Compost Teas Reduce <i>Verticillium dahliae</i> Growth In Vitro via Direct and Volatile Effects

ABSTRACT Verticillium wilt of potatoes is caused by the fungus Verticillium dahliae , which leads to significant economic losses in potato production. The fungus is soil‐borne and can remain for up to 14 years in the soil or previous crop residues as hard‐to‐suppress microsclerotia. Compost teas (CTs) have been shown to suppress a variety of pathogenic fungi in food crops and are a sustainable option for pathogen suppression and crop nutrient provision. CTs are prepared by extracting compost in water and allowing it to brew for a period of time. In this study, we used a factorial experiment of two composts with four combinations of additives and aeration time to prepare eight CTs. Each CT was used after 3, 6 and 10 days of brewing. We tested the direct and volatile effects of unsterilised and filter‐sterilised CTs on the mycelial growth of V. dahliae in vitro. Unsterilised CTs inhibited V. dahliae growth by 90.1% via direct suppression and 71.7% via volatile suppression. Sterilised CTs inhibited V. dahliae growth by 79.3% via direct suppression and 26.6% via volatile suppression. CT efficacy was least at 3 days of brewing time and greatest at 6 days. Scanning electron microscopy revealed mycoparasitism by several fungal species on V. dahliae , indicating that mycoparasitism may be a source of CT efficacy as a biocontrol agent. This study demonstrates that, with further development, CTs have the potential to be a sustainable solution for suppressing verticillium wilt in potatoes.