Fungal leaf spots are perhaps the most prevalent plant diseases in the Northeast. They are generally favored by cool, wet weather. Although unsightly in appearance, most leaf spot diseases result in little damage to the host plant. Following are descriptions of some of the more common ones.

Frogeye Leaf Spot of Apple and Crabapple:

Pathogen: Botryosphaeria obtusa

This leaf spot disease is a minor problem on most crabapple trees. Small brown spots with purple borders develop on leaf surfaces following moist spring weather. Some spots enlarge, developing irregular brown lobes. The original spot turns gray, causing the appearance of a "frog eye."

B. obtusa also causes black rot cankers on weakened crabapple tree limbs, especially following winter injury. Infection may be superficial (confined to the bark) or may extend down into the wood and grow perennially. Prune out and destroy branches with black rot cankers to prevent release of spores.

Oak Leaf Blister:

Pathogen: Taphrina caerulescens

Oak leaf blister is a common disease which primarily affects red oaks, although it can be a problem on white oaks. Symptoms appear as blister-like, circular bulges on upper leaf surfaces (depressions when viewed from the lower leaf surface) which turn from yellow to brown as they age. Spores are produced on the upper leaf surface in midsummer, giving the leaves a powdery appearance. Some of these spores become lodged in bud scales where they overwinter. The following spring, the spores germinate and infect leaves as they unfurl. The disease may be severe when cool, wet weather prevails. As leaves mature, they become more resistant to infection. Controls are generally not warranted and fungicide recommendations are rarely made except in some nursery/garden center situations.

Downy mildew of cranberrybush viburnum:

Pathogen: Plasmopara viburni

This is one of the few significant downy mildew diseases of ornamentals. Symptoms appear as light, greenish spots on the upper leaf surface that coalesce, forming angular patches bordered by veins. Infected leaf tissue often reddens, then browns and shrivels. Corresponding areas on the lower leaf surface are covered with downy grayish-white fungal mycelium.

The fungus reproduces rapidly and multiple infections occur during periods of leaf wetness and cool to warm (although not hot) conditions. This disease is diagnostically different from powdery mildew of viburnum in that the fungus is present on lower leaf surfaces. Promote dry leaf surfaces to control this disease. When fungicides are required, apply a labeled product containing mancozeb to the undersides of leaves.

Guignardia blotch of Aesculus:

Pathogen: Gunignardia aesculi

This disease creates a serious aesthetic problem on most horse chestnut and buckeye species. However, since much of the annual growth is completed before symptoms become severe, damage to the plant is minimal. Leaf symptoms first appear as water-soaked areas which turn reddish-brown to brown with yellow borders. These lesions coalesce, causing large blotches which curl the leaves. By August the whole plant appears scorched. The fungus overwinters in fallen leaves, producing spores for new infections in spring, so removal of leaves should be thorough. As with other leaf spot diseases, infection is enhanced by moist conditions. Improve air circulation to hasten leaf drying. If pesticide treatment is desired, apply a labeled fungicide containing mancozeb or chlorothalonil at bud break during wet springs. Reapply at intervals specified on the label until conditions are no longer moist. For new plantings, select plants with resistance to Guignardia blotch such as bottlebrush buckeye (Aesculus parvifolia).

Tar Spots of Maple:

Pathogens: Rhytisma acerinum, Rhytisma punctatum

These dramatic, but inconsequential, diseases of numerous maple species cause tar-like spots on leaves which can reach almost 24 mm (1 inch) in diameter. Symptoms first appear as yellowish spots on the upper leaf surface. Later in the summer, a black, tar-like mesh of fungal and leaf tissue develops within the yellow spot. Some early leaf drop may occur but is not considered serious. Fungicide treatment is usually not necessary.

Phyllosticta leaf spot of maple:

Pathogen: Phyllosticta minima

This disease affects a number of maple species, particularly silver, red, Japanese and Amur maple. Leaf spots are roughly circular, tan in color with purple to red borders. Later in the season, black fruiting bodies of the fungus arranged in rings appear within the lesions. Since damage to the plant is minimal, fungicides are rarely recommended.

Control:

Cultural management is generally sufficient to keep these diseases in check. Leaf spot fungi overwinter in and produce spores for new infections on the fallen leaves, which should be gathered in the fall and removed from the site or destroyed. Infection occurs when leaf surfaces are moist; improve air circulation by thinning crowded plants and pruning overly dense growth. Avoid overhead irrigation, or water only in the morning to ensure leaves dry quickly.

Adapted from Stephen Nameth, C. Wayne Ellett and Jim Chatfield, Ohio State University Extension, 1999