Schlechtendalia 28 (2014) 41 First report of Erysiphe baptisiae on Baptisia in UK with emended description Roger T. A. COOK, Alick HENRICI & Uwe BRAUN Abstract: Cook, R. T. A., Henrici, A. & Braun, U. 2014: First report of Erysiphe baptisiae on Baptisia in UK with emended description. Schlechtendalia 28: 41–48. Erysiphe baptisiae has been found for the first time in UK. Taxonomy and morphology of this species are discussed, and an emended description and illustrations are provided. Zusammenfassung: Cook, R. T. A., Henrici, A. & Braun, U. 2014: Erstangabe von Erysiphe baptisiae auf Baptisia in Großbritannien mit emendierter Beschreibung. Schlechtendalia 28: 41–48. Erysiphe baptisiae wurde erstmalig in Großbritannien gefunden. Taxonomie und Morphologie dieser Art werden diskutiert, ergänzt durch eine emendierte Beschreibung und Abbildungen. Key words: powdery mildew, Baptisia australis, chasmothecial appendages, Erysiphe (Microsphaera) rayssiae, Erysiphe baptisiicola, distribution of conidial widths. Published online 9 Dec. 2014 Introduction Baptisia (false indigo) is a genus of shrub-like fabaceous perennials, natives of North America with lupin-like flowers used in gardens or landscapes. On 13 August 2014 at the Royal Botanic Garden, Kew, a heavy infection of a powdery mildew was found on a clump of Baptisia Purple Smoke, a natural hybrid between B. australis and B. alba distributed recently by North Carolina Botanical Garden. Greyish white patches of thin mycelium, sometimes covering both leaf surfaces, bore scattered groups of fruiting bodies (chasmothecia). One week later, a survey of the Order Beds at Kew revealed two plants of one of the parents, B. australis, about 100 m from the first site. One of these, dated 1969, was entirely healthy. The other, dated 2011, had thin mildew mycelium on a few shaded leaves which by 2 October 2014 bore a few chasmothecia. Only two powdery mildews are presently recognised on Baptisia, viz. Erysiphe baptisiae confined to Europe (Germany, Romania, Switzerland) and E. baptisiicola U. Braun confined to North America (Braun et al. 2010, Braun & Cook 2012). On first inspection, there appeared to be two types of chasmothecia; one with mainly unbranched, hyaline and aseptate appendages conforming to the description of E. baptisiae (Fig. 1) and another with 0–6 times dichotomously branched appendages that were either pale and aseptate (Fig. 2, above) or mid brown and septate at the base (Fig. 2, below). The branching was either more or less symmetrical (Fig. 3, above) or very asymmetrical (Fig 3, below). Appendages are critical in diagnosis, but a mixture of two species is ruled out, because characteristics of conidiophores and conidia (Table 1A-B) consistently conformed to E. baptisiae, apart from many foot cells being distinctly ‘kinked’ at the base (Fig. 4). Conidial width, also critical for identification of a species, showed little variation with a sharp peak at 12 μm (Fig. 5). By contrast, conidial lengths and length/width ratios were both more variable and did not have single peaks (Fig. 6A-B). However, the absence of a second peak in conidial width strongly supported the presence of a single species. In conclusion it can be said that the British collections on Baptisia can be assigned to E. baptisiae and represent first records of this species from UK. This powdery mildew belongs to the E. trifoliorum (Wallr.) U. Braun complex in E. sect. Microsphaera and is distinguished from other species in the group by both anamorph and teleomorph characters. The British collection clearly shows that conidia are narrower, its chasmothecial appendages are not strongly directional and have frequently branched apices that agree well with a Romanian mildew labelled E. rayssiae (Mayor) U. Braun & S. Takam. on B. australis (Eliade, 1990). Since E. rayssiae is restricted to Spartium junceum, the Romanian mildew was assigned to E. baptisiae by Braun & Cook (2012). However, this material was unavailable for re-examination by Braun et al. (2010) and Braun & Cook (2012) and so details