nail galls and other plant galls

On my walk this morning, I saw bizarre lipstick-red protrusions on leaves near the bottom of the tree, where bright green new shoots had sprouted. The tree had kind of sticky heart-shaped leaves, some of them about the size of the palm of my hand. Are these insects? A fungus or disease?

 

Your photos show new growth on a linden tree, possibly large-leaved linden (Tilia platyphyllos). The red things are called nail galls (Eriophyaes tilia) and they are caused by the red nail gall mite.

According to Margaret Redfern’s book Plant Galls, this type of gall (in the form of a pouch) is “initiated in the spring by the fundatrices, females that have overwintered in cracks and crevices in the bark or under the scales of dormant buds.” The mite will wander over a new leaf’s underside and feed on individual cells which then collapse and die. The leaf domes up into a pointed pouch around that area. When the pouch is partly formed, the female mite lays her eggs inside it. The larvae hatch and feed there. Tilia nail galls have a thick nutritive layer, and each gall can contain 100-200 mites by summer. In fall, they disperse and overwinter.