Efficacy of a Magnolia officinalis bark extract against grapevine downy mildew and apple scab under controlled and field conditions

In organic agriculture, the control of several diseases is largely depending on copper fungicides. Yet, copper can accumulate in the soil if the annual input exceeds annual uptake by plants, which can have a negative impact on soil fertility. Its use should thus be avoided or reduced. The aim of the present study was to evaluate the efficacy of a bark extract of Magnolia officinalis Rehder and Wilson to control three pathogens including Plasmopara viticola (Berk. & M.A. Curtis) Berl. & De Toni (causing grapevine downy mildew), Venturia inaequalis (Cooke) G. Winter (causing apple scab), and Phytophthora infestans (Mont.) De Bary (causing potato and tomato late blight) under controlled and field conditions, and to identify compounds responsible for the antifungal activity of the extract. Under controlled conditions, M. officinalis bark extract showed a mean efficacy of 97% (P. viticola) and 93% (V. inaequalis) at 1 mg mL−1, and EC50 between 0.14 and 0.20 mg mL−1. Efficacy against P. infestans was comparatively low (52% at 1 mg mL−1). Magnolol and honokiol were identified as the main active compounds, both with EC50 ≤ 0.08 mg mL−1 against P. viticola and V. inaequalis. Under field conditions, preliminary formulations reached efficacies up to 71% at 1–2 mg plant extract mL−1 against grapevine downy mildew, whereas activity against apple scab could not be confirmed. Magnolia officinalis is a promising candidate for the development of a sustainable plant protection product against grapevine downy mildew due to a combination of good efficacy, high availability of the raw material at affordable prices, reasonable extraction efficiency, and expected low human toxicity due to its longstanding use in traditional Chinese medicine.