Plant of the Week – 16 November – Alder tongue (Taphrina alni) (fungus)

We all know that fungi are not plants, but the Botanical Society of Scotland promotes the study of cryptogams and so fungi (as honorary plants) are very much within our remit.

At this time of year it’s easy to see the cones on the almost-leafless alder trees. This year’s immature greenish-yellow cones are visible alongside last year’s open brown ones. The sight of these cones are likely familiar to many and, as such, it can be easy to walk past the trees without paying too much attention to them – but it is worth stopping next time and having a closer look. You might find a dried, brown, twisted shape emerging from some of those cones. These are clearly not part of the tree – so what are they?

What is this?

These strange objects are galls caused by the fungus Taphrina alni, known as alder tongue. It is a fungus in the Ascomycota that lives as a plant pathogen, forming galls on the female reproductive strobili (‘cones’) of Alnus glutinosa (common alder) and A. incana (grey alder). Galls on plants can be formed by different organisms – mites, wasps, fungi, viruses – which provoke a growth reaction in the plant when they infect or infest its tissues. What you see is actually plant tissue, rather than a fungal fruiting body. The reproductive structures (asci) of the fungus are found within the structure of the galls.

The galls formed by T. alni can be several centimetres long and are smooth-edged, grooved and generally curved to form all sorts of shapes. They sprout from between the cone scales, and there may be a single one or several on a cone. When they form early in the season, they are green, like the scales they emerge from. However, for a few weeks in late summer they change colour to a noticeable red, tinged with yellow. This red shade, coupled with their shape, has earned them the name alder tongue. Apparently, they are also known as ‘languets’, a word describing something resembling a tongue.

Large, single alder tongue emerging from a cone

I was introduced to this fungus a few years ago and now tend to scan alder trees for it as standard, but I’d managed to consistently miss the red phase every summer since I was made aware it existed…until this year! I came across them unexpectedly while walking through a young woodland and meadow in August; my eye was caught by a splash of colour in the alders as I passed and, lo and behold, there they were. I was surprised (and pretty delighted) by how many there were.

This species has been found in several European countries; for example, the UK, Germany, Switzerland, Norway, Denmark and Ireland. It is also present in the USA and Canada. The distribution of this fungus appears to have been increasing in the UK in recent years, so it is worth recording if you do come across it. This raises an interesting question: why is the range increasing? Are conditions becoming more amenable to the fungus’s spread and growth, or has its spread been assisted in some way – or has the apparent expansion in range been caused by an increase in recording?


Britain’s Plant Galls: A photographic guide by Michael Chinery. Wild Guides Ltd., 2011
Alder tongue gall on Encyclopedia of Life
The Wildlife Trust of South and West Wales: Alder tongue

– Heather Forbes

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