Isolation, Molecular Identification, and Antifungal Activity of Stenotrophomonas hibiscicola and Chryseobacterium cucumeris against Soil-Borne Fungi

Objective: This study aimed to isolate, characterise, and evaluate plant growth-promoting rhizobacteria from Cucurbit rhizosphere soils in Duhok, Iraq, assessing their antifungal activity against major soil-borne fungal pathogens. This work reports the first isolation of S. hibiscicola and C. cucumeris from Iraqi agricultural soils.Materials and Methods: Rhizobacterial isolates were screened for plant growth-promoting traits, including HCN production and phosphate solubilisation. Antifungal activity against Fusarium oxysporum, Rhizoctonia solani, and Macrophomina phaseolina was evaluated using dual-culture assays. Seed treatments on cucumber germination and plant growth under pathogen stress were assessed.Results: C. cucumeris exhibited strong HCN production, whereas S. hibiscicola exhibited weak production; neither demonstrated phosphate solubilisation on NBRIP agar. Both isolates exhibited significant antagonistic activity against all tested pathogens. S. hibiscicola exhibited the highest inhibition against F. oxysporum (55.83%), R. solani (55.00%), and M. phaseolina (52.23%), whereas C. cucumeris exhibited the highest inhibition against F. oxysporum (48.83%). C. cucumeris seed treatment accelerated germination, achieving 100% by day 3 compared with day 5 in the control. Both isolates improved seedling establishment and suppressed pathogen-induced growth. C. cucumeris consistently enhanced plant biomass under fungal stress, recording the highest shoot dry weight (87.25 mg) under F. oxysporum stress and increasing root dry weight under R. solani and M. phaseolina treatments.Conclusion: Both S. hibiscicola and C. cucumeris demonstrated strong antifungal and plant growth-promoting properties despite the absence of phosphate-solubilising activity. These indigenous rhizobacteria show potential as sustainable biocontrol agents and biofertilizers for cucumber production. Furthermore, this first report expands knowledge of regional microbial diversity.