When I arrived home Sunday night, my neighbor Harry came over, anxious to once again play "Stump the Master Naturalist" Another neighbor's cedar tree was covered with 40+ bright orange growths, visible from across the street. As usual, Harry had done his homework. Fortunately, I knew this one.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.
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| Rust gall in first year |
The two year life cycle of this fungus is complex, requiring it to attack cedar one year, apples the next and them back to cedars. On the red cedar they form perennial galls on twigs that mature in two years. When spring rains hit as we have seen this week, the pits of the gall swell and extrude multiple bright orange jelly-like horns. They occasionally can cause significant damage to the junipers. The spores these galls produce can only infect trees of the rose family, which include apple trees. 
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.
