Bacillus velezensis UTB96 mitigates saffron fusarium corm rot and promotes plant growth and phytochemical quality

Saffron corm rot, predominantly associated with Fusarium species, is a major constraint to saffron production and may adversely affect both yield and spice quality. In this study, the causal agents of saffron corm rot were isolated from diseased corms collected in Iran, and identified based on morphological characteristics and pathogenicity tests. Three pathogenic isolates were characterized as Fusarium oxysporum , F. solani , and F. culmorum . The biocontrol potential of Bacillus velezensis strain UTB96 was subsequently evaluated through dual-culture and volatile organic compound assays, greenhouse experiments, and a field trial. Under in vitro conditions, UTB96 significantly inhibited the mycelial growth of all three Fusarium isolates, with the greatest inhibition in the dual-culture assay observed against F. solani (68.35%). Volatile compounds produced by UTB96 also suppressed fungal growth, with inhibition rates of 59.28, 66.00, and 73.00% against F. oxysporum , F. culmorum , and F. solani , respectively. In planta experiments, corm treatment with UTB96 significantly reduced disease severity caused by all three pathogens compared with infected controls, with the highest reduction recorded against F. solani (70.74%). In addition, UTB96 significantly enhanced leaf length, leaf number, sprouts number, root length, and corm number, whereas daughter-corm weight was not significantly affected. Field application of UTB96 improved important yield-related traits, including stigma length, stigma weight, and flower weight. Moreover, according to ISO3632-based measurements, the treatment significantly increased crocin and picrocrocin contents at P < 0.01 and safranal content at P < 0.05. Correlation analysis revealed strong positive associations between agronomic traits and crocin and picrocrocin, while safranal was significantly correlated with the other quality indices but not with the measured yield traits. Overall, B. velezensis strain UTB96 demonstrated substantial antifungal, disease-suppressive, and plant growth-promoting activities, highlighting its promise as a multifunctional bioinoculant for sustainable saffron production.