First Conclusive Report of an Erysiphe sp. Causing Powdery Mildew on Moth Vine (Araujia sericifera) in Australia and Worldwide

Abstract

Moth vine (Araujia sericifera, Apocynaceae), native to South America, has been introduced as an ornamental plant to many parts of the world and has become an invasive environmental weed in some countries such as Australia and New Zealand. In May 2017, heavy powdery mildew infection was observed on moth vine in Ravensbourne National Park, Queensland, and also at two sites in Sydney, NSW, Australia. Powdery mildew mycelium covered large parts of the upper surfaces of older leaves with other parts of the plants not exhibiting disease symptoms. Conidiophores consisted of foot-cells, somewhat curved to sinuous at the base, (19-) 21 to 33 × 7 to 10 µm, followed by one or two cells of approximately the same length, each 14 to 25 (-29) × 7 to 10 µm, and conidia produced singly. Conidia were subcylindrical or ellipsoid-ovoid, (27-) 29 to 41 × 16 to 21 μm, and produced germ tubes apically or subapically, with lobed ends. Hyphal appressoria were lobed to multilobed. The sexual morph was not found. Based on these characteristics, the powdery mildew fungus was identified as an Erysiphe sp. (Braun and Cook 2012). Specimens from the three collection sites were deposited in the Queensland Plant Pathology Herbarium as BRIP 66127-66129. The rDNA ITS region was sequenced for each specimen as described by Kiss et al. (2018). The 552-bp long ITS sequences (deposited in GenBank as MG551719–21) were identical to several others including LC009920, LC010042, and LC010073, available in GenBank as E. aquilegiae, E. knautiae, and E. euphorbiae, respectively. Pathogenicity tests were done twice on small asymptomatic A. sericifera plants collected in Queensland. In each test, six plants at the 6 to 10 leaf stage were removed from the soil and maintained with their roots in water. Three plants were inoculated by dusting conidia from field collected plants onto the leaves; the three other plants were not inoculated. Plants were maintained separately under transparent plastic covers for 3 weeks. The first symptoms appeared on the inoculated leaves 7 to 10 days post inoculation. All the noninoculated plants remained asymptomatic. Light microscopy and ITS sequencing confirmed the identity of the pathogen. To our knowledge, this is the first report of a conclusively identified Erysiphe sp. infecting A. sericifera globally. So far, powdery mildew has been reported on A. sericifera twice: an unidentified Oidium sp. was listed on this weed in Australia (Simmonds 1966), and Oidium cf. insolitum was reported on A. sericifera in Argentina (Waipara et al. 2006). This latter fungus is very different from any Erysiphe species (Braun and Cook 2012). Several powdery mildews infecting diverse plant species share ITS sequences that are 97 to 99% similar, or identical, to that of E. aquilegiae (Takamatsu et al. 2015), just like the fungus reported here. This group needs a taxonomic re-evaluation (Takamatsu et al. 2015). Also, additional studies are needed to determine the host range of this Erysiphe sp. and its impact on A. sericifera populations.