Juniper Rust Gymnosporangium sp.

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.

Class Pucciniomycetes
Order Pucciniales
Family Pucciniaceae
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

Juniper Rust Gymnosporangium sp.