Impact of the antibacterial chemicals on antagonistic Bacillus spp. consortium and the pathogenic soil fungi: Phytophthora sp. and Rhizoctonia sp.

Abstract

Soil-borne fungi, particularly Phytophthora sp. and Rhizoctonia sp., are major causes of agricultural yield losses. The management of soil-borne diseases commonly relies on bactericidal chemicals, which may adversely affect beneficial microorganisms, especially Bacillus spp. This study aimed to determine the effects of bactericidal chemicals on Bacillus spp. and their efficacy in controlling Phytophthora sp. and Rhizoctonia sp. The effects of bactericidal chemicals on antagonistic bacteria were evaluated by measuring inhibition zones (clear zones) and bacterial turbidity using three isolates of Bacillus subtilis and one isolate of Bacillus velezensis. The results showed that all Bacillus spp. isolates were most strongly inhibited by the combination of Gentamicin and Oxytetracycline. In contrast, SiO2 + Al2O3 + CaO, Polyoxyethylene, ZnO, and Kasugamycin exhibited progressively lower inhibitory effects on bacterial growth, respectively. The four Bacillus isolates were subsequently tested for compatibility using the cross-streak method. No antagonistic interactions were observed among the isolates, indicating that they were compatible and could be combined into a Bacillus consortium. The consortium was then evaluated in combination with bactericidal chemicals at a concentration of 2,000 ppm. against Phytophthora sp. and Rhizoctonia sp. The combined treatment with SiO2 + Al2O3 + CaO completely inhibited fungal mycelial growth. Polyoxyethylene, ZnO, and the combination of Gentamicin and Oxytetracycline also exhibited high antifungal activity; however, these treatments completely inhibited the growth of the Bacillus consortium. In contrast, Kasugamycin did not inhibit the growth of the Bacillus consortium, allowing its combined application with the bacteria to suppress fungal growth by 56.67–68.89%. The results also demonstrated that Rhizoctonia sp. was more sensitive than Phytophthora sp. to treatments combining bactericidal chemicals with the Bacillus consortium. Overall, the combined application of Si2O + Al2O3 + CaO, Polyoxyethylene, ZnO, and Kasugamycin with the Bacillus consortium effectively controlled both Phytophthora sp. and Rhizoctonia sp.