Sustainable control of onion Fusarium basal rot using seed-applied Trichoderma spp.

Fusarium Basal rot disease (FBR) poses a global threat to onion production, with damage occurring throughout the entire crop cycle. The present study evaluated Trichoderma-based protection against major Fusarium pathogens in Israel through a series of experiments, from in vitro antagonism and seed assays to a semi-field pot trial spanning a full growing season. In confrontation assays, 10 out of 15 Trichoderma strains exhibited statistically significant inhibitory activity against the Fusarium spp., with inhibition levels varying by strain and antagonistic mechanism. Among the most effective isolates, T. asperellum (P1), T. longibrachiatum (T7407), and T. beinertii (T14707) consistently demonstrated significant antagonism through both secreted metabolites and volatile compounds. Unlike other Trichoderma strains, these three caused moderate effects on seed germination and early growth. Seed assays further revealed cultivar-dependent sensitivity, with the Orlando yellow variety more susceptible than the Maadim red, and Neocosmospora falciformis emerging as the most aggressive pathogen. Seed coatings with selected Trichoderma strains, tested under semi-field conditions, significantly promoted plant growth, with effects evident as early as mid-season. Shoot fresh biomass increased by 54–131% compared with untreated controls, accompanied by parallel improvements in other growth parameters. At the season’s end, T. asperellum treatments showed consistent positive effects, significantly enhancing shoot weight (28–119%) and bulb weight (15–157%) under pathogen stress. These improvements were accompanied by up to 36% reduction in foliar symptoms and 59% suppression of infection (tracked by qPCR). Overall, the study demonstrates the promise of Trichoderma-based seed treatments as an effective and sustainable strategy for managing onion FBR.