Lecanora (?) sp. from Denmark

Your enquiry prompted me to re-examine a specimen which I had, somewhat tentatively, identified as Lecidea exigua. There are many similarities between my material and yours but perhaps not an exact match. I am not providing a definite determination (I am hesitant even about naming my own material) but, in the absence of suggestions from others, perhaps my observations will provide another line of investigation.

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My specimen (above) is from the smooth bark of an apple tree in a Kent orchard. It has apothecia (250-)300-350 microns in diameter. The disc has a slight pruina due to coarse epithecial granules. The thallus is indistinctly warted, sits on a slight black prothallus (visible where the thallus is thin) and forms conspicuous black lines where it meets neighbouring lichen thalli. The thallus shows a patchy UV+ orange fluorescence, it is K+ yellow (yellow turning slightly orange as in yours). It is K/UV+ yellow (perhaps slight green hint). I suspect this K/UV+ yellow is indicating the presence of a xanthone (rather than atranorin). It is C- but I think the K reaction is slightly deepened more orange when C is applied to the spot (i.e. probably KC+ orange).

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Above are three sections in water and one in K. The coarse epithecial granules dissolve in K. My specimen had rather inconspicuous small crystals in the exciple, mainly towards inner parts, soluble or mainly soluble in K. The ascospores look a bit fatter than in your material (typically e.g. 11 x 5.5) but similar dimensions.

I was particularly interested to see what the ascus tips were like in my specimen. The afl website states that L. exigua has a Lecidea-type ascus but this is not illustrated and may be an assumption (the same page states that Lecidella elaeochroma has a Lecanora-type ascus which is incorrect, an error also in LGBI2, L. elaeochroma having a Lecidella-type which is more like a Bacidia- or Biatora-type).

https://www.afl-lichenologie.fr/Photos_AFL/Photos_AFL_L/Lecidea_exigua.htm

The asci of my material have similar ascus tips to your material, and rather similar to those in Lecidella as shown below.

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I’m sorry that I can’t give any definite identification but I was struck by some of the similarities between your material and my specimen which I have tentatively concluded must be L. exigua. L. exigua is supposed to grow mainly on smooth bark of deciduous trees but there may be overlap of species on deciduous trees and the smooth bark plates of old conifers. I am not even certain that our material is the same taxon but seems to be the ‘same sort of thing’. Perhaps there is more than one taxon that has been named as L. exigua (it seems to be a rather poorly understood lichen with only scanty published descriptions).