Efficacy of a bacterial preparation of Aneurinibacillus migulanus against downy mildew of cucumber (Pseudoperonospora cubensis)

A bacterial preparation of Aneurinibacillus migulanus white (a phenotype isolated in our laboratory after subculturing of A. migulanus WT in TSB medium) was investigated in the host-pathogen system cucumber (Cucumis sativus) and downy mildew (Pseudoperonospora cubensis). With a 1:1 diluted non-separated liquid culture (containing gramicidin S attached to the spores and a biosurfactant released into the broth), an efficacy above 90% was achieved, when applied protectively. Curative or systemic effects were not found. It appeared, that the culture had no influence on the pathogen once it had established in the plant. In vitro, the non-separated liquid preparation of A. migulanus white and the separated spores (both 1:1 diluted) almost completely inhibited the release and the survival of zoospores of another oomycete, Phytophthora infestans. On leaf discs, released and encysted zoospores of P. cubensis were found, but only few germ tubes were formed. In vitro growth of mycelia of P. infestans was inhibited by pure gramicidin S from a concentration of 25 μg ml−1. With 100 μg ml−1 an inhibitory effect of about 85% compared with the water control was recorded. The treatment of cucumber plants with liquid culture of A. migulanus E1 (1:1 diluted; no production of gramicidin S, only biosurfactant) significantly controlled P. cubensis by reducing the time of leaf wetness. For a drying time of plants of 30 min before incubation at 100% relative humidity, the efficacy was 71% when calculated against the water control of the same drying time. Overall, we demonstrated that both, gramicidin S containing spores and the biosurfactant of A. migulanus white liquid culture contributed to the control of P. cubensis on cucumber, based on different modes of action. Spores carrying gramicidin S exhibit a direct effect both, on mycelia as well as on sporangia and zoospores of the oomycetes P. cubensis and P. infestans. The supernatant worked mainly by the effect of faster drying of the leaf surface. We conclude that a liquid culture of A. migulanus has a high potential to control downy mildew on cucumbers and other oomycete diseases.