The disease results from the combined action of the beech scale insect and a pathogenic fungus, Nectria coccinea. Most affected beech end up succumbing to the disease, either directly or as a result of being attacked by other pathogens. The beech scale insect is part of the scale family. In mid-summer, the female deposits her eggs (asexual reproduction) in the bark fissures. The larva hatches and stays in the same place or migrates to other cracks. In fall, the nymph becomes stationary again and secretes a woolly envelope. This woolly envelope makes the tree look like it is covered with snow. The scale insect over winters in the bark of the tree. The fungal spores are disseminated by rain splash or by the wind and penetrate into the tree through wounds created by the scale insect. The fungus first causes a depression in the bark of the affected region and cankerous blisters of various sizes also form. On severely affected trees, there are so many cankers that they end up merging.
Tree mortality is often caused by other pathogens, such as Hypoxylon fungi, for example, or other insects.