Microemulsion preparation of Waltheria indica extracts and preliminary antifungal mechanism exploration

Waltheria indica L. is a traditional officinal plant with high medicinal values. This work aims to develop a stable microemulsion of W. indica extract and study its antifungal activity against Phytophthora capsici Leonian. The results indicated that the optimal compositions of W. indica microemulsion (WIM) were 5 % crude extract of W. indica, 10 % ethyl acetate, 5 % ethanol as solvents, 5 % pesticide emulsifier 601# (601), 10 % pesticide emulsifier BY (El-40) and 65 % water, and the microemulsion presented an average particle diameter with 1047 nm, the polydispersity index was 0.19. The W. indica microemulsion (WIM) could effectively inhibit the different growth and development stages of P. capsica in vitro, and the EC50 of WIM inhibit P. capsici mycelial growth and sporangium formation and zoospore production were 0.228 and 1.923 and 1.786 μg/mL and exhibited significantly protective activity against P. capsica in pot experiments. In addition, the WIM treatment caused severe destruction to the cell membrane of P. capsici that contents of the cells leaked out. The relative conductivity, N-Acetyl-D-Glucosamine (D-GlcNAc) contents, and malondialdehyde (MDA) concentration were also significantly increased after WIM treatment. Besides, WIM inhibited the activities of P. capsici antioxidant enzymes (SOD, POD, CAT), thus increased the content of hydrogen peroxide, further aggravated the damage of the membrane system. In general, these findings provided a scientific basis for WIM as a biological agent against P. capsici.