White, waxy, sap-feeding aphids colonise cracks, crevices and pruning cuts on the bark of apple trees in the second half of spring. They then spread to new growth and create soft swellings. If the bulges split open in frosty weather, infections, such as canker, can set in.
Congregations of fluffy, white, waxy aphids cause localised swellings on branches. If the swellings split open in winter, infections can set in.