Leaf Spots on Shade Trees
Plant Health Problems
For more information, see the fact sheet on Fungal Leaf Spots of Trees and Ornamentals.
Diseases caused by Fungi:
This disease is widespread on shade trees. Large irregular areas on leaves turn reddish-brown, and may include a purplish discoloration that runs along the veins. The fungus fruits abundantly in little pale pink masses. Petioles may also be attacked. From a distance, infected trees may look scorched and significant premature leaf-drop may occur. However, these diseases are considered more aesthetic than life-threatening to the health of the tree.
Rake and dispose of the fallen leaves to reduce the chance of infection in the following season, and maintain vigor of the trees with adequate water during dry periods and fertilization in the spring. Since this disease is usually not a serious problem for the health of the trees, chemical controls are usually not necessary.
Purple eye, Phyllosticta.
Small grayish-tan spots with purple borders are scattered over the leaves. Tiny black dots in the spots are fruiting bodies of the fungus.
Control is as for anthracnose.
Tar spot, Rhytisma acerinum.
This fungus produces raised black spots that look like blobs of tar on the leaves. It is unsightly, but not serious. Tar spots are most prevalent on maples.
Raking and disposing of fallen leaves is sufficient for control. Maintain vigor of the trees with adequate water during dry periods and fertilization in the spring.
Powdery mildew, Microsphaera.
Whitish mealy appearance of upper surface of leaves is characteristic of infection by this fungus on species in all the groups. In the fall, small black dots which are the fruits of the fungus may be found embedded in the white mycelium.
Although widespread and common, since this disease is usually not a serious problem for the health of the trees, chemical controls are usually not necessary. Maintain vigor of the trees with adequate water during dry periods and fertilization in the spring.
Diseases caused by Physiological/Environmental Factors:
Leaves of shade trees are subject to sunburning, especially if hot sunny days follow a prolonged period of cloudy weather in spring. Sunburning resembles anthracnose infection and can be differentiated only by laboratory examination. Sunburning usually occurs on the south or southwest side of a tree, especially on those in exposed locations. Early infection by a wilt fungus in a large tree, or a girdling root, may resemble this trouble.
For more information, see the fact sheets on:
Drought and Its After-Effects on Trees and Shrubs
Drought Stress, Tree Health, and Management Implications
Recognizing Tree Diseases and Stress Factors
Solving Abiotic Tree Care Problems