Purple Bordered Leaf Spot

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Phyllosticta minima is a common and widespread fungus. It occurs in eastern North America and west to the Great Plains. It causes a disease known as Purple Bordered Leaf Spot on maples. It infects mostly Amur, Japanese, red, and silver maple, but also mountain and sugar maple, and in other areas, Tartarian and sycamore maple.

Infections are most common in years with a wet spring and early summer. The infection is most severe on the bottom third of the tree, where there is more moisture. Some infected leaves may eventually turn brown and drop off the tree. Rarely, the infection is severe enough to cause partial defoliation in late summer, but most trees are able to withstand the infection.

Infestation first occurs in the spring, when rain and wind causes spores to be splashed and blown onto new leaves. The infected leaf develops small, round, yellowish-green spots (lesions), less than ¼″ in diameter. The lesions eventually turn tan and have distinct purple, red, or brown margins. Tiny, black, pimple-like fruiting bodies (pycnidia) form in each lesion and can be seen with a hand lens. The pycnidia are usually arranged in a circle in the middle of the lesions. The dead tissue in the middle of the lesion sometimes breaks away, leaving a small hole.

The pycnidia overwinter in the leaf litter and produce spores the following spring. Control involves removing leaves with spots from the tree, and raking up and removing fallen leaves. That prevents further infections in the current growing season, and reduces the number of infected leaves that will overwinter.

Maples

Spring to fall

Sources

4, 7, 26, 29, 30, 77.

Common

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