First report of rhododendron powdery mildew on Rhododendron spp. in the Czech Republic

Since July 2003, rhododendron bushes have frequently been observed infected with powdery mildew in the Czech Republic (Olomouc, Brno, Ostrava, and Zlin). Characteristic powdery mildew symptoms were observed on varieties such as Rhododendron mollis (syn. Azalea mollis) and R. luteum affecting both upper and lower leaf surfaces, sometimes causing purpling of infected areas. Seed pods of these species were also affected (Fig. 1). On some varieties such as R. impeditum, R. nipponicum, R. racemosum, R. calostratum, R. catawbiense and R. smirnowii only seed pods were observed infected with the fungus.

White superficial colonies, with abundant sporulation, developed amphigenously on affected parts of the plant. Conidiophores were unbranched, cylindrical, 36-90 µm, composed of a twisted or kinked to flexuous basal cell and one to three cylindrical cells (Fig. 2A). Appressoria were mutilobed to moderately lobed, often in pairs. Conidia devoid of fibrosin bodies were oblong to cylindrical or somewhat doliiform, 20-34 x 12-17 µm, producing germ tubes at the basal end (Fig. 2B-D). Chasmothecia were not observed. Based on the anamorphic morphological features seen and previous reports of rhododendron powdery mildew in Europe, this Oidium subgen. Pseudoidium conforms to an Erysiphe sp., most probably E. azaleae (U. Braun) U. Braun & S. Takam. (Braun, 1987; Inman et al., 2000; Piatek, 2003; Shin & Mulenko, 2004). Pathogenicity was confirmed by gently pressing diseased leaves onto three, healthy R. mollis plants. Five non-inoculated plants served as controls. Inoculated and non-inoculated plants were maintained in a garden where temperature ranged between 18 and 30°C. After 12 days, typical powdery mildew symptoms appeared on the leaves of inoculated plants. No symptoms were observed on control plants.

Rhododendron powdery mildew, probably introduced from North America or Asia (Inman et al., 2000), has been recorded in England, Germany, Switzerland (Inman et al., 2000), and Poland (Piatek, 2003; Shin & Mulenko, 2004) over recent years. Its identification in the Czech Republic indicate that the fungus is rapidly spreading eastwards in Europe. Representative specimens are kept in SMK (Korea University) and in the Department of Botany collection at Palacky University in Olomouc.