Late blight, caused by Phytophthora infestans, is one of the most destructive diseases in potato cultivation. Chemical fungicides are currently the primary means of controlling it, but they pose significant issues, including environmental pollution and the development of resistant strains. Plant jiaosu (PJ), derived from the direct fermentation of various plants, plays a vital part in various fields such as environmental protection, agriculture, daily chemicals, and livestock. This study illustrates that PJ, fermented from lettuce leaves, significantly inhibits the growth of P. infestans. An isolated anti-oomycete bacterium, designated X-1, has 100% homology to Bacillus subtilis in the NCBI database, shown through 16S rDNA sequence analysis. B. Subtilis X-1 effectively inhibits the hyphal growth and sporangia germination of P. infestans, induces morphological changes in the hyphae, and can promote the growth of potato. PJ exerts a synergistic effect with the chemical fungicide Infinito (the main active ingredients are fluopicolide and propamocarb hydrochloride). In summary, this study provides a novel approach for the reutilization of fruit and vegetable waste and offers an environmentally friendly and effective alternative to chemical fungicides for controlling potato late blight.