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Gymnosporangium cornutum/clavariforme

Gymnosporangium clavariiforme Common Name: Tongues of Fire (suggestion only see below) (Old scientific names include: Podisoma clavariaeforme; Tremella clavariaeforme;   Tremella clavariaeformis; Podisoma clavariiforme; Podisoma juniperi-communis)

Gymnosporangium cornutum Common Name: Crown of Thorns (suggestion only)

There are four established Gymnosporangium species in the UK;  they are rusts and their life cycles alternate between different host plants, in this case between juniper and a range of  rosaceous trees and shrubs. Two species, G. clavariiforme and G. cornutum, have juniper (Juniperus communis) as one of their hosts and are visible on the juniper in the first half of the year on damp days. The other two species, G. confusum and G. sabinae, are on Juniperus sabina and are not presented here. The alternate host for G. clavariiforme is hawthorn with structures found on the underside of leaves and on berries; the alternate host for G. cornutum is rowan with structures again found on the underside of leaves and on berries.

It is the structures of G.cornutum on rowan leaves, like little upside down, spiky crowns that can most easily be found in the summer and autumn months and this is the main source of records for G. cornutum (the yellow gelatinous blobs in the spring on juniper are less commonly recorded). Do check out the rowans - its amazing what turns up when you know where to look! Checking the underside of hawthorn leaves can also be productive for G. clavariiforme although the structures are not quite so distinct.

G. clavariiforme is most commonly recorded in the spring when it produces amazing bright orange, structures on juniper known as telial horns. These horns grow in dense clusters and each individual horn can reach a length of around 1cm, expanding in damp conditions. When flushing, it can appear from a distance as though the tree is on fire and it has been suggested that a Middle Eastern species of Gymnosporangium may have been responsible for the story of the burning bush in the Bible. In dry weather, these structures disappear and all that is visible is a slightly swollen area on the host stem. If you have a juniper wood near you, it would be well worth checking this out on a damp day in April or May.

A word about names. Rusts illustrate perfectly the complications of naming some fungi because rusts come with more than one stage in their life cycle - each looking very different. In some cases, where a relationship was not previously recognised, different stages of the same species have been given different scientific names. Should different stages all have the same scientific name and if so, which of the already named stages should take priority? 

Something similar applies to the common names of species with different stages in their life cycles - should we name for a visually distinctive stage, for example 'tongues of fire' which only relates to the appearance of the telial stage of G. clavariiforme? Conversely, the distinctive stage of G. cornutum is the aecial stage when 'crown of thorns' is a great fit (although the existing use of this name for a starfish might mean that we have to think of something else anyway - any offers?). 

Rust fungi are basidiomycetes but only distantly related to mushrooms and toadstools.  More informationabout some of these terms and fungal lifestyles


G. cornutum has been recorded from February to May in its rather unobtrusive telial stage on juniper (blobs of orangey jelly) but for its distinctive aecial stage on rowan, August and September are the most productive months.

G. clavariiforme has been recorded from April and May in its distinctive telial stage on juniper; the less distinct aecial stage is mostly found in August and September on hawthorn.

The telial stages on juniper of both species occur in the spring and can most readily be seen on damp, rainy days when the telial stage is hydrated.


Like all rusts, Gymnosporangium is parasitic, in other words, feeding from a live host. In most cases, there is little damage done to the hosts although sometimes apparently the production of haws can be limited.

Searches for the different stages of the fungus are clearly limited to habitats that support juniper, hawthorn or rowan. It is of interest to me that in many areas where the distinctive telial stage of G. clavariiforme is relatively common on juniper, the alternate host, hawthorn is not present in any quantity. Occurrence is differently skewed with G. cornutum - the telial stage is relatively uncommonly recorded on juniper whilst the aecial stage seems to be on every other rowan leaf examined! Any ideas as to why this might be are very welcome!          


Records of these species can be generated from either stage of their life cycles. The majority of Scottish records of G. cornutum are from the aecial stage on rowan leaves. Records of G. clavariiforme are mostly be from the telial stage on juniper. This may go some way to explaining the more limited distribution of the latter.

Please remember to submit your records to your local recording group or via the Scottish Fungi online recording form.

Many thanks to Alan Watson Featherstone from Trees for Life for the excellent photos in this profile.

Gymnosporangium clavariforme on juniperGymnosporangium clavariforme on juniper
Gymnosporangium clavariforme on juniper

Gymnosporangium clavariforme on hawthorn
Gymnosporangium clavariforme on hawthorn
Gymnosporangium clavariforme on hawthornGymnosporangium clavariforme on hawthorn

Gymnosporangium cornutum on rowan
Gymnosporangium cornutum on rowan
Gymnosporangium cornutum on rowan

Gymnosporangium clavariiforme

Gymnosporangium cornutum

The National Biodiversity Network Atlas records from all available datasets are shown on the above maps (see terms and conditions at http://data.nbn.org.uk). Data providers and the NBN Trust bear no responsibility for any further analysis or interpretation of the information in the map.