Optimization of d-pinitol extraction from vegetable soybean leaves and its potential application in control of cucumber powdery mildew

•Twelve soybean cultivars were screened to obtain the highest amount of d-pinitol.•The extraction condition of d-pinitol from soybean was optimized by RSM.•The control effect against cucumber powdery mildew was tested in growth chamber.•The control effect was tested in the greenhouse in four provinces over two years.

d-pinitol is a bioactive compound with an important application in both food and nonfood industries. In the present study, the leaves of twelve vegetable soybean cultivars that are usually treated as agricultural waste were screened for the presence of d-pinitol. The results showed that the d-pinitol content in these 12 cultivars ranged from 1.32 to 3.04% (w/w) of dry weight, and the cultivar Z98002 was found to contain the highest amount of d-pinitol. Response surface methodology (RSM) was used for optimization of d-pinitol extraction conditions by the Box–Behnken design. The predicted extraction efficiency, which is at least 1.3-fold that of the previously reported typical method, reached to 3.27% under the following optimal conditions: temperature of 65.5 °C, extraction time of 86.8 min, and dilution rate of 1:10. The test in the growth chamber showed that the efficacy of soybean extracts in cucumber powdery mildew control was dose-dependent on the d-pinitol concentration, and the formulated extracts clearly increased the control compared to the crude extract. This effect of the d-pinitol formulation in a greenhouse was confirmed in four provinces of China over a period of two years. Our present study provides the optimized extraction conditions for a selected agricultural waste—vegetable soybean—to obtain a high yield of d-pinitol. Results from the growth chamber test and the greenhouse bioassay will be useful in exploring a low-cost phytochemical fungicide for cucumber powdery mildew control.