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On a hiking trip to the Alps I found trees whose leaves showed those spots you see on the picture. I am curious about what the cause is.

Higher resolution pictures of front and back side.

enter image description here

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This site is not for specific medical or botany advice. Please consult your local botanist for a specific diagnosis. –  Mew Sep 27 '13 at 8:49
    
I was just kidding –  Mew Sep 28 '13 at 2:03
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1 Answer

up vote 4 down vote accepted

This is a "Tar Spot" disease usually found in Europe and North America. It mostly affects the Maple tree leaves. Tar spot is caused by 'Rhytisma acerinum' a plant pathogen fungus. This pathogen does not seem harm to tree but disturbs the leaves as it finds a suitable condition in summer with bit of wetness. It enters the leaves through stoma and then creates yellow lesions of various sizes over the leaf area which later gradually turns into brown-black tar colored spot. It reduces the photosynthesis process of leaves and thus creating more wide dark spots on the leaves. After sometime the leaves will fall.

Entire detail is available in the Wikipedia link with the detail of the pathogen. (Add some more info if you find this detail not enough)

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Thanks - very insightful! –  Raffael Sep 26 '13 at 19:49
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