Mussel scales are up to 3mm long and brownish black in colour. Their shape is similar to that of the shellfish known as mussels. Heavily infested plants will lack vigour and stems may dieback, especially on box. On apple, the scales sometimes spread onto fruits. Old dead scales often remain attached, so the scales can be seen throughout the year.
Light infestations of mussel scale are of little consequence but heavy attacks can affect the host plant's growth. When mussel scale spreads onto apple fruits it makes them less palatable, although the scales can be wiped off.