associated bacterium, Bombella apis , to suppress the growth of fungal pathogens and ultimately protect bee brood from infection. Our results showed that strains of B. apis inhibit the growth of two insect fungal pathogens, Beauveria bassiana and Aspergillus flavus , in vitro . This phenotype was recapitulated in vivo ; bee broods supplemented with B. apis were significantly less likely to be infected by A. flavus . Additionally, the presence of B. apis reduced sporulation of A. flavus in the few bees that were infected. Analyses of biosynthetic gene clusters across B. apis strains suggest antifungal candidates, including a type 1 polyketide, terpene, and aryl polyene. Secreted metabolites from B. apis alone were sufficient to suppress fungal growth, supporting the hypothesis that fungal inhibition is mediated by an antifungal metabolite. Together, these data suggest that B. apis can suppress fungal infections in bee brood via secretion of an antifungal metabolite. IMPORTANCE Fungi can play critical roles in host microbiomes (5–7), yet bacterial-fungal interactions are understudied. For insects, fungi are the leading cause of disease (5, 8). In particular, populations of the European honey bee ( Apis mellifera ), an agriculturally and economically critical species, have declined in part due to fungal pathogens. The presence and prevalence of fungal pathogens in honey bees have far-reaching consequences, endangering other species and threatening food security (1, 2, 9). Our research highlights how a bacterial symbiont protects bee brood from fungal infection. Further mechanistic work could lead to the development of new antifungal treatments.
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- NSF-PAR ID:
- 10329369
- Editor(s):
- Graf, Joerg
- Date Published:
- Journal Name:
- mBio
- Volume:
- 12
- Issue:
- 3
- ISSN:
- 2150-7511
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1128/mBio.00503-21
