Willow trees are subject to a number of diseases and insect pests, although these pests usually affect the leaves and twigs first, rather than the bark. Inspect the tree for additional signs of disease, and consider other causes of the peeling bark as well.
The three most common diseases in willow are crown gall, black canker or scab. Crown gall is a root disease that causes galls, or growths, to form along the lower portion of the trunk. These galls are like tumors and may completely engulf the roots, killing the tree. Black canker causes large black areas to form on leaves and twigs. The infected areas may ooze pink spores. Scab or blight cause young leaves to blacken and die. Twigs may die as well. None of these diseases typically cause peeling bark.
Poplar and willow borers tunnel through the inner layer of the bark, causing the bark to crack or split. Look for other signs of poplar borers, such as piles of sawdust, swelling branches or twig dieback. Prune out diseased branches and spray the tree with permethrin to kill borers.
Sunscald occurs in the winter when bright sunlight reflects off snow. The sunlight creates heat, which causes the cells within the willow tree trunk to become active. When nighttime temperatures plummet, the cells may freeze and rupture, causing damage to the tree. Willow trees with sunscald have yellow or red patches on the trunk, usually on the south or southwestern side of the tree. The patches may later peel or crack. Willow trees recover from sunscald, although the cracks make the tree more vulnerable to insects and disease. Paint the tree trunks with diluted, white paint in early winter to prevent sunscald, which is most common on young willow trees with thin bark.
Another possible cause of peeling bark is injury caused by excavation, animals or lawn mowers. Wrap the tree trunk loosely in chicken wire to prevent squirrels and cats from climbing it. Mulch the base of the tree to avoid lawn mower damage in the future.