Utah Plant Disease Control No. 26
September 1998

Aspen Leaf Spot

Many trees in the genus Populus (which includes aspen) and also in Salix (the willows) are affected by a leaf spot caused by several species of the fungus Marssonina. These trees commonly occur in Utah, and the leaf spot is seen frequently in home plantings and in the canyons and other native habitats. In aspen, Marssonina infections result in leaf spotting and premature defoliation.

SYMPTOMS

Leaves are first infected in early spring. Fungal spores are spread by wind and rain to young emerging leaves where small, dark brown flecks with light centers develop. These spots gradually enlarge and coalesce to form large necrotic (dead) blotches on the leaves. Areas around the dark spots usually turn chlorotic or yellow. Within the infection sites, fungal fruiting bodies called acervuli may form. These produce white masses of spores that push through the leaf epidermis and are easily spread by water to cause new infections.

Leaf petioles and terminal shoots are also susceptible to infection. They develop the same type of lesion. Heavy infections of Marssonina can cause premature defoliation in the spring and again in the fall if frequent rains occur. Repeated spring defoliation in several successive years may result in the death of the tree. Fall infections and defoliation are not considered to be detrimental to the tree.

Quite often, symptoms of iron deficiency will mimic those of aspen leaf spot. In general, iron deficient leaves are lighter green or yellow overall and not just around the dead areas as in leaf spot. Necrotic lesions associated with iron deficiency are usually limited by the veins while those of leaf spot cross over the veins. It is common to see both conditions on the same leaves.

CONTROL

The fungus survives in infected leaves and twigs over the winter. To reduce the fungal inoculum for next year, fallen leaves may be raked up in the autumn and composted or discarded. Sprinkler irrigation of foliage should be strictly avoided since the splashing water spreads the fungal spores and increases the relative humidity in the planting. Avoid close planting. Allow space for air circulation. Prune out lower branches to improve air circulation and reduce humidity. Trees that are deficient in iron are more susceptible to leaf spot. Therefore, the correction of iron chlorosis by avoiding overirrigation and treating with chelated iron products will improve tree vigor and make trees more resistant to the fungus.

Some selections or clones of aspen are resistant to leaf spot and should be used when available. However, the susceptibility of trees available in the nursery is usually unknown or variable. If you find that your trees are infected every year, then you might consider removal and replacement with a known resistant selection or another species.

Protective fungicide sprays may be applied at bud break and repeated at 10-14 day intervals as long as wet weather continues in the spring. Dry weather slows the spread of spores so fungicides are unnecessary at these times. The following fungicides are registered for use on shade trees. Apply the fungicide solution until it begins to drip from the leaves.

Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician

Listing of commercial products implies no endorsement by the authors or the Utah State Cooperative Extension Service. Criticism of products not listed is neither implied nor intended. Persons using such products are responsible for their use according to the current label directions of the manufacturer. Pesticide labels are legal documents, and it is a violation of federal and state laws to use a pesticide inconsistent with its labeling. The pesticide applicator is legally responsible for its prope
use. Always read and follow the label.

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