Some diseases that affect sycamore trees are anthracnose, powdery mildew and bacterial leaf scorch. Anthracnose is caused by the fungus Apiognomonia veneta, and it appears on trees in four separate phases. The first phase, twig blight, begins in the spring before the tree's leaves begin to emerge. The fungus kills the tips of small branches that are less than a year old.
The second phase of anthracnose is bud blight. In April and May, the emerging leaf buds die due to the girdling caused by the cankers that form on the tree's branches. During the blight's third phase, any new shoots and young leaves that managed to grow suddenly die. In the final phase, leaves shrivel, turn brown and fall from the tree. In extreme cases, the infection can cause trees to lose all of their leaves repeatedly in a single season.
Powdery mildew forms on leaves and twigs as patches of white to gray powdery material that is composed of spores and threadlike strands of the fungus Microsphaera. Leaves that become infected turn yellow, lose their shape and fall from the tree. The growth of new leaves is typically stunted, and shoots begin to grow within the patches of fungus. These new growths are a second type of spore that adapt to survive the winter. These spores cause a new infection on the tree when spring begins.
Bacterial leaf scorch is transferred to the tree by insects that ate infected plants. The bacteria causes irregularly shaped sections of dry, brown tissue to form on the edges of the tree's leaves and between the veins of the leaves. Leaves that are infected also begin to curl inward, but they typically remain on the tree.