Uniting the Role of Entomopathogenic Fungi against Rhizoctonia solani JG Kühn, the Causal Agent of Cucumber Damping-Off and Root Rot Diseases

Beauveria bassiana and Metarhizium spp. are entomopathogenic fungi with potential applications beyond insect pest control, including plant disease suppression, plant growth promotion, and rhizosphere colonization. This study investigated the plant growth-promoting characteristics and extracellular enzyme activities of Metarhizium spp. and B. bassiana in relation to phytopathogen interactions and plant growth. Additionally, the efficacy of these fungi in mitigating damping-off and root rot caused by Rhizoctonia solani on cucumber plants was evaluated in vitro and in vivo. Results indicate that B. bassiana and M. anisopliae produce indole-3-acetic acid, hydrocyanic acid, and hydrolytic enzymes. Seed treatment with these fungi significantly reduced disease severity (3.85%-1.86%, respectively) and enhanced germination parameters [germination percentage (85.33%-86%, respectively), germination index (10.67-12.29, respectively), seedling length vigor index (86.41-109.44, respectively), and seedling weight vigor index (30.24-37.57, respectively)] compared to the control positive. Both fungi demonstrated high inhibition rates of R. solani mycelial growth (93.90%-90.46%, respectively). Greenhouse trials revealed that preventive treatments using B. bassiana and M. anisopliae increased catalase (104.40- 105.52 units/mg protein/min, respectively), (4.58-5.77 units/mg protein/min, respectively), superoxide dismutase (40.65-41.74 units/mg protein/min, respectively), and polyphenol oxidase (0.539-0.559 units/mg protein/min, respectively) activities, as well as total phenolic (2.60-2.65 mg/g, respectively) and total sugar content (2.23- 2.16 mg/g, respectively) in cucumber plants. Consequently, disease severity (9.51-6.99%, respectively) was reduced, and plant height (93.76-98.76 cm, respectively) increased compared to the positive control. These fi nd- ings suggest that B. bassiana and M. anisopliae can enhance plant growth, stimulate plant defense mechanisms, and effectively control damping-off and root rot diseases, making them promising candidates for biological control strategies.