Nigrospora oryzae, a newly identified pathogen, is responsible for poplar leaf blight, causing significant harm to poplar growth. Here, we describe, for the first time, a biological control method for the control of poplar leaf blight via the applications of 3 dominant Trichoderma strains/species. In this study, dominant Trichoderma species/strains with the potential for biocontrol were identified and then further characterised via dual culture assays, volatile organic compounds (VOCs), and culture filtrates. The biocontrol efficacy of these strains against N. oryzae was found to exceed 60%. Furthermore, the reactive oxygen species (ROS) content in Populus davidiana × P. alba var. pyramidalis (PdPap) leaves pretreated with these Trichoderma strains significantly decreased. Furthermore, pretreatment of PdPap with a combination of these Trichoderma (Tcom) resulted in 9.71-fold and 1.95-fold increases in peroxidase (POD) and superoxide dismutase (SOD) activity, respectively, and 3.87-fold decrease in the MDA content compared to controls. Moreover, Tcom pretreatment activated the salicylic acid (SA) and jasmonic acid (JA) pathway-dependent defence responses of poplar, upregulating pathogenesis-related protein (PR) and MYC proto-oncogene (MYC-R) by more than 12-fold and 17.32-fold, respectively. In addition, Trichoderma treatments significantly increased the number of lateral roots, aboveground biomass, and stomata number and density of PdPap, and Tcom was superior to the single pretreatments. The soil pH also became weakly acidic in these pretreatments, which is beneficial for the growth of PdPap seedlings. These findings indicate that these dominant Trichoderma strains can effectively increase biocontrol and poplar growth promotion.
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