Preliminary Study on the Biocontrol Efficacy and Antimicrobial Mechanism of Pseudomonas eucalypticola NP-1 Against Gray Mold in Eucalyptus Seedlings

Botrytis cinerea is a globally distributed phytopathogenic fungus that causes gray mold in eucalyptus seedlings, posing a severe threat to eucalyptus nursery production. Pseudomonas eucalypticola Liu et al. NP-1 is an endophytic bacterium isolated from eucalyptus with broad-spectrum antifungal activity. In this study, the fermentation broth extract of strain NP-1 was prepared using the organic solvent extraction method. The inhibitory effects, antifungal mechanisms, and biocontrol efficacy of the extract against B. cinerea were investigated. The results suggested that the NP-1 extract effectively inhibited mycelial growth, conidial germination, and germ tube development of B. cinerea. The EC50 and EC90 values for mycelial inhibition were 110 μg/mL and 332 μg/mL, respectively, while those for conidial germination inhibition were 126 μg/mL and 310 μg/mL. Microscopic and ultramicroscopic observations indicated that while the mycelial structures in the control and EC50 groups remained intact, the EC90 treatment significantly was associated with protoplasmic aggregation, leakage, and cavitation, suggesting potential structural damage to the fungal cells. In vitro and in vivo biocontrol assays showed that the control efficacy against gray mold reached 90.0% on detached eucalyptus leaves and 93.3% on eucalyptus seedlings. These findings elucidate the biocontrol potential of NP-1 and lay a foundation for the development of bio-based pesticides.