In 2022, EFSA was mandated by the European Commission's Directorate-General for Health and Food Safety (M-2022-00070) to provide technical assistance on the list of Union quarantine pests qualifying as priority pests, as specified in Article 6(2) of Regulation (EU) 2016/2031 on protective measures against plant pests. As part of Task C, EFSA conducted comprehensive expert knowledge elicitations for candidate priority pests on the lag period, rate of expansion and impact on production (yield and quality losses) and the environment. This report provides the rationale for the dataset on Bretziella fagacearum, delivered to the European Commission's Joint Research Centre, to feed the Impact Indicator for Priority Pest (I2P2) model and complete the pest prioritisation ranking exercise.