Antibacterial Activity and Mechanisms of Ethanol Extracts from Scutellaria baicalensis Georgi and Magnolia officinalis against Phytophthora nicotianae

Phytophthora nicotianae causes substantial economic losses in most countries where tobacco is produced. At present, the control of P. nicotianae mainly depends on chemical methods, with considerable environmental andhealth issues. We investigated the effects of ethanol extracts from Scutellaria baicalensis Georgi (SBG) and Magnolia officinalis (MO). On mycelial growth, sporangium formation, and zoospore release of P. nicotianae. Bothextracts inhibited the growth of P. nicotianae, with mycelial growth inhibition rates of 88.92% and 93.92%, respectively, at 40 mg/mL, and EC50 values of 5.39 and 5.74 mg/mL, respectively. The underlying mechanisms were theinhibition of sporangium formation, the reduction of zoospore number, and the destruction of the myceliumstructure. At an SBG extract concentration of 16.17 mg/mL, the inhibition rates for sporangia and zoospores were98.66% and 99.39%, respectively. At an MO extract concentration of 2.87 mg/mL, the production of sporangiaand zoospores was completely inhibited. The hyphae treated with the two plant extracts showed different degreesof deformation and damage. Hyphae treated with SBG extract showed adhesion and local swelling, whereas treatment with MO extract resulted in broken hyphae. Mixture of the extracts resulted in a good synergistic effect.