Mandevilla splendens (Hook.) Woodson is a tropical plant belonging to the Apocynaceae family and grown in Italy as an ornamental. It is generally potted and used to create small barriers on terraces and gardens. During February 2003, severe outbreaks of a previously unknown powdery mildew were observed in a commercial glasshouse located at Albenga (northern Italy) where 30% of the plants were affected by the disease. Infected young leaves were covered on both sides with white mycelia. Mycelia were more evident on the lower surface of older leaves. As the disease progressed, infected leaves turned reddish and eventually became yellow and died. Powdery mildew infections sometimes cause leaves to distort and have reduced growth. Conidia were hyaline, ellipsoid or cylindrical, sometimes appeared to adhere in chains, measured 9 to 15 × 14 to 28 μm (average 12 × 21 μm), and did not show fibrosin bodies. Foot cell was cylindric and appressorium lobed. Cleistothecia were not observed. The pathogen was identified as Oidium sp. subgenus Pseudoidium (1,2,3). Pathogenicity was confirmed by gently pressing diseased leaves onto young leaves of healthy, 1-year-old M. splendens plants grown in 3.5 liter pots. Three plants were inoculated, while three noninoculated plants served as controls. After inoculation, plants were maintained in a growth chamber at 18°C (12-h light, relative humidity >75%). After 60 days, powdery mildew symptoms were observed on inoculated plants. Noninoculated plants remained healthy. The pathogenicity test was carried out twice. To our knowledge, this is the first report of powdery mildew on M. splendens in Italy, as well as in the world. Specimens of this disease are available at the DIVAPRA Collection at the University of Torino.
References: (1) R. Belanger et al., eds. The powdery mildew A comprehensive treatise. The American Phytopathological Society, St Paul, MN, 2002. (2) U. Braun. Nova Hedwigia, 89:700, 1987. (3) R. T. A. Cook et al. Mycol. Res. 101:975, 1997.