Screening antifungal compounds for inhibitory action against Ascosphaera apis, the causative agent of chalkbrood

Despite the fungi being a large kingdom, with estimates of over 2 million extant species, they are relatively understudied. Less than 200,000 species of fungi have been formally described. Among the 19 currently recognized phyla, the Ascomycota is the largest. The phylum is composed of a diverse group of fungi, including the genus Ascosphaera- a group of spore cyst fungi- associated exclusively with bees. While most of the Ascosphaera are saprophytes, Ascosphaera apis is a notable entomopathogen and causative agent of chalkbrood disease in the honey bee (Apis mellifera). Currently, there are no effective treatments for chalkbrood. Recent surveys demonstrate that commercial beekeepers are experiencing staggering honey bee colony losses, likely related to pests, pathogens, pesticides, and poor nutrition. Reports of honey bee losses highlight the need to further study honey bee-associated pathogens like A. apis. In this study, we evaluate the inhibitory activity of five antifungal compounds, propiconazole, chlorothalonil, benzimidazole, iprodione, and thymol- against A. apis, using a modified Kirby – Bauer assay. Additionally, the safety of these fungicides will be tested on honey bee adults and larvae reared in vitro. We expect to find several fungicides in different chemical classes that are able to significantly restrict the growth of chalkbrood. Future work will find ways to safely administer these fungicides in the field to see if chalkbrood infection can be reduced safely and quickly for beekeepers.