Spiraea japonica is an old-fashioned shrub widely grown in Italy in parks and gardens. In the summer of 2003, severe outbreaks of a previously unknown powdery mildew were observed in some parks and gardens in the city of Torino (northern Italy). Infected leaves became covered on both sides with a white mycelium. As the disease progressed, infected leaves turned reddish and eventually dropped prematurely. The presence of powdery mildew infections on leaves sometimes caused their distortion and growth reduction. Frequently, mycelium was observed also on the stem. Conidia were hyaline, ellipsoid, cylindrical, or dol iform, born in chains, measured 21.0 to 38.4 × 10.8 to 14.4 mm, and showed fibrosin bodies. Cleistothecia were not observed. The pathogen was identified as Oidium subgenus Fibroidium (1,2). Pathogenicity was confirmed by spraying leaves of healthy potted S. japonica plants with a conidial suspension (105 conidia per ml) prepared in sterile water from diseased leaves. Three plants were inoculated and three noninoculated plants served as control. The artificial inoculation was carried out twice. After artificial inoculation, plants were maintained in a growth chamber at 25°C. After 20 days, powdery mildew symptoms developed. Microsphaera alni and Podosphaera oxyacanthae were described as causal agents of powdery mildew on S. japonica in the United States (3), while Sphaerotheca spiraeae was considered the causal agent of a powdery mildew observed in Japan (4) and more recently in Poland.
References: (1) U. Braun and S. Takamatsu. Schlechtendalia 4:1, 2000. (2) R. T. A. Cook et al. Mycol. Res. 101:975, 1997. (3) P. Pirone. Diseases and Pests of Ornamental Plants. John Wiley and Sons, NY, 1978. (4) K. Sawada. Rev. Appl. Mycol. 31:577, 1952.
