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I found a seemingly diseased tree when I was out playing tennis yesterday... What is growing on this tree leaf? Is this a disease? If so, is it contagious? I have zero knowledge in botany, but I'm curious about what is happening to these trees. :) enter image description here

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Briefly, these look like insect galls; which are reactions of plant tissue caused by parasitic insects (often wasps) laying eggs inside the leaf. I'm sure someone can answer in more detail, though!

Cf. How Do Galls Form?

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Would the tree die because of this? Most of its leaves are growing "insect galls"... –  StephenChen Aug 26 '13 at 6:21
    
I imagine that it depends on the galling species and tree species, but most leaf galls don't hurt trees –  Oreotrephes Aug 27 '13 at 2:49
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