Trust is crucial for successful collective action. A prime example is collective commercialization of agricultural produce through producer organizations. We conduct a cluster-randomized controlled trial in rural Senegal in which we vary the number and the type of smallholder farmers – members and/or leaders of local producer organizations – invited to a three-day training on collective commercialization. We use this variation to identify effects on intra-group trust, both direct treatment effects of having participated in the training and spillover effects on farmers who did not partake. Looking at different measures of trust in leaders' competence and motives and of trust in members we find that participating in the training significantly enhances both trust in leaders and trust in members. For trust in leaders, we also find a strong spillover effect. Our findings suggest that relatively soft and non-costly interventions such as a group training appear to be able to positively affect trust within producer organizations.