Peronospora grisea Unger

Peronospora grisea Unger, Bot. Ztg. 5 (18): 315. (1847). Fig. 9 ≡ Botrytis grisea (Link) Fr., Systema Mycologicum 3: 396. 1832. = Peronospora verna Gäum., Annls mycol. 16 (1 / 2): 198. 1918. Description. Lesions on leaves greyish, reddening to dark brown when dried, often vein-delimited. Down present on the lower leaf surface, greyish-white, consisting of scattered to dense felt-like conidiophore outgrowth. Conidiophores hyaline, straight to curved, thin-walled, 250–335 µm long, av. 293 µm; trunk 122–207 µm long, av. 165 µm, 6.5–8 µm broad, sometimes slightly swollen to up to 13 µm at the base, ratio of the total length to trunk length 1.5–2, callose plugs absent. Branching subdichotomous in 4–7 orders, with sub-straight to curved branches, gradually attenuate, often at right angles. Ultimate branchlets sub-straight to curved, paired branchlets differing in length, with the longer ones 9–14 µm long, the shorter ones 6–10 µm long, with a ratio of the longer to the shorter ultimate branchlet of 1.2–1.65, base 2–2.5 µm broad, apex slightly sharp. Conidia ovate to ellipsoid with a brown colour, 20–25 µm long, 15–18 µm broad, length-to-breadth ratio 1.25–1.5, basal part of the conidia mostly protruding. Oospores not observed. Type host. Veronica beccabunga (Unger 1847). Confirmed additional host. Veronica serpyllifolia. Reported hosts. Veronica beccabunga (Unger 1847; Gäumann 1923; Stevenson 1926; Riethmüller et al. 2002), V. arvensis, V. hederifolia, V. verna (de Bary 1863), V. peregrina (Swingle 1887), Veronica serpyllifolia (de Bary 1863; Voglmayr 2003). Reported distribution. Germany (de Bary 1863; Stevenson 1926; Riethmüller et al. 2002), USA (Swingle 1887), Austria, Croatia, Denmark, France, Hungary, Ireland, Netherlands, Norway Russia, Sweden, Switzerland, UK (Fries 1832; Gäumann 1923; Voglmayr 2003). Specimens examined. GERMANY. Saxony-Anhalt: Bergwitz, on living leaves of Veronica serpyllifolia, 16 Apr 2002, H. Jage (GLM-F 076812); Bülzig, on living leaves of Veronica beccabunga, 28 Apr 2004, H. Jage (GLM-F 064183); Hayn, on living leaves of Veronica serpyllifolia, 20 Apr 2005, H. Jage (GLM-F 075766); Lodersleben, on living leaves of Veronica beccabunga, 9 Apr 2004, H. Jage (GLM-F 063053); Lützen-Hohenmölsener Platte, Gieckau east, Nautschke Creek, on living leaves of Veronica beccabunga, 14 Jun 2005, H. Richter (GLM-F 075648); Morungen, on living leaves of Veronica beccabunga, 21 Apr 2005, A. Hoch (GLM-F 075774); Neckendorf, Helme, on living leaves of Veronica beccabunga, 1 Sep 2002, H. Jage (GLM-F 076527); Obhausen, on living leaves of Veronica beccabunga, 3 May 2005, H. Jage (GLM-F 075736); Straßberg, on living leaves of Veronica beccabunga, 10 Sep 2005, H. Jage (GLM-F 075832); Wörlitz, Auwiese, on living leaves of Veronica serpyllifolia, 20 May 2002, H. Jage (GLM-F 078065); Saxony: Torgau, on living leaves of Veronica serpyllifolia, 3 May 2002, H. Jage (GLM-F 076783); SWEDEN. Uppsala: Uppsala-Norby, on living leaves of Veronica serpyllifolia, 16 May 2007, H. Boyle (GLM-F 079274). Notes. It is surprising that P. grisea on Veronica beccabunga (the type host of P. grisea) and P. verna on Veronica serpyllifolia (the type host of P. verna) are genetically identical in LSU D 1-3, LSU D 6-8, COX 1, COX 2, and RPS 10, with only a single base difference in the ITS and NAD 1 regions. Given the principle of priority by chronological order, P. verna is relegated to synonymy with P. grisea here. However, it would be meaningful to investigate the relationships between the populations of both hosts to determine how far they are genetically separated enough to be considered independent subspecies. In any case, the pathogens on these two hosts might provide a model for studying speciation processes in downy mildews in particular, and obligate biotrophic pathogens in general. Furthermore, this finding emphasises the limitations of relying solely on host-specificity for species identification of the pathogens. Although in most cases Peronospora species are highly host-specific, there are some exceptions due to recent host jumping (Thines 2019; Ploch et al. 2022) or multiple colonisations of the same host species (Mu et al. 2024 a). One specimen, WU-MYC 0022901, parasitic on Veronica serpyllifolia, was also identified as P. grisea in Voglmayr’s 2003 study, based on ITS sequences (AY 198241), although Veronica serpyllifolia was the type host for P. verna (Gäumann 1918). The differences observed by Gäumann can likely be attributed to the effect of the host matrix, as it has been previously shown in Pseudoperonospora cubensis, where the host matrix can influence several morphological characteristics of the pathogen significantly (Runge et al. 2012). Morphologically, P. grisea is characterised by its rather short conidiophores and trunks, with average lengths of 293 µm and 165 µm, respectively.