First Record of Erysiphe elevata on Plumeria rubra in Taiwan

Abstract

Plumeria rubra L., native to tropical Central America, is a widely cultivated ornamental woody plant. In two municipalities in northern Taiwan we observed brown discolorations irregularly following veins on the abaxial surface of green or yellowing lower (old) leaves of two outdoor plants of P. rubra. The upper side did not show symptoms or diffuse yellow spots. About 80% of leaves exhibited superficial mycelia and conidiophores typical of a powdery mildew. Specimens collected from a garden plant in Taipei City on 31 May 2018, and from a pot plant in Taoyuan City, Zhongli District, on 7 June 2018, were deposited under the collection numbers R. Kirschner 4647 and 4651 in the National Museum of Natural Science, Taichung, Taiwan (TNM). Fresh specimens were mounted in approximately 10% KOH and observed with light microscopy. The mycelium was hypophyllous, forming irregular patches. Hyphae were hyaline, walls thin, smooth, 3 to 8 μm wide, with hyphal appressoria single or opposite in pairs, nipple shaped or lobed. Conidiophores were 35 to 76 μm long, 7 to 9 μm wide, erect, straight or curved at the base, mostly three-celled, occasionally one- or two-celled. Foot cells were rough at the base, 24 to 35 μm long, 6 to 8 μm wide, with basal septum mostly even with the surface of the supporting hypha. Conidia were solitary, ellipsoidal to cylindrical, smooth, 28 to 35 μm long, 13 to 16 μm wide, and germination was not observed. The specimens were compared with a specimen (R. Kirschner 1945, TNM) of E. elevata on Catalpa sp. (Bignoniaceae), collected in Germany, Frankfurt am Main, Bergen-Enkheim, 18 July 2004. The anamorphic specimens from Catalpa sp. in Germany and P. rubra from Taiwan were morphologically indistinguishable. The internal transcribed spacer (ITS) region of rDNA of both Taiwanese specimens was amplified with the primers ITS1F/ITS4 and sequenced. The 660-bp-long ITS sequences of R. Kirschner et al. 4647 (GenBank MH507182) and 4651 (GenBank MH507183) were 100% identical to each other and 99 to 100% identical to the 10 available sequences of E. elevata in GenBank (Meeboon and Takamatsu 2017), whereas the similarity was 98% or lower for other species. Erysiphe sp. (labeled as E. elevata in GenBank under MF092833) recorded on Plumeria pudica by Suarez et al. (2017) had a similarity of 97% (582/597 bp). The sequence MF092833 contained seven unclear positions (“N”), indicating technically insufficient quality. Because of the high similarity of sequences and anamorph morphology, the specimens on P. rubra from Taiwan are conspecific with E. elevata. It seems to be limited to the lower side of the older leaves, where it forms inconspicuous mycelia so that the brown scribble-like discolorations along the leaf veins are more diagnostic symptoms than the mycelial colonies. Powdery mildew of Plumeria leaves is often reported anecdotally, but confirmed reports are lacking. Recently, Suarez et al. (2017) identified the causative agent as the genus Erysiphe on P. pudica in the United States. Because P. rubra was introduced to Taiwan in the 17th century and the first hybrid in 1967 (Hsueh and Yang 2009), the pathogen may have gone unnoticed for a long time. According to our observation, however, the powdery mildew appears to be less frequent than the recently reported rust Coleosporium plumeriae (Chung et al. 2006) and might have been recently introduced. The fungus may be involved in an accelerated leaf-fall, particularly in connection with additional stress factors such as drought or coinfection with the rust fungus.