At least 20 different fungi and one bacterium are pathogenic on maple foliage. Fortunately, leaf diseases seldom detract from the aesthetic values of maples, and even on those rare occasions when they do, they do not threaten the overall health of affected trees.

Most of the fungi that cause foliage diseases are extremely sensitive to environmental conditions, and higher disease incidence can be expected in years when spring and early summer weather is cool and wet for prolonged periods of time. Leaves in lower crowns and on north sides of trees are affected more often because they remain wet longer in the morning (following dew) and after rainy periods. The powdery mildews are exceptions in that they are favored by shade and dry foliage; they are most serious in mid- to late summer.

Leaf diseases can be prevented some what by pruning or thinning to improve air circulation through crowns. In nurseries, planting trees further apart also will help speed drying of foliage after it has been wetted. Overwintering sites for many leaf disease fungi are eliminated by collecting and disposing of fallen leaves in autumn. However, because these fungi are such prolific spore producers, a few missed leaves can provide sufficient spores to start a new epidemic. Protective fungicides give good control of some maple leaf diseases, but they must be applied before infection by pathogens occurs. Timing fungicide applications to immediately precede infection periods often is difficult, but failure to do so can result in wasted chemicals, time, and money.

1) Gray mold spot, bull's eye spot
Cristulariella depraedens red, slv, sug, str, mtn, nwy, jpn, syc
C. moricola red, slv, sug, blk, box, jpn, syc

Diseases caused by the two species of Cristulariella are indistinguishable from one another on maple leaves. Lesions have light brown centers with darker margins and may have concentric zones of alternating light and dark tissue. In some cases, the lesions are nearly perfect circles, but in others they are irregular and may spread over entire leaves. Both fungi overwinter on remnants of leaves infected the previous year, and they begin to liberate spores in early spring. If conditions are favorable (cool and wet), infection of new leaves occurs readily, and subsequent generations of spores may be produced and cause disease throughout the summer. These diseases usually go unnoticed until early June when spots are numerous and badly infected leaves start to drop.

2) Anthracnose
Discula sp. red, slv, sug, box, nwy

The pathogen spends the winter in fallen leaves and begins to produce its spores in the spring as new maple leaves are expanding. Infections occur on or near major leaf veins and cause irregularly-shaped areas of dead tissue around them. When weather is favorable, additional crops of spores are produced from minute structures on killed tissues. A high incidence of lesions may result in defoliation starting in mid- to late July.

3) Phyllosticta spot, purple eye
Phyllosticta minima widespread, all species

Symptoms of this disease are circular "eyes" with light brown centers and red dish purple to brown borders. Soon after leaf spots are visible, tiny (less than l/2 mm) black, spore-producing pustules appear in their centers. Most infections take place early in spring on tender, young leaves, but prolonged rainy periods later in the season allow continued reinfection.

4) Tar spot
Rhytisma americanum red, slv, sug, mtn
R. punctatum red, slv, sug, blk, box, mtn

5) Leaf blister (extremely rare)
Taphrina dearnessii red, mtn
T. sacchari sug

Areas of leaves infected with either of these fungi produce many extra leaf cells and the cells are larger than normal. A result of this unusual development is that parts of infected leaves become distorted and blackened and resemble shallow blisters; hence, the name of the disease. Spores of the pathogens are liberated from diseased leaves in the summer, and they lodge among bud scales where they lie dormant until new leaves emerge the following spring. Then, infection takes place and the fungi begin another cycle.

6) Powdery mildew
Phyllactinia guttata sug. box
Uncinula circinata red, slv, sug, box

Powdery mildew fungi overwinter as dormant spores in the protective environment afforded by bud scales or in spore-producing structures on twigs and leaf debris. Although new leaves become infected as they emerge in the spring the diseases may not be evident until mid summer, when they appear as a powdery, white coating on upper and/or lower leaf surfaces. With some powdery mildews, minute black spheres, visible with a magnifying glass, may be intertwined with the powdery material. Though these spheres represent another phase in development of the fungus, their contribution to epidemic development is of minor importance.