Several different fungi cause canker and dieback on poplars. Lombardy poplars are especially vulnerable. The fungi enter the tree through a wound or, in some cases, through the leaves, killing the surrounding healthy tissue. A dark sunken canker develops in the wood and expands through it in all directions. If the fungus infects the leaves first, it grows down through the leaf stems and forms cankers on the twigs. The canker cuts off the flow of nutrients and water to the twigs or branch, causing the leaves to turn yellow. Twig or branch dieback follow if the canker girdles the wood. The tree may wall off the spreading fungus by producing callus tissue, a rapid growth of barklike cells. If the expanding canker is stopped before it covers half the diameter of the trunk, the tree usually survives. However, the fungus may grow faster than the callus, or the tree may not produce a callus, resulting in the death of the branch or the whole tree.
Prune off dead twigs and small cankered branches, cutting well below the canker. Remove and destroy severely infected trees. To prevent the development of new cankers, avoid wounding trees. Keep trees vigorous by fertilizing and watering. For more information about fertilizing and watering, see Nitrogen and About Watering.
Plant Care for Populus