Bacillus-based product protects soybean, improves grain yield and produces metabolites able to promote plant growth and antagonize pathogenic fungi

Soybean is frequently affected by pathogenic fungi, which are usually managed by chemicals. Foliar sprays with biological agents have been introduced as new allies to reduce plant diseases. This study verified foliar sprays of Bacillus subtilis-based product applied for three consecutive cropping seasons associated or not with chemical fungicides to control Asian rust (Phakopsora pachyrhizi) and the effect of B. subtilis-produced metabolites against pathogenic fungi and on soybean growth. Field treatments were as follows: (i) Foliar application of B. subtilis-based product, followed by chemical sprays; (ii) chemical management alone; (iii) B. subtilis-based product alone, and (iv) control treatment with water sprays. In the three years, B. subtilis-based product alone or followed by chemicals reduced at least 75% the Asian rust severity, similar to chemical management. Moreover, soybean grain yield increased when B. subtilis-based product was followed by chemicals. Cell-free filtrates and volatile organic compounds (VOCs) from B. subtilis isolate or from the formulated bioproduct were tested against fungi pathogens Fusarium cf. incarnatum/equiseti and Macrophomina phaseolina. All filtrates and VOCs emission reduced more than 40% the mycelial growth of both fungi. Also, the filtrates stimulated the soybean growth at a rate similar to the bioproduct, indicating the potential of the B. subtilis compounds to control fungi pathogens and promote plant growth. Therefore, the B. subtilis-based product showed a satisfactory protection and yield increment for soybean and B. subtilis-produced metabolites antagonized pathogenic fungi and stimulated the soybean plant growth.