Honokiol inhibits growth and improves control efficiency against Phytophthora nicotianae through disrupting redox homeostasis and energy metabolism

Tobacco black shank, caused by Phytophthora nicotianae, is one of the most destructive fungal diseases affecting tobacco plants and is responsible for significant economic losses. Honokiol, as a botanical fungicide, is preferred over chemical fungicides to control tobacco black shank due to it being less toxic to the environment. This study evaluated the in vivo and in vitro antifungal activity of honokiol against Phytophthora nicotianae. Compared with the control, significant growth inhibition was observed upon honokiol treatment from 45% to 76%, respectively, at different concentrations. In vitro experiments indicated that honokiol disrupted hyphal morphology and increased membrane permeability in Phytophthora nicotianae. Honokiol treatment disrupted the cell membrane structure and resulted in increased sugar, protein, and relative conductivity-related contents. In addition, honokiol treatment reduced catalase and adenosine triphosphatase activity and increased superoxide dismutase activity, resulting in reactive oxygen species accumulation. The maximum inhibition of adenosine triphosphatase activity (3.98 mol/g protein) was achieved at 120 min with 54 µg/mL of honokiol. Moreover, in vivo analyses showed that honokiol protected tobacco seedlings from Phytophthora nicotianae and reduced the abundance of Fusarium pathogens; the highest control efficiency was recorded as 91.67% on day 5. Results suggest that honokiol can suppress tobacco black shank development, possibly by directly suppressing pathogen growth, and it possesses a remarkable biocontrol potential for Phytophthora nicotianae.