First Report of Powdery Mildew Caused by Podosphaera xanthii on Glandularia tenera in China.

Abstract

Glandularia tenera (syn. Verbena tenera) is an herbaceous perennial ornamental plant used in gardens as an edging plant with beautiful white, red, or purple flowers. In autumn 2020 and 2021, severe powdery mildew infection was observed on G. tenera cultivar Xianghe in Renming Botanical garden in Shangqiu, Henan province, China (34.4568° N, 115.6640° E). Approximately 80% of leaves on each plant were symptomatic, and about 90% of the plants were infected. Powdery mildew colonies appeared as white spots on the adaxial surface of the leaves and stems of the plants in the initial infection stage. Later, mycelial growth was amphigenous, thick, forming irregular white patches, and effused to cover the whole leaf surface. Finally, leaves turned yellow and senescence occurred. Samples of symptomatic leaves were stained with trypan blue and examined under a Leica DM2500 microscope. Microscopic observations showed that conidia on infected leaves were hyaline and ellipsoid to oval with fibrosin bodies, measured 25 to 37 × 14 to 23 μm with a length/width ratio of 1.4 to 2.0. Conidiophores were unbranched, straight, 80 to 210× 10 to 14 µm in size, and produced two to five immature conidia in chains. Foot cells of conidiophores were cylindrical with slight constrictions at basal septa, and followed by one to three short cells. Fungal hyphae were septate, branched, and flexuous to straight and 4 to 7µm wide with indistinct to slightly nipple-shaped appressoria. Chasmothecia were not observed. These morphological characteristics were identical with the previous description of Podosphaera xanthii (Castagne) U. Braun & Shishkoff (Braun and Cook 2012). To confirm the identification, the sequence of ITS1-5.8s-ITS2 region of rDNA for the isolate SQVT was amplified from conidia collected from infected leaves with universal primers ITS1 and ITS4, sequenced and analyzed using the BLASTn search of GenBank. Amplicon was 565 bp (OM293967) and showed 99.82% similarity with sequence of P. xanthii from Eclipta prostrate (MT260063) in China (Xu et al. 2020), from Youngia denticulate (AB040351) in Japan (Hirata et al. 2000), and 99.65% with sequence of P. xanthii from V. brasiliensis in Korea (Cho et al. 2014). The domains D1 and D2 of the 28S rDNA for the isolate SQVTPX-1 was amplified with primer NL1/NL4. Amplicon was 613 bp (ON259308) and showed 100% similarity with sequence of P. xanthii from V. brasiliensis (AB936277) (Meeboon and Takamatsu, 2015). Pathogenicity tests were conducted by gently pressing the infected leaves onto leaves of five healthy G. tenera cultivar Xianghe plants. Five non-inoculated plants served as controls. Plants were maintained in a greenhouse at 25 ± 2°C. Eight days after inoculation, symptoms similar to those observed under natural conditions developed on the inoculated leaves of G. tenera plants, whereas the control plants remained symptomless. The fungus on inoculated leaves was morphologically identical to that first observed in the field. P. xanthii is a cosmopolitan powdery mildew fungus, parasitic on numerous plant species, especially Cucurbitaceae and Compositae plants. The pathogen has been reported infecting V. bonariensis (Hong et al. 2021), V. × hybrida in China (Zhuang 2005), and V. brasiliensis in Korea (Cho et al. 2014). Interestingly, G. tenera plants infected by P. xanthii were adjacent with V. × hybrida plants infected by P. xanthii in Renming Botanical garden. Incidence of P. xanthii on G. tenera add information on pathogen's host range and help us develop comprehensive survey and effective management of the disease. To our knowledge, this is the first report of P. xanthii on G. tenera in China (Farr and Rossman 2021). Braun, U., and Cook, R. T. A. 2012. Taxonomic Manual of the Erysiphales (Powdery Mildews). CBS Biodiversity Series No. 11. CBS, Utrecht, the Netherlands. Cho, S. E., et al. 2014. Plant Dis. 98: 1159. Farr, D. F., and Rossman, A. Y. 2021. Fungal Databases, Syst. Mycol. Microbiol. Lab., ARS, USDA. Hong, Q. Q., et al. 2021. Plant Dis. 105: 3297. Hirata, T., et al. 2000. Can. J. Bot. 78: 1521-1530. Meeboon, J. and Takamatsu, S. 2015. Mycoscience . 56: 243-251. Xu, D. D., et al. 2020. Plant Dis. 104: 3263. Zhuang, W. Y. 2005. Fungi of northwestern China. Mycotaxon, Ltd., Ithaca, NY.