Peronospora arvensis Gäum., Annales Mycologici 16 (1–2): 198. 1918.

Fig. 4

Description.

Lesions on leaves at first chlorotic, later often darkening to become reddish, and 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, 287–406 µm long, av. 347 µm; trunk 168–260 µm long, av. 214 µm, 7–9.5 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 4–6 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 10.5–17.5 µm long, the shorter ones 6.5–11.5 µm long, with a ratio of the longer to the shorter ultimate branchlet of 1.3–1.95, base 2–3 µm broad, apex slightly sharp. Conidia broadly ellipsoid to ellipsoid with a pale brown to dirty grey colour, 25–28 µm long, 19–21.5 µm broad, length-to-breadth ratio 1.25–1.4, basal part of the conidia mostly protruding. Oospores 34–41 µm in diameter.

Type host.

Veronica hederifolia (Gäumann 1918).

Reported hosts.

Living leaves of Veronica hederifolia (Gäumann 1918, 1923; Stevenson 1926; Riethmüller et al. 2002; Göker et al. 2003, 2007), V. sublobata (Ellis 2025), V. triloba (Voglmayr 2003) and V. triphyllos (Gäumann 1918, 1923; Stevenson 1926).

Reported distribution.

Germany, France, Switzerland, Austria, Hungary, Denmark, Sweden. Poland, Austria (Gäumann 1923; Stevenson 1926; Riethmüller et al. 2002; Göker et al. 2003, 2007; Voglmayr 2003).

Specimens examined

(all on Veronica hederifolia). GERMANY. Hessen: Rheingau, Rüdesheim am Rhein, on living leaves of Veronica hederifolia, 2 Apr 1916, A. Volkart (ZT 81629); Saxony: Görlitz-Rauschwalde, 24 Apr 2005, S. Hoeflich (GLM-F 062270); Kunnersdorf, 7 May 2008, H. Boyle (GLM-F 090770); Leipzig, 17 May 1985, H. Jage (GLM-F 069391); Leipzig, 11 Mar 1990, H. Jage (GLM-F 079120); Lodenau, 29 Apr 2008, H. Boyle (GLM-F 081506); Podrosche, 24 Apr 2008, H. Boyle (GLM-F 081427); Prositz, 1 May 1998, H. Jage (GLM-F 073863); Thümmlitz, 16 Apr 1993, H. Jage (GLM-F 068053); Saxony-Anhalt: Dessau, 24 Apr 2002, H. Jage (GLM-F 076685); Field between Wallwitz and the Petersberg, 5 May 1878, Oertel (ZT 81621); Gänsefurth, 27 Mar 2004, H. Jage (GLM-F 063010); Gänsefurth, 12 Mar 2004, H. Jage (GLM-F 063062); Hohenkirchen, 14 Apr 2001, H. Jage (GLM-F 074167); Klein Wanzleben, 4 May 2001, H. Jage (GLM-F 074159); Klitzschena, 29 Apr 1992, H. Jage (GLM-F 067424); Könnern, 4 May 2003, H. Jage (GLM-F 073461); Meuro, 17 May 1986, H. Jage (GLM-F 068191); Roßla, 20 Apr 2005, H. Jage (GLM-F 075761); Schadeleben, Cemetery, 22 Apr 2001, H. Jage (GLM-F 074206); Seeburg, 1 May 2001, H. Jage (GLM-F 074186); Wittgendorf, 14 Apr 2001, H. Jage (GLM-F 074174).

Notes.

Peronospora arvensis is widely distributed in Europe and has been reported from Veronica hederifolia, its type host, V. sublobata (Ellis 2025), V. triloba, and V. triphyllos (Gäumann 1918, 1923; Stevenson 1926; Riethmüller et al. 2002; Göker et al. 2003, 2007; Voglmayr 2003; this study). These host species are very difficult to distinguish, and identification errors are common. In fact, many of the specimens included in this study were initially misidentified as V. sublobata. However, they only had two nucleotide differences compared to the ITS sequences of Veronica hederifolia in GenBank, and based on the host DNA barcode (matK), they are identical to Veronica hederifolia. Based on our phylogenetic analyses, the Peronospora species parasitising Veronica triphyllos represents a distinct previously undescribed taxon, and P. arvensis is largely confined to its type host, Veronica hederifolia. One P. arvensis specimen, WU-MYC 0022883, collected from Austria in 1999, was recorded infecting Veronica triloba (Voglmayr 2003). However, based on the host ITS and matK sequences, two specimens (WU-MYC 0032791 and WU-MYC 0032807) from Veronica triloba actually represent a previously undescribed species distinct from P. arvensis, therefore, the host of specimen WU-MYC 0022883, identified as Veronica triloba, is likely a misidentification. This highlights the importance of DNA molecular identification for hosts that are not easily identified, as it is crucial for determining the species specificity of Peronospora. Phylogenetic analyses indicate that P. arvensis is closely related to the Peronospora sp. on Veronica triloba and P. veronicae-cymbalariae on Veronica cymbalaria. Due to the lack of specimens of Peronospora sp. on Veronica triloba, this new species has not been described in this study. Despite this, when compared with P. arvensis, P. veronicae-cymbalariae still had larger conidia, 32 × 23 µm on average, and shorter conidiophores and trunks, 246 µm and 117 µm in breadth on average, respectively.