Combined Application of Biochar, Arbuscular Mycorrhizal Fungi, and Trichoderma harzianum Affects Fusarium Development and Soil Properties in Cotton

Abstract The increase in Fusarium spp. infections in cotton production necessitates integrated biological strategies that enhance disease resistance and improve soil functionality. This study evaluated the combined application of biochar, arbuscular mycorrhizal fungi (AMF), and Trichoderma harzianum to alleviate Fusarium pressure in cotton and reinforce beneficial soil–plant interactions. Field trials were conducted under a randomized complete block design with four replications to evaluate the individual and combined effects of 2% biochar (weight/weight), commercial AMF, and Trichoderma harzianum Rifai strain KRL-AG2. Over a two-year period, the combined application of biochar, commercial AMF (Endo Roots Soluble, water-soluble formulation), and Trichoderma harzianum Rifai strain KRL-AG2 increased cotton growth and development and reduced disease severity. The study focused on field-based indicators, including plant growth, disease severity, and soil physicochemical properties. This process significantly increased the number of leaves (up to 83 leaves per plant), shoot length (up to 92.9 cm), and diameter (up to 15.21 mm), pod formation (up to 27.9 pods per plant), and photosynthetic efficiency (up to 58.46%) while improving soil nutrient content (N, P, Fe) and AMF colonization (up to 95%). The combined application of biochar, AMF, and Trichoderma harzianum reduced the severity of Fusarium disease by 67.5% and increased the availability of macro- and micronutrients in the soil, including nitrogen, phosphorus, and iron. Therefore, combining microbial biostimulants with organic regulators is an effective strategy for the biological control of Fusarium -related diseases and sustainable cotton production.