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. :)
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!