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Answer

The rust colored spots on your apple tree are most likely either Apple Scab (Venturia inaequalis) or Cedar Apple Rust (Gymnosporangium juniperi-virginiana) neither will kill the tree but it may affect some of the appearance and some of the fruit.

Apple Scab will present its self with olive green to brown leaf spots with an irregular or feathered edge which eventually become a dark brown to black, the infected leaves often drop prematurely. Infected fruit may have corky raised spots and become misshaped if severally infected. To control apple scab fungicide application should begin in early spring and continue as need if the spots are present through August. It is important to clean up and discard infected leaves to prevent the spread. In home orchards fungicide application if often not necessary.

Cedar Apple Rust presents its self with leaves that develop yellow, then bright orange spots with a red border, small raised black dots may be present on the upper side of leaf spots, fringed tubes may be seen on the underside of the leaf spots. The fruit may develop rough brown spot but they do not extend into the fruit. The best defense against Cedar apple rust is to plant resistant varieties.

Thanks for the question, happy gardening!

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