Search for Natural Products as Substitutes for Synthetic Pesticides in Agriculture

There is an increasing demand for environmentally friendly alternatives to synthetic pesticides in agriculture. In this context, a collaboration was established between the Research Institute of Organic Agriculture (FiBL, FiBL, Research Institute of Organic Agriculture) and the Division of Pharmaceutical Biology at the University of Basel with the aim of discovering alternatives to copper in organic agriculture. An in-house library of more than 3000 extracts from plant and fungal origin was screened in vitro against three severe agricultural pathogens, grapevine downy mildew (Plasmopara viticola), potato late blight (Phytophthora infestans), and tomato late blight (Pseudomonas fluorescens). Among the hits, Sumatra benzoin (the resin from Styrax benzoin) and a leaf extract of Inga sapindoides exhibited promising activity.
The methanolic leaf extracts Inga sapindoides exhibited good in vitro and in vivo activity against grapevine downy mildew (Plasmopara viticola). An extract concentration of 1 mg/mL reduced leaf surface infestation on Chasselas grapevine seedlings by 96%, which was comparable to a standard copper treatment. The activity could be assigned to an early eluting broad peak by HPLC-based activity profiling. Targeted purification by a combination of open column chromatography and HPLC on columns with various chemistries afforded a series of unique triterpenoid saponins with a monoterpene moiety and a p-methoxy cinnamoyl residue. The isolated compounds inhibited the formation of P. viticola zoospores in vitro with MIC100 values of 3 to 6 μg/mL.
Ethanolic solutions of Sumatra benzoin resin (Styrax benzoin) displayed potent antifungal activity in vitro and in vivo. The activity could be assigned to a main phenolic constituent, p-coumaryl cinnamate. Sumatra benzoin and p-coumaryl cinnamate may have the potential to be developed as a new agricultural fungicide. In order to obtain comprehensive data on the composition of the resin, a chromatographic profile was recorded for a commercial reference sample using various HPLC detectors. A total of 13 compounds including a new cinnamic acid ester derivative were conclusively identified by NMR analysis after preparative isolation. The isolated compounds account for an estimated 90% of the resin according to 1H NMR analysis of the reference resin. p-Coumaryl-cinnamate and the main triterpenoid, sumaresinolic acid, were quantified by HPLC analysis. In addition, a qualitative and (semi)-quantitative profile was established in a large set of resin samples of different quality grades which were purchased from various suppliers in Sumatra. While the qualitative profiles of most samples were similar, significant differences were observed in the relative contents which underscores the need for a thorough quality assessment of the resin.