Chitosan molecular weight affects biocontrol efficacy of chitosan-Persian gum-encapsulated Bacillus subtilis PVRC001 against Tomato yellow leaf curl virus

Tomato yellow leaf curl virus (TYLCV), a devastating Begomovirus, causes severe global yield losses. Conventional control relying on resistant cultivars and synthetic insecticides is increasingly compromised by resistance development and environmental concerns, highlighting the urgent need for sustainable, eco-friendly alternatives. This study evaluated the biocontrol efficacy of Bacillus subtilis strain PVRC001 encapsulated in chitosan–Persian gum complexes formulated with chitosan of four different molecular weights: ultra-low (ULMWCC, ∼20 kDa), low (LMWCC, ∼50 kDa), medium (MMWCC, ∼150 kDa), and high (HMWCC, ∼1250 kDa). Under greenhouse conditions, these formulations—along with non-encapsulated bacteria (BS) and an untreated control—were tested in TYLCV-infected tomato plants using viruliferous whiteflies as vectors. Encapsulation of B. subtilis PVRC001 in chitosan–Persian gum complex significantly enhanced plant growth and TYLCV suppression compared to the non-encapsulated treatment. The MMWCC treatment outperformed all other groups, resulting in the highest enhancements in plant height, fresh weight (increased by 54.3%), and leaf area, as well as the greatest reduction in disease incidence (48.27%) and severity (lowest AUDPC). Both MMWCC and BS reduced viral accumulation and activated host defense mechanisms, elevating the activity of phenylalanine ammonia-lyase (PAL), peroxidase (POD), and superoxide dismutase (SOD), and upregulating defense-related genes (ICS1, PR1, PR2, and ERF-A2). These results indicate that encapsulating B. subtilis PVRC001 in medium molecular weight chitosan (∼150 kDa) maximizes its protective effects, presenting a sustainable strategy for TYLCV management through improved biocontrol agent delivery, plant growth enhancement, and induction of systemic resistance.