The red galls on the top of the leaf are caused by the leaf’s reaction to damage inflicted by gall mites feeding. In response to the grazing, the leaf makes a little dome (the gall) and later on the mite lays its eggs inside that gall, where the larvae will hatch out and continue feeding and growing, perpetuating the infestation.
The white fluffy looking coating between the major veins on the underside of the leaf is also caused by the leaf reacting to a gall mite. This time, it is a felt gall mite. Apparently a chemical in the mites’ saliva somehow stimulates the tree to produce lots of tiny hairs: which are what the fluffy stuff actually is – tiny little hairs. Within the hairs the mites can feed and breed with a degree of protection.
Perhaps surprisingly, the galls will have very little impact on the tree’s overall health and growth. Both the felt and the red blister galls are caused by Aceria mites.
As well as the galls, this leaf is host to a huge number of aphids at various stages of growth: the little creamy egg shapes are very tiny aphids, then there are all sizes up to fully grown adults, plus there are the ghosts – the white skins that have been shed as the aphids grew.
It seems tough for this tree, but it should be fine in the long run:)