The bountiful spring rains have brought us our first report of a sighting of telial horns (see picture below) of the fungal pathogen Gymnosporangium juniperi-virginianae. This pathogen causes the disease called cedar apple rust. As the disease name implies, this fungus infects both cedar trees (Juniperus spp) and apples trees (including crabapple trees).
If you have susceptible apple and crabapple trees, the appearance of the telial horns marks the beginning of the production of spores that can be dispersed long distances from the cedar trees to apples and crabapples located 1 mile from the infected cedar. The horns will "inflate and deflate" following rain events up to 10 times during the spring season. The microscopic fungal spores capable of infecting apple produced in the horns are released into the wind.
If you are concerned you can prune and destroy the galls on the cedar trees.
Your crabapple and apple trees, and mostly your local commercial apple grower will be grateful for the destruction of those galls!