Size 2-4 cm in diameter. This is a fungi plant gall which alternately infects Juniperus (Junipers) and trees like Hawthorn, Apple, or Pear. Junipers are the primary hosts, and the fungus forms a ball which produces a set of orange tentacle-like spore tubes called telial horns. These horns expand and have a jelly-like consistency when wet. The spores are released and travel on the wind until they infect the secondary hosts like Hawthorn where fungus produces tiny rust-like pimples on the leaves. It also infects the fruit, which grows tubes which carry the spores. The spores must then infect a Juniper to complete the life cycle.
It is either Gymnosporangium clavariiforme or Gymnosporangium confusum, depending on the Juniper host. A microscope is required to determine actual species.
The rust horns appear on Junipers in the spring. Fairly common and widespread in Britain. Native.
Genus Gymnosporangium R. Hedw. ex DC.
Photos: On Hawthorn (Crataegus monogyna), showing telial horns (spore tubes) and possible Cecidomyiidae (gall midge) larvae. Back garden, South Staffordshire. June 2021. © Peter Hillman