Rheum officinale free anthraquinone composition for preventing plant powdery mildew and nano emulsion preparation

The invention belongs to the technical field of biopesticides, and particularly relates to a natural-proportion composition of rheum officinale free anthraquinone as well as a formula of a nano emulsion preparation of the composition, a preparation process of the composition and application of the composition for preventing powdery mildew. Monomer emodin, aloe-emodin, chrysophanol, emodin monomethyl ether and rhein in the rheum officinale free anthraquinone composition are extracted from the traditional Chinese medicine rheum officinale and provided with a natural proportion with a fixed range. The rheum officinale free anthraquinone composition and the nano emulsion preparation thereof can be used for preventing the plant powdery mildew. The free anthraquinone in the rheum officinale free anthraquinone nano emulsion disclosed by the invention is from the traditional Chinese medicine rheum officinale, the extracting process is strong in applicability, the raw material obtaining cost is low, the nano emulsion preparation is spontaneously formed, the process is simple, the quality is controllable, the large-scale production is easy, and an excellent prevention effect is shown for the plant powdery mildew.