In vitro evaluation of the effects of some plant essential oils on Ascosphaera apis, the causative agent of Chalkbrood disease

Ascosphaera apis is one of the major fungal pathogens of honey bee broods and the causative agent of Chalkbrood disease. The factors responsible for the pathogenesis of Chalkbrood disease are still not fully understood, and the increasing resistance of A. apis to commonly used antifungal agents necessitates a search for new agents to control this disease. The in vitro antifungal activities of 27 plant essential oils against two isolates of A. apis (Aksu-4 and Aksu-9) were evaluated. Out of the 27 plant essential oils tested, 21 were found to be effective in killing both isolates of A. apis. Based on their minimum fungicidal concentration (MFC) values, the effective oils were grouped into three categories: highly effective, moderately effective and minimally effective. Mountain pepper oil, Kala Bhangra oil, spearmint oil, babuna oil, betel leaf oil, carrot seed oil, cumin seed oil and clove bud oil were highly effective, with MBC values between 50.0 μg/mL and 600.0 μg/mL. Mountain pepper was the most effective essential oil, with an MBC value of 50.0 μg/mL. Citral and caryophyllene containing oils were the most effective with MIC 50 ppm. The essential oils tested exhibited significant antimicrobial activities against both strains of A. apis, and they may contain compounds that could play an important role in the treatment or prevention of Chalkbrood disease of honeybee.