Trichoderma biofertilizer (mixTroTha) mediates Malus sieversii resistance to Alternaria alternata

•Trichoderma biofertilizer (mixTroTha) can inhibit the growth of pathogens.•mixTroTha induced the systemic resistance of Malus sieversii.•mixTroTha enhanced the antioxidant ability of M. sieversii following pathogen attack.•mixTroTha promoted the growth and increasednumbers of lateral roots.•mixTroTha improved the soil metabolism and microbial community structure. As a biocontrol agent, Trichoderma can ameliorate fungal phytopathogen infection of plants by inducing plant resistance, promoting plant growth, and improving the soil environment. Trichoderma rossicum T7 (TroT7) and Trichoderma harzianum T2 (ThaT2) can significantly inhibit the growth of Alternaria alternata, Fusarium spp., and Cytospora mandshurica, with inhibition rates ranging from 34% to 58%. Therefore, spores of TroT7 and ThaT2 were mixed as a biofertilizer (mixTroTha). Soil irrigated with mixTroTha had a decreased population of phytopathogens. Following pretreatment with mixTroTha, the leaves of two-month-old Malus sieversii seedlings infected by A. alternata had improved cell membrane permeability and decreased numbers of diseasespots (by 16.67%, in the laboratory). The leaves of three-year-old M. sieversii seedlings also had 6.71% fewer diseasespots (in the field). The mechanism might involve the increased jasmonic acid content, which upregulated the expression of PDF1.2 (encoding plant defensin 1.2), by 12.4-fold at 72 h compared with the control. In addition, following Trichoderma colonization of the apple tree rhizosphere, the fungal communities changed significantly: 12 of 14 pathogenic genera showed a lower species abundance, and 4 of 5 biocontrol genera had a higher species abundance. However, the bacterial communities did not change significantly. Moreover, exposure to mixTroTha promoted the growth of lateral roots of three-year-old M. sieversii seedlings, and also accelerated soil metabolic processes, further increasing the contents of available nutrients in soil. Thus, mixTroTha could mediate M. sieversii seedling resistance to A. alternata via diverse methods.