First Report of Powdery Mildew Elicited by Podosphaera fusca (Synonym Podosphaera xanthii) on Euryops pectinatus in...

Euryops pectinatus Cass. is an evergreen shrub that is planted extensively in Spain for landscape use. In 2007, powdery mildew outbreaks were observed on E. pectinatus in several nurseries located in the Axarquia area (Malaga, southern Spain). Fungal growth appeared as typical, white, powdery mildew colonies that were restricted to upper leaf surfaces. Initially, individual colonies were small and nearly circular, but later enlarged and coalesced to cover the whole leaf surface. With progress of the disease, all green parts (leaves, stems, and petioles) were covered with a white mycelium. Newly developed leaves especially became rapidly infected. Diseased leaves ultimately dried up and senesced, making nursery plants aesthetically unattractive and unsaleable. Conidiophores were erect, had crenate edge lines, cylindrical foot cells that measured 37.5 to 45.0 × 10.0 to 12.5 μm, and were followed by one to three shorter cells. Conidia were hyaline, ellipsoid to ovoid, borne in chains, and measured 27.5 to 35.0 × 12.5 to 17.5 μm. Conidial length-to-width ratios ranged from 1.6 to 2.4. Conidia possessed conspicuous fibrosin bodies and from their sides produced short germ tubes. No chasmothecia were found. The nuclear rDNA internal transcribed spacer (ITS) regions were amplified by PCR and sequenced (GenBank Accession No. EU424056). On the basis of morphological characteristics of the imperfect state and ITS sequence data, this powdery mildew was identified as Podosphaera fusca (Fr.) U. Braun & N. Shishkoff (1), this isolate belonging to ITS haplotype 15 (group III) (3); this group is considered a separate species, P. xanthii (Castagne) U. Braun & N. Shishkoff by some authors (2). Pathogenicity was confirmed by gently pressing diseased leaves onto leaves of healthy E. pectinatus plants. Plants were incubated in a growth chamber at 25°C, and after 14 days, powdery mildew colonies developed. A similar disease of E. pectinatus was observed in 1999 in California (4). P. fusca parasitizes a large number of asteraceous species including field marigold (Calendula arvensis) and fleabane (Erigeron sp.) weeds, as previously reported in the same area, and ornamentals such as Calendula officinalis, Chrysanthemum spp., and Gerbera spp., which are also grown in the same nurseries and frequently attacked by powdery mildew. References: (1) U. Braun and S. Takamatsu. Schlechtendalia 4:1, 2000. (2) U. Braun et al. Schlechtendalia 7:45, 2001. (3) T. Hirata et al. Can. J. Bot. 78:1521, 2000. (4) G. S. Saenz et al. Plant Dis. 84:1048, 2000.