Dual Field Effects of Trichoderma Trunk Injections on Chestnut Trees: Suppression of Nut Rot and Enhanced Control of the Asian Chestnut Gall Wasp

The European chestnut (Castanea sativa Mill.) is under increasing threat from the combined impact of the emerging fungal pathogen Gnomoniopsis castaneae, the causative agent of chestnut brown rot, and the Asian chestnut gall wasp (Dryocosmus kuriphilus), which is one of the most destructive invasive pests in chestnut orchards. This study evaluated the efficacy of endotherapy involving the injection of a multispecies Trichoderma-based biocontrol formulation (T. harzianum, T. viride, and T. atroviride) into the trunks of trees in two chestnut-growing areas in Tuscany, Italy. Independent trials were conducted using either a single spring application or two applications (autumn and spring) to assess the effects on brown rot, the gall wasp and its parasitoid, Torymus sinensis, as well as on major nut-feeding insects. Trunk injection significantly reduced the incidence of G. castaneae from 48.1% to 61.7% in untreated trees to 16.6%–28.1% in treated trees. The double application strategy showed promising results compared to untreated trees: increased larval mortality of D. kuriphilus, reduced bud infestation, and enhanced parasitisation by Torymus sinensis. These results demonstrate that trunk-injected Trichoderma is an effective, environmentally sustainable tool for the integrated protection of chestnut, suppressing brown rot, and supporting the biological control of the Asian gall wasp simultaneously.