You picked up a brown bur oak leaf and get grossed out that it looks like it's molding.
But those fuzzy spots on the leaf aren't mold; in fact, they're galls.
A gall is a growth on a plant that develops due to insects that majority of the time. A female gallmaker insect --- of which there are about 1,500 different species --- will select a certain type of plant and a certain place on that plant, whether it be a leaf, twig or branch, and will lay an egg in that part of the plant.
From there, science hasn't quite found an answer of what happens. It is thought that something in the female's saliva or in the egg or larvae then prompts the plant tissue to grow around the newly-forming insect.
The larvae grows inside its new home, which has strong walls, is rich in protein and provides food and protection until it is full grown and emerges during the summer or the following spring.
The fuzzy gall on oak leaves is a woolly oak leaf gall. They can be as large as three-fourths of an inch and are often bright pink or yellow, fading to brown in the fall. It is created by the cynipid wasp, also called a gallfly, and don't sting humans. Most wasps, like bees, are actually solitary, non-stinging species.
Oaks are especially susceptible to galls, and you might see a variety of galls on one tree.
Another kind of gall often seen in northwest Iowa is on the stem of goldenrod. You might find a round, spherical protrusion about halfway up the stem, and these are caused by the goldenrod gall fly.
In the spring, the female fly deposit eggs in goldenrod stems, usually only one gall making it per stem, even if she lays more than one egg in each plant. The egg hatches in several days and begins to feed. The plant increases cell production around the site of the injury and grows around the larvae. Yet again, the gall acts are both a food source and protection.
In other places, other species of insects will produce differently-shaped galls on goldenrod. In fact, galls have such distinctive appearances that they can be used to identify the insect species inside.
Most galls do no seriously affect the health of their hosts.