Biological control of Fusarium wilt in Phalaenopsis orchids by Trichoderma harzianum isolates

Moth orchids (Phalaenopsis spp.) are widely cultivated ornamental plants due to their high flower production and long flowering period. This study aimed to identify fungal agents associated with Phalaenopsis orchids grown in Türkiye and to evaluate the biocontrol potential of Trichoderma harzianum (TH) isolates against pathogenicity-confirmed Fusarium oxysporum isolates. Fusarium spp. were the dominant fungal agents, and F. oxysporum was one of the most common species. In addition, Rhizoctonia solani, Sclerotinia sclerotiorum, and Macrophomina phaseolina were identified as important phytopathogens. Samples were collected from Antalya, Mersin, Yalova, and Istanbul. A total of 15 T. harzianum isolates and three pathogenicity-confirmed F. oxysporum isolates were used in in vitro assays. Based on these results, selected isolates and a commercial T. harzianum product were further tested in in vivo experiments under controlled conditions. In vitro assays showed that volatile metabolites of TH isolates reduced the colony diameter of F. oxysporum by 54.1%, while hyperparasitic effects resulted in up to 76.36% inhibition. In vivo applications reduced disease severity by 77.3% for orchid-derived isolates and 81.8% for the commercial product. Both treatments also increased root length, root fresh weight, and shoot biomass. These results demonstrate that Trichoderma harzianum isolates are effective in the biological control of Fusarium oxysporum-related diseases in orchid cultivation.