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Shade Trees:
Leaves

Spots/Blotches: Fungal Leaf Spots

Fungal Leaf Spots Picture

Leaf Spots of Shade Trees

Most established shade trees and shrubs produce more leaves than they need for normal growth. Unless severe leaf defoliation takes place, enough leaves are usually left for healthy growth. On young trees or newly planted trees leaf loss is more detrimental. When a tree or shrub loses most of its leaves its food reserves are depleted which may cause dieback, decline, and/or death. The majority of leaf spotting diseases are favored by cool, wet, spring weather. (see photo above - apple scab) There are numerous leaf spotting diseases that occur on shade trees and shrubs, but few are lethal. Leaf spots caused by fungi often can be distinguished by their fruiting structures and pattern of lesion development.

There are several descriptive categories of leaf spot diseases that affect shade trees. One of the most common leaf spot categories are the "anthracnose leaf diseases" (oak anthracnose, sycamore anthracnose). These diseases are characterized by discrete lesions that are usually found along leaf veins. In severe cases these lesions may run together and kill the entire leaf. Young leaves may become distorted as healthy tissue continues to grow around dead areas. Under optimum disease conditions the entire tree may be defoliated in the spring or early summer. Infection may proceed from the leaf blade down the petiole (leaf stem) into the small twigs at the tips of the tree branches. The pathogen may over winter in the twigs until the infection cycle starts over the next spring.

Other leaf spot diseases include leaf blisters (oak leaf blister) which are characterized by raised, wrinkled or puckered areas of the leaf. Initial symptoms may begin as light green or yellow areas before blisters form and eventually fall out leaving holes in the leaf. Rust diseases can also form leaf spots which are characterized by yellow-orange powdery areas usually on the lower leaf surfaces. "Rusted areas" can give the leaves a yellow-orange appearance. Severe infections will cause defoliation.

Management strategies: In most cases as mentioned above leaf spots on mature trees will not be a major problem threatening the health of the tree. The best management practices for leaf spotting diseases involve pruning and removal of dead twigs during the winter or dry summer months. Mature trees can be thinned for better air circulation throughout the canopy. Rake and remove infected fallen leaves in the fall and plant resistant varieties when available.

It is generally not economical to spray large mature shade trees for leaf spot diseases nor is it effective once disease symptoms are noticeable. Young trees or newly planted trees may need fungicide sprays to prevent excessive leaf loss until they become established in the landscape.

Keywords: leaf spot, anthracnose, spots, blister, disease, fungal

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Photo Gallery

Picture of Phylosticta on sycamore.

Phylosticta on sycamore.

Picture of Tar spot on maple.

Tar spot on maple.

Picture of Oak anthracnose.

Oak anthracnose.

Picture of Sycamore anthracnose.

Sycamore anthracnose.

Picture of Oak leaf blister.

Oak leaf blister.

Picture of Ash rust.

Ash rust.

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