First report of powdery mildew (Oidium sp.) on greater celandine (Chelidonium majus)

Since 2004, symptoms of powdery mildew infection were observed on greater celandine (Chelidonium majus), a common weed of shaded habitats, in several places in Hungary. Sporulating mildew mycelia covered large areas of the upper, and rarely the lower, leaf surfaces (Fig. 1), in addition to the petioles and unripe capsular fruits.

The pathogen was identified as a powdery mildew anamorph belonging to the genus Oidium subgenus Pseudoidium. Conidia were cylindrical to doliiform, measured 25-48 x 10-20 µm and were produced singly on 80-150 µm long conidiophores consisting of a foot-cell measuring 20-40 x 7-10 µm, followed by two or three shorter cells (Fig. 2). During conidial germination, short germ tubes were produced apically terminating in lobed appressoria. Hyphal appressoria were lobed or multi-lobed (Fig. 3). The teleomorph stage was not found. Specimens were deposited in the herbarium of Martin Luther University, Halle, Germany (HAL 1842 F).

No well-founded report of powdery mildew on C. majus is known worldwide (Braun, 1987; 1995). To contribute to a more precise identification of the novel powdery mildew anamorph, DNA was extracted from its mycelium, the internal transcribed spacer (ITS) region of the ribosomal DNA was PCR-amplified, cloned and sequenced as described in Landwehr et al. (2002) for Glomus. The ITS sequence (GenBank accession number DQ665673) was 99% similar to those determined in O. neolycopersici infecting tomato in Europe and North America (Kiss et al., 2005). However, our cross-inoculation tests using conidia of O. neolycopersici and those of the C. majus mildew showed that these pathogens could infect their host plants only. The tests were repeated three times in two greenhouses using healthy potted C. majus and tomato plants kept in isolation after artificial inoculations. Non-inoculated plants served as controls. These results confirmed the pathogenicity of the Oidium sp. on C. majus.