In Vitro and Greenhouse Evaluation of Fungicides and Bacillus Antagonists Against Diplodia corticola (Botryosphaeriaceae, Botryosphaeriales) on Quercus suber

Cork oak (Quercus suber) forests are threatened by emergent fungal pathogen Diplodia corticola, which causes significant economic and ecological losses. This study evaluates the efficacy of synthetic and natural fungicides, as well as Bacillus antagonistic agents, against this phytopathogen in vitro and in vivo. Eighteen fungicidal agents were tested across three concentrations, whereas the bacterial antagonistic agents Bacillus amyloliquefaciens and a mixture of B. amyloliquefaciens + Bacillus mojavensis were tested at a fixed concentration. The assayed chemicals, including penconazole, clove oil, vanillin, and belthanol, showed 100 ± 0.0% radial growth inhibition (n = 24) and conidiation (n = 24), highlighting their potential as alternatives to benomyl and methyl thiophanate (Restricted in the European Union). In vivo assays further validated the efficacy of these agents in reducing symptom incidence and seedling mortality in cork oak seedlings. Similarly, the Bacillus-based treatments showed 47.6 ± 0.9% (n = 35) in vitro antagonistic effects and in vivo application on seedlings (n = 470) significantly reduced disease symptoms and supported physiological stability (GLMs with Tukey HSD post hoc). The study aimed to evaluate chemical, natural and biological control agents against this pathogen to identify effective management alternatives for forest nurseries and cork oak.