A spring rain, like in the last few, days typically produces the Cedar Apple Rust Gall. In Missouri they are commonly seen on our Eastern Red "Cedar" trees (actually a Juniper- Juniperus virginiana - there are no native cedars in North America.) Unlike many other galls, these are due to a fungus infection which are caused by the fungus called, appropriately enough, Gymnosporangium juniperi-virginianae.
|Rust gall in first year|
When they spread to apple or crab apple trees, the leaves develop yellow spots. If there are heavy rains such as this week, the leaves and buds can be severely attacked, affecting the health and production of the tree. With dry weather, the tree may lose most of its leaves. Later in the summer the leaves produce rust-colored spores that infect only a juniper host, completing the cycle.
See the Missouri Botanical Garden site for treatment options.
A more complete description including a graphic demonstration of the fungal life cycle is at this Wikipedia Site.