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Potential of soil-derived fungal biocontrol agents applied as a soil amendment and a seed coating to control Verticillium wilt of sugar beet

ABSTRACT Verticillium dahliae (Vd) is an emerging threat to sugar beet production. Control measures such as fungicides are not available and the utilisation of resistant cultivars is very limited. Hence, we explored the potential of two soil-derived fungal biocontrol agents (BCAs), Fusarium oxysporum F2 (FoF2) and Verticillium tricorpus 1808 (Vt1808), against Verticillium wilt of sugar beet. Pathogenicity tests revealed that Vd caused over 90% disease incidence and severity and led to a significant yield reduction, whereas BCAs neither inhibited nor promoted plant growth. Germination rate was higher in BCA-treated seeds compared to untreated ones. Viability of both BCAs was significantly reduced after six months of storage in liquid methylcellulose formulation (MC), while BCA concentrations remained stable on stored seeds treated with MC. In contrast, Vt1808 produced in sand-rye flour formulation remained 100% viable after storage. Similarly, post-storage analysis of the FoF2 talcum powder formulation revealed improved colony forming units, but increments were not significant. In vitro, both BCAs caused no growth inhibition zones and only insignificant Vd growth reductions were observed. In the greenhouse, soil amendment with a higher dose of FoF2:Vt1808 mixture resulted in substantial reductions of disease severity on the crown (33.3%), disease incidence (55.6%) and disease severity (68.8%) on the beet as well as an improved yield (32.9%). In contrast, seed coating did not reduce symptoms on the crown and on the beet. In the field, both BCAs did not provide significant disease reductions. Nevertheless, a promising result was achieved with the application of FoF2. Despite the need for improvement of the biocontrol activity, the results of this study demonstrate the suitability of the optimised BCA formulations for utilisation in commercial sugar beet production.