Approximately 100 years after the first introduction of Dutch elm disease (DED) in Europe, an effective commercial biocontrol product is available to protect susceptible elm trees against DED transmission by beetles. Injection of trees with conidiospores of the fungus Verticilium albo-atrum isolate WCS850, product name DutchTrig®, reduces infection to less than 0.2% of treated trees. This biocontrol agent, its mode of action, application, efficacy and limitations in controlling DED are described in this chapter.