Friends or foes: co-application of Trichoderma- and Bacillus-based products with chemicals to manage soybean foliar diseases

Bacillus spp. and Trichoderma spp. are typically applied to control soybean diseases. However, the impact of combining these biocontrol agents with each other, or with commonly used fungicides is not well studied in soybean. Therefore, we investigated the efficiency of bioproducts based on Trichoderma asperellum and Bacillus subtilis against soybean pathogens, and the compatibility between bioproducts and between bioproducts and fungicides. Between the bioproducts, the compatibility was assessed by applying cell-free filtrates from one microorganism to the other. Filtrates of T. asperellum reduced the B. subtilis growth, while filtrates of B. subtilis did not affect T. asperellum. Furthermore, filtrates and volatile compounds of T. asperellum and B. subtilis were toxic to the pathogens Corynespora cassiicola, Cercospora sojina and Septoria glycines. In the field, foliar sprays were performed using B. subtilis-based product, T. asperellum-based product, systemic and multisite fungicides, bioproducts and fungicides co-applied or alternated. For two seasons, treatment using B. subtilis on early stages followed by systemic fungicides and treatment using T. asperellum sprayed over crop debris followed by B. subtilis and later systemic fungicides on leaves efficiently reduced foliar diseases. Co-application of bioproducts did not improve their efficiency. Grain yield increased by 25% when using chemical fungicides or B. subtilis compared to the control. B. subtilis- and T. asperellum-based products were efficient against soybean pathogens, but their co-application did not improve the control. Instead, using T. asperellum over crop debris followed by foliar application of B. subtilis and systemic fungicides increased the yield and reduced soybean foliar diseases.