Investigation of the effects of Bacillus subtilis and Bacillus thuringiensis as Bio-agents against powdery mildew (Podosphaera xanthii) disease in zucchini (Cucurbita pepo L.).

Fungi, bacteria, and viruses cause highly devastating diseases in species of the Cucurbitaceae family. Powdery mildew, a fungal disease, is one of the most important diseases of cucurbits. The pathogen, Podosphaera xanthii, is the most common causal agent of powdery mildew disease within cucurbits. The aim of this study was to investigate the effectiveness of the combined formulations of two biological agents, B. subtilis and B. thuringiensis, in combating powdery mildew disease, which represents a significant threat to C. pepo cultivation in Kayseri, Türkiye. The efficacy of six different treatments in controlling the disease agent P. xanthii was evaluated in susceptible zucchini genotypes. It was found that full-dose bacteria dilution application, before and after powdery mildew infection, as well as three- and five-fold bacteria dilutions application significantly prevented (1-2 scale value) powdery mildew disease on infected zucchini plants than the control application. There was a decrease in vegetative growth in the control-treated crops while plant growth increased significantly in bacterial-treated crops. Also, our findings showed that combined formulations made from Bacillus subtilis (61.29e and 3.3a strains) and Bacillus thuringiensis (2B3-1 and 2B2-2 strains) significantly increased the synthesis of plant defense enzymes such as DPPH, antioxidant, proline production, total phenolic substance, and total flavonoid content. The application of B + PM resulted in the highest enzyme contents, quantified as follows: 22.91 mg AAE/g antioxidant, 2.01 mg/g KU proline, 10.03 mg GAE/g TPC, and 7.756 mg CE/g TFC. These enzymes may have played vital roles in triggering zucchini defense mechanisms, thereby significantly preventing powdery disease in the bacteria-treated crops.