An In Vitro Evaluation of Trichoderma harzianum as a Biocontrol Agent for Managing Fungal Diseases in Rosa grandiflora

The global rose industry is severely threatened by fungal diseases, which are traditionally managed using chemical fungicides. These fungicides, however, pose significant risks to environmental integrity and human health, highlighting the urgent need for sustainable alternatives. This study aimed to evaluate the antagonistic potential of Trichoderma harzianum—a soil-borne beneficial fungus—as a bio-control agent against key fungal pathogens of roses. Trichoderma harzianum was isolated from garden, agricultural, and forest soils in Jandool Valley, Khyber Pakhtunkhwa (KPK), Pakistan (34.88° N, 71.66° E). The fungal pathogens targeted in this study included Alternaria alternata, Aspergillus niger, Diplocarpon rosae, Exserohilum rostratum, Fusarium oxysporum, and Lasiodiplodia theobromae all were isolated from infected tissues of Rosa grandiflora at the University of Malakand, Dir Lower, KPK, Pakistan (34.6690° N, 72.0622° E). Antagonistic activity was assessed via dual culture assays and cell-free filtrate tests. In dual culture assays, T. harzianum exhibited significant inhibitory effects on all tested pathogens, with inhibition rates ranging from 51.13 to 69.33%. The highest inhibition was observed against Diplocarpon rosae (69.33%), followed by Lasiodiplodia theobromae (68.37%), Alternaria alternata (62.82%), Fusarium oxysporum (59.10%), Aspergillus niger (51.35%), and Exserohilum rostratum (51.13%). Cell-free filtrate tests further confirmed metabolite-mediated suppression of pathogens, with inhibition rates between 54.49 and 70.88%. Additionally, Nutrient Agar was identified as the optimal growth medium for T. harzianum, as indicated by the highest growth slope (16.93). Statistical analysis validated the efficacy of T. harzianum (p < 0.0001), with strong correlations (R2 = 0.6762–0.8721) confirming the role of its secondary metabolites in pathogen suppression. These findings demonstrate that T. harzianum is a viable, eco-friendly alternative to chemical fungicides for rose disease management. To maximize its practical utility, further field trials are recommended to refine application protocols for sustainable rose farming systems.