Broad spectrum activities of essential oils in the lab fail to translate to blueberry disease control for organic producers in the field

Blueberry production in the southeastern U.S. is challenged by various fungal diseases, especially in organic systems. Essential oils (EOs), derived from plants, offer a potential sustainable alternative to synthetic fungicides. In this study, we evaluated the in vitro and in vivo efficacy of several EOs against key blueberry fungal pathogens including Colletotrichum, Corynespora, Lasiodiplodia, and Neofusicoccum isolates. While tea tree oil showed very limited antifungal activity; thyme, oregano, savory, and cinnamon oils demonstrated varying, but promising antifungal activity against all fungal isolates in vitro. The EC50 values varied significantly across the isolates and oils, with thyme and cinnamon showing the widest ranges (up to 692 and 636 µL L⁻¹), while oregano and savory remained more consistent. In contrast, EO-based products failed to provide significant disease control compared to untreated controls in field trials. Additionally, concerns regarding phytotoxicity arose from repeated applications and higher concentrations. Other organic fungicides, such as polyoxin D zinc salt (OSO) and copper-based products, showed promise in vivo against Anthracnose, but their overall effectiveness varied. These findings highlight the complexities of translating in vitro efficacy to field performance and emphasize the need for further research to optimize the use of EOs and other organic fungicides in blueberry production.