Chlorostroma vestlandicum (Xylariales): first records of the rare ascomycete in Russia

In this paper, for the first time in Russia we report on four new occurrences of the rare xylarialean ascomycete Chlorostroma vestlandicum revealed in the course of mycological surveys of the Northwestern European Russia. Description and colour illustrations of the material studied are provided. Dutch Elm Disease as the major possible threat for the species is discussed. The species is recommended for inclusion in the next editions of the regional Red Data Books of Leningrad, Novgorod, and Pskov Regions.

In this paper, we report four new occurrences of C. vestlandicum revealed in the course of mycological surveys of the Northwestern European Russia and provide a description and illustrations of the material studied.

Material and Methods
The material was collected during a series of mycological surveys of Leningrad, Novgorod, and Pskov Regions of Russia and identified by E. S. Popov. The coordinates are in WGS84 geographic coordinate system. The macro-morphological characters were described according to the observations of fresh and dry material as well as the analysis of the photos. Capitalized colour names and corresponding colour codes given are those of Kornerup, Wanscher (1978). The micromorphological structures were examined in dried material. Fragments of stroma and perithecia were mounted in 5 % KOH and tap water for standard light microscopy. To test the amyloid reaction of asci's walls and apical rings Melzer's reagent (MLZ) and Lugol's solution (IKI) without and with KOH pre-treatment were used. The number of measured morphological structures (n) is given in parentheses after corresponding measurements. The images of micro-morphological characters were captured with an AxioCam MRc 5 digital camera on an AxioImager A1 microscope (Carl Zeiss, Göttingen, Germany). The studied specimens are deposited in the Mycological Herbarium of the Komarov Botanical Institute (LE), Russian Academy of Sciences. Stromata when fresh peltate-pulvinate to turbinate, centrally attached, 0.5-1.5 cm in diam., 1-3(-5) mm thick. Outline circular, oblong or irregular. Upper surface smooth or undulating, dark olive gray (3F2-3) to dark olive brown (4F3). Margin adnate or free, rounded or sharp, often lobed or undulate, sometimes whitish pruinose. Perithecial contours indistinct. Ostioles hardly visible, 20-50 (-65) mkm (n = 10) diam., numerous, plane or slightly umbilicate, concolorous or slightly darker than the stroma surface. Lower side sterile, black. Stroma interior solid, soft corky in texture, lighter than the surface, mustard yellow (3B6-7) to grayish yellow (4B3-4). Extractable pigments in 10 % KOH dark green (29D6-7).
Chlorostroma vestlandicum is a highly distinctive stromatic ascomycete. It can be easily recognized in field by the naked eye due to its mustard yellow stroma interior and frequent association with Hypoxylon vogesiacum on larger-sized wood debris of U. glabra. The connection between C. vestlandicum and Hypoxylon vogesiacum is well-established (Nordén et al., 2014). The last species is known in Leningrad and Pskov Regions and quite regularly, although not often observed on dead elm logs, especially in riparian forests (author's own observations). In two of the studied specimens (LE 323571, LE 321716), the stromata of C. vestlandicum were clearly growing on H. vogesiacum, in the other (LE 323494), the fungus was growing on blackened decorticated wood with remnants of some stromatic xylariaceous fungi on some distance, which is likely to be H. vogesiacum. The fourth specimen (LE 323710) contains a piece of corticated wood overgrown with mosses, and stromata of C. vestlandicum are erumpent from the cracks in the bark; no signs of Hypoxylon were seen.
Ulmus glabra declines drastically due to Dutch Elm Disease. For a long time, the disease was localized in the south of European Russia, but since the end of the 20th century, there has been a massive dying of elms in the central and northwestern regions (Gninenko et al., 2020). During last decades, the decline of elms has become disastrous in parks of Saint Petersburg and its suburbs (Firsov, Bulgakov, 2017). The tree is assessed as vulnerable in Europe (Rivers et al., 2019). While there are no reliable data on decline of wild wych elm populations in Northwestern European Russia, the rarity of C. vestlandicum and its narrow substrate specialization allow us to conclude that the Dutch Elm Disease in the future may become a crucial threat for C. vestlandicum.

Conclusions
Chlorostroma vestlandicum is classified as Endangered in Norway (Brandrud et al., 2015). The new occurrences of C. vestlandicum reported in our study are more than 1000 km away from Norway populations and significantly expand the known range of the species. Considering that these three collections were made in different areas of European Russia in a short time, it could seem that the species occurs in favorable biotopes more regularly than one could expect. Nevertheless, its narrow substrate specialization and confinement to rare plant communities with elm and other broadleaved trees allow us to recommend it for inclusion in the next editions of the regional Red Data Books of Leningrad, Novgorod, and Pskov Regions. Further targeted studies of wych elm habitats are needed to clarify the status of local populations of C. vestlandicum and their potential threats in European Russia for making a decision on the including the species in the national Red Data Book and developing the necessary conservation actions.