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Peronospora aquatica Gäum., Annales Mycologici 16 (1–2): 199. 1918. Fig. 3 Description. Lesions on leaves at first yellow to brown then darken to become reddish, diffuse to vein-delimited. Down present on the lower leaf surface, consisting of scattered to dense felt-like conidiophore outgrowth. Conidiophores hyaline, straight to curved, thin-walled, 276–420 µm long, av. 348 µm; trunk 146–234 µm long, av. 190 µm, 7.5–11 µm broad, sometimes slightly swollen to up to 17 µm at the base, ratio of the total length to trunk length 1.5–2, callose plugs absent. Branching subdichotomous in 4–8 orders, with sub-straight to curved branches, gradually attenuate, often at acute angles. Ultimate branchlets sub-straight to curved, paired branchlets differing in length, with the longer ones 13–21.5 µm long, the shorter ones 8–13 µm long, with a ratio of the longer to the shorter ultimate branchlet of 1.5–2, base 2.5–3.5 µm broad, apex slightly sharp. Conidia ellipsoid to elongated with a pale brown to reddish colour, 29.5–37 µm long, 18.5–24 µm broad, length-to-breadth ratio 1.45–1.7, basal part of the conidia mostly protruding. Oospores 28–33 µm in diameter. Type host. Veronica anagallis-aquatica (cited under its orthographic variant Veronica anagallis Cham. & Schltdl.) (Gäumann 1918). Reported distribution. Austria, Germany, Poland, Switzerland, Sweden, United States, China, Denmark, France, Latvia, Lithuania, Romania, Russia (Gäumann 1923; Yu 1998; Göker et al. 2003, 2007). Specimens examined. GERMANY. Saxony-Anhalt: Helme-Unstrut Buntsandsteinland, on living leaves of Veronica anagallis-aquatica, 29 May 2003, H. Jage (GLM-F 073366); Querfurt Plate, Lodersleben, at the castle, in the Querne, on living leaves of Veronica anagallis-aquatica, 6 May 2005, H. Jage (GLM-F 075951); Trebitz, Krähenberg, on living leaves of Veronica anagallis-aquatica, 22 Aug 1996, H. Jage (GLM-F 069669). SWEDEN. Gotland: unknown, on living leaves of Veronica anagallis-aquatica, 20 Jul 1910, T. Vestergren (ZT 81575). Notes. Peronospora aquatica is widely reported to be distributed across Europe, Asia, and North America, where it parasitises Veronica anagallis-aquatica (= Veronica anagallis Cham. & Schltdl.) (Gäumann 1918; Gäumann 1923; Yu 1998; Göker et al. 2003, 2007). Phylogenetic analyses reveal that P. aquatica has a closely related sister species, P. sagittaria, which was previously undescribed and parasitises Veronica catenata. Although the two species are morphologically similar, P. sagittaria can be distinguished by its smaller conidia (32 × 21 µm on average), longer conidiophores and trunks (405 µm and 243 µm long on average, respectively), as well as narrower trunks and conidiophore bases (11 µm and 8 µm broad on average, respectively).