Rediscovery of Erysiphe clandestina on elm (Ulmus) in Germany

Erysiphe clandestina (syn. Uncinula clandestina), the powdery mildew pathogen of elm, has until now been regarded as extinct in Germany since 1941 (Scholler, 1999; U. Braun, pers. comm. 2004). In October 2004, heavily-infected elm trees were observed in two parks and a riparian forest in the city of Leipzig (Province Saxony) and shortly thereafter at two localities within a riparian forest near Schkopau (Province Saxony-Anhalt). The thin, white mycelium was amphigenous, in four cases with abundant chasmothecia. At one locality near Schkopau the infection was sparser.

Conidiophores are erect and form solitary, ± cylindrical to ellipsoid conidia. The diameter of the chasmothecia ranges between ca. 80 and 120 µm. The appendages are equatorial, ± evenly distributed, with the uncate apex typical for the section Uncinula. The chasmothecia contain several (mostly 3 or 4) 2-spored asci, spore size ~ 20-30×10-20 µm. The morphology, as well as the host, are in accordance with Braun's descriptions (Braun, 1995). The park specimens are deposited at the herbarium of the State Museum of Natural History Görlitz (GLM) (accessions 53805 and 53848), while that of the Leipzig riparian forest in the herbarium Klenke, and those from Saxony-Anhalt in the herbarium Richter. In November 2004, a duplicate collection of GLM 53848 was made and presented to the Botanische Staatssammlung München for distribution in the exsiccata set “Microfungi exsiccati”.

Re-inoculation tests were not carried out, since beyond the dense coverage of the leaves with ascocarps of the elm-specific obligate biotrophic parasite, no further specific symptoms were observed.

Germany has been devoid of this fungus for more than sixty years. However, some recent findings are known for southern European countries (e.g. Italy, pers. comm. D. Triebel, Botanische Staatssammlung München). Two collections from Geneva, Switzerland, are deposited at the Herbarium of Halle/S. (HAL) and there are two undated records from an anonymous collector, det. Ellis, listed for Britain (Cooper, 2003). Recent observations are also known from central and southern Ukraine and from Georgia (Minter, undated).

Though powdery mildews do not usually pose a serious threat to trees, it would be desirable to follow the further development of Erysiphe clandestina, especially in view of the general disease situation of elm trees in Germany and neighbouring countries due to Ophiostoma novo-ulmi.