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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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I'm just wondering if you still play tennis in that location to confirm if you see any Juniperous virginiana (eastern red cedar) nearby. – theforestecologist Feb 27 at 14:02
up vote 11 down vote accepted

This is actually not a gall as other answers have suggested. This is likely a fungus called Cedar-apple rust (Gymnosporangium juniperi-virginianae).

The fungus only thrives in the presence of both Juniperus virginiana (Eastern red cedar) and apple (Malus spp.) trees.

The leaf in the picture belongs to some species of the apple genus and the growths are aecia of G. juniperi-virginianae. You can read about this interesting fungi through Rutger's Plant & Pest Advisory here.

You can see a picture of the fungi here, and I've included Sabrina Tirpak's (Rutgers PDL) photo of its aecia below:

Gymnosporangium juniperi-virginianae-Sabrina Tirpakhttp://plant-pest-advisory.rutgers.edu/3859/

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What a fascinating organism! – augurar Jan 21 at 7:15
    
Right?! I've never seen the tentacled glob that grows on the Juniper – theforestecologist Jan 21 at 14:12
    
That's so cool :) – AleksandrH Jan 22 at 1:13

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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