Polystigma fulvum Pers. ex DC., Fl. franç., Edn 3 (Paris) 5/6: 164 (1815)
Index Fungorum number: IF189406
Parasitic on Prunus padus. Leaf spots developing on living leaves, scattered, irregular, swollen, composed of pigmented outer layers and a hyaline inner layer. Sexual morph: Ascostromata irregularly-shaped, dark orange-brown on upper side and yellowish brown on lower surface, forming an inner hyaline layer within and throughout the leaf tissue with a high number of ascomata which appear like reddish orange spots 67–126 µm diam. on the strongly raised adaxial surface, ostioles nearly inconspicuous and somewhat sunken. External layer of stroma 40–60 µm thick at the adaxial surface and thinner at the abaxial one. Ascomata 194–218 µm high, 230–246 µm diam (x̅ = 204.9 × 237.7 µm, n = 10), almost sphaerical with the walls differentiated from stromatal tissue. Ostiole 37–54 µm × 26–87 µm (x̅ = 43.5 × 54.5 µm, n = 8), almost inconspicuous, periphysate. Periphyses up to 18 µm long and 2–5 µm wide at the base, gradually becoming narrow at the apex. Peridium 16–31 µm thick, hyaline, adequately developed, fusing with the stromatal tissue, both of which are filled with shiny yellow material. Paraphyses up to 78 µm long, rather sparse, gradually tapering towards the apex, very thin-walled, strongly inflated between the septa. Asci 50–110 (–133) µm × 10–15 (–18) µm (x̅ = 94.8 × 13 µm, n = 58), 8-spored, unitunicate, narrowly clavate with long stalk (to ca. 52 µm), the apex obtuse, 1–4 µm × 2–5 µm diam. (x̅ = 2.2 × 3.1 µm, n= 14) with a J-refractive apical annulus. Ascospores 8–15 µm × 3–8 µm (x̅ = 11.9 × 5 µm, n = 97), arranged biseriately or irregularly seriate, cylindrical, ellipsoidal to obovoid, hyaline, aseptate, regularly guttulate, comprising a faint gelatinous sheath, no appendage. Asexual morph/ Andromorph: Polystigmina pallescens (illustrated in Suzuki et al. 2008).
Polystigma fulvum (Material examined – BELARUS, Minsk region, Maladzyechna District, Sychevichi Village, near children's health camp “Brigantina”, deciduous forest on Prunus padus (Rosaceae), 15 Oct. 2017, A.K. Khramtsov T-2083, MFLU 18-0261). a Swollen leaf spots on Prunus padus L. b Close-up of ascomata. c Vertical section through ascoma. d Close-up of inconspicuous ostiole. e Peridium of ascoma. f Asci. g Ascospore. h Faint sheath around ascospore. Scale bars: a = 200 μm, c = 50 μm, d, f = 50 μm, e = 20 μm, g, h = 5 μm (image from Hyde et al. 2020).