Powdery Mildew of Taraxacum mongolicum Caused by Podosphaera macrospora in China

Taraxacum mongolicum is a perennial herbaceous plant belonging to the Asteraceae family. It is widely planted in China for its use as a vegetable plant as well as its commercial and medicinal values. In August 2017, approximately 33.5% of the dandelion plants in a 3-ha commercial field in Bijie, Guizhou Province of China were symptomatic of a powdery mildew infection. Observed symptoms included circular to irregular shaped white patches of fungal colonies on the petioles and adaxial surface of the leaves. Colonies on leaves commonly appeared white from April to October and dissipated afterward. Irregular spot and/or necrotic symptoms with violet to wine red surfaces were also frequently observed on the surface of overwintered leaves. Severe infections caused premature senescence of the leaves. Chasmothecia were observed only on leaves that had senesced and died. A voucher specimen (PG-1) was deposited in the laboratory of Guizhou Academy of Tobacco Science. Morphological characters of the pathogen were examined microscopically. Conidiophores were erect, straight, measured 80 to 220 × 10 to 15 μm, and were composed of a subcylindrical foot cell followed by one to three short cells. Indistinct appressoria were formed on ectophytic mycelia. Conidia were ellipsoid, doliiform to limoniform, and ranged in size from 26 to 40 × 15 to 20 μm. These morphological characteristics were consistent with those of the anamorph of Podosphaera macrospora U. Braun & V. Kumm (sexual stage name of Sphaerotheca alpina) (Ale-Agha et al. 2008). To confirm identification, primers ITS1 and ITS4 were used to amplify the ITS1-5.8s-ITS2 region (ITS) of rDNA of PG-1, and the polymerase chain reaction product was sequenced (White et al. 1990). The ITS sequence was deposited to GenBank as MG171179.1 and was nearly identical to the ITS sequences of several P. macrospora accessions in GenBank (99.6% similarity to KP966080.1 and KP966082.1). Pathogenicity of the fungus was confirmed in the greenhouse by performing Koch’s postulates as follows. Six 1-month-old T. mongolicum plants were inoculated by gently dusting conidia from an infected plant onto the leaves. Six plants not inoculated in the greenhouse were regarded as the control. After inoculation, plants were maintained at 25°C, 100 lux, >70% relative humidity, and 16-h light/day and were monitored for symptoms. The experiments were repeated twice. After 9 days, inoculated plants developed powdery mildew symptoms similar to those on diseased plants in the field, whereas control plants remained symptomless and free of powdery mildew signs. P. macrospora is a cosmopolitan powdery mildew fungus, parasitic on numerous plant species (Ale-Agha et al. 2008). It also has been reported on Saxifraga tricuspidata in Canada and on Saxifraga odontoloma in North America (Braun and Cook 2012). Previously, powdery mildew on T. mongolicum in China has been reported to be caused by Sphaerotheca fuliginea (Amano 1986) and Sphaerotheca fusca (Zhuang 2005). To the best of our knowledge, this is the first report of P. macrospora causing powdery mildew on T. mongolicum in Guizhou Province of China. Owing to the abundant and prolific production of conidia and the rapid, aerial dissemination of this inoculum, P. macrospora has the potential to cause significant economic losses to and become a limiting factor in the production of dandelion in Guizhou Province of China.