New insights into the anti-oomycete mechanisms of a leaf extract from Glycyrrhiza glabra L. against Pseudoperonospora cubensis

Cucurbit downy mildew, caused by Pseudoperonospora cubensis, is responsible for high economic losses worldwide in cucumber production. Synthetic pesticides or copper-based products are still important tools to manage the disease. However, the pathogen has developed resistance against common fungicides rather quickly and there is a need for alternative plant protecting agents. Glycyrrhiza glabra L. leaf extract is known for its antifungal activity and was highly effective in former bioassays and semi-commercial trials against downy mildew of cucumber. To elucidate the active ingredients and the mode of action, licorice leaf extract was here fractionated into five fractions (F1-F5) with a newly developed and optimized separation process via flash chromatography (FC). The crude extract (P1) and fraction F1 inhibited the zoospore release from sporangia, the zoospore germination and germ tube development of P. cubensis almost completely on two cucumber cultivars, one tolerant and one susceptible to the pathogen. Infestation rates were reduced between 73% and 96%. F1 contained three previously reported antifungal polyphenols glabranin, pinocembrin and licoflavanon. Here, we report an additional new compound, namely naringenin. Further, F5 is found here to show some protective effects against P. cubensis, either based on direct fungicidal or indirect effects via the host plant. The presented results showed that licorice leaf extract can serve as an alternative plant protection agent, able to manage P. cubensis infestation on cucumber cultivars with differing levels of susceptibility by interfering with important early stages in the pathogen development.