Spent Mushroom Substrate Improves the Performance of Trichoderma harzianum for the Biocontrol of Rice Blast

Trichoderma harzianum, a beneficial rhizosphere ascomycete, is widely used for the control of a variety of plant diseases. However, powdered T. harzianum inoculant (PTH) suffers from poor root-colonization capacity and variable biocontrol efficacy, which limits its field application. In this study, an antagonistic T. harzianum strain TH10 was isolated from marine algal sediments. Using spent mushroom substrate (SMS) as the carrier for TH10, we developed an SMS-based T. harzianum inoculant (STH) for rice blast control. Scanning electron microscopy (SEM) revealed dense fungal mycelial networks on root surfaces of STH-treated rice, and these hyphae were presumed to be T. harzianum. At the three-leaf, tillering, and jointing stages, shoot height and root length of Pyricularia oryzae-infected rice treated with STH increased by 0.56–26.48% and 25.02–29.51% relative to those treated with PTH, respectively. Furthermore, STH increased the 1000-grain weight by 23.70% relative to the untreated control and by 12.99% relative to PTH, and improved grain number per panicle and panicle weight. STH exhibited a biocontrol efficacy of 51.34 ± 6.39% under greenhouse conditions, and reduced panicle-blast severity to 14.60 ± 2.68%. Transcriptomic data suggest that SMS may promote cuticular lipid biosynthesis in root tissues, and these lipids might in turn serve as carbon sources for T. harzianum. Most defense-related pathways, including phenylpropanoid biosynthesis, effector-triggered immunity (ETI), and MAPK cascades, were significantly upregulated in rice blast-infected shoots. These pathways may be associated with the accumulation of the immune-signaling molecules abscisic acid (ABA) and salicylic acid (SA), a response accompanying enhanced disease resistance in rice.