Cedar Apple Rust

The bizarre looking orange growths in these photos are reproductive structures of the cedar apple rust fungus. I took these pictures while I was walking along a road near Hammonton, New Jersey, in a light spring rain.

They are usually called galls. They don't always look like this; like Gremlins, they change when wet.

The cedar apple rust fungus cycle takes two years. Air-borne spores are released from infected apple leaves in late summer. Spores settle from the air onto junipers. Eastern red cedar, Juniperus virginiana, is a common host.

The spores germinate and infect the juniper tree (confusingly called red cedar). Galls grow on the tree in the following spring. First-year galls are pitted brown balls. In the second spring, the galls swell when they get wet. Gelatinous orange tendrils grow from them, quickly like mushrooms, pushing out of the surface pits. When it stops raining, the tendrils dry out and shrink into brown threads. When the galls get wet again, they swell and grow more. The gall eventually becomes a glob of two-inch long tendrils about the size overall of a small orange. It releases spores during dry periods for a few weeks.

Wind carries the spores away. If they fall on apple leaves, they can germinate and infect the tree. Pale yellow spots appear on infected apple leaves. These "rust spots" mature by summer and eventually open to release spores that are carried by the wind to junipers. The cycle repeats.

The spores produced on apple leaves don't infect apple trees. The spores produced in juniper galls don't infect junipers. The fungus needs both hosts to live.