In organic agriculture, there is a growing demand to replace copper by environmentally safer substitutes. In this context, an in-house library of more than 3000 extracts of plant and fungal origin was screened against the major plant pathogens Venturia inaequalis, Phytophthora infestans, and Plasmopara viticola.
As one of the hits, the ethyl acetate extract from inflorescences of Verbesina lanata Rob. & Greenm. (Asteraceae) showed significant inhibitory activity in vitro against P. viticola with a MIC100 value of 35 μg/mL. In vivo activity was assessed on grapevine seedlings. A dry formulation with 50 mg/g of extract was developed to enhance the solubility. A 5 mg/mL solution of the formulation lowered leaf surface infestation to 18% compared to 79% infestation with the blank formulation.
With the aid of HPLC-based activity profiling, the activity could be correlated with a series of lipophilic compounds. Preparative isolation by a combination of chromatographic techniques, including silica gel column chromatography and preparative HPLC, afforded eight new and eight known eudesmane sesquiterpenes. Nine compounds were obtained in sufficient quantities to be tested in vitro, and were found to inhibit the growth of P. viticola with MICs100 ranging from 4 to 50 μg/mL. The two major compounds 1 and 2 showed MICs100 of 31 μg/mL and 5 μg/mL, respectively.
Our results demonstrate that plant derived compounds could serve as potential alternatives to copper in organic farming.
