Enigmatic lichen from Denmark

As soon as I glanced at your overview photos I thought the pattern of pale grey areoles looked familiar and my thoughts went straight to Verrucaria ochrostoma. This was formerly considered a very rare species in Britain but it is now accepted that it is actually a common colonist of calcareous substrata, including mortar and concrete. The nature of the cortex in V. ochrostoma, with a distinct epinecral layer, is much as you have described. However, I have never seen pycnidia on any of the specimens of V. ochrostoma that I have examined. I wonder whether the pycnidia belong to something else, perhaps even to a lichenicolous fungus? In the close up from your overview, I can see a couple of small black structures on the edge of the little cluster of algal units making up the main areole (upper left) and the top of a larger brownish structure (above and right of centre). I wonder if the smaller black structures are the pycnidia that you have found and that you haven’t properly investigated what I think might be the perithecia belonging to V. ochrostoma.

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Perhaps we have an arrangement as in my drawing but with some unknown pycnidia on the edges of some of the areoles?

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Here is a link to more information and images of V. ochrostoma:

http://fungi.myspecies.info/taxonomy/term/7172/media

I offer this as merely a suggestion of something to investigate.

So we are left with various possibilities:

1. my suggestion of V. ochrostoma is incorrect.

2. it is V. ochrostoma and the pycnidia belong to a lichenicolous fungus.

3. it is V. ochrostoma and the pycnidia belong to the lichen.

I slightly favour the third possibility at the moment, partly because the pycnidia are usually positioned where I would expect the perithecia of V. ochrostoma would normally be (surrounded by the little algal units). Although I have never seen pycnidia in V. ochrostoma, and they are not mentioned in the literature, that doesn’t mean that they definitely don’t exist. The 2009 ‘Flora’ tells us that the conidia of the rather similar V. viridula are “straight to slightly curved, 7-10 x c. 1 um.” Unfortunately it can be frustratingly difficult to tell whether pycnidia belong to a lichen or whether they are a lichenicolous fungus.

In Eastern England we have many buildings with sloping limestone windowsills. Many are 19th C. and include church and municipal buildings. These are often dominated by Verrucaria ochrostoma. If you have similar buildings in Denmark it might be worth keeping this in mind.

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V. ochrostoma also turns up commonly on sloping cement on walls, for example the little slopes often found towards the tops of modern suburban walls. A wall such as the one below has a suitable little sloping fillet of cement below the topmost black bricks.

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V. ochrostoma has been a remarkably overlooked lichen. When I hosted a BLS meeting in 2012 I pointed out good material to the great and good of the lichen community and nobody present could name it. In the end, on a strong hunch, I made a trip to the Natural History Museum in London to examine Borrer’s 19th C. type specimen and this reassured me that this common colonist which I was finding regularly really is V. ochrostoma. I also noticed that the tops of the low walls outside the museum, where visitors queue to get in, were dominated by V. ochrostoma where it had sat completely unnoticed. I went to Scotland a few years ago and within a very short time I was able to find first Scottish records for both V. ochrostoma and V. obfuscans. Last year I confirmed the first record of the latter for the Netherlands from a few ‘crumbs’ scraped from a tomb. I would be surprised if you don’t have plenty of occurrences of both species in Denmark.