First Report of Powdery Mildew Caused by Golovinomyces bolayi on Torenia fournieri in Korea

Abstract

Torenia fournieri E. Fourn., known as torenia or clown flower, is an annual plant in the Linderniaceae. In October 2014, typical powdery mildew symptoms were observed on torenia plants of an unknown cultivar in a private garden in Seongnam (37°26′50″N; 127°10′02″E), Korea. In August 2019, dozens of torenia of an unknown cultivar planted in shady borders of a roadside garden in Seoul (37°34′46″N; 127°01′25″E), Korea, were found to be infected with a powdery mildew with 100% disease incidence. Signs first appeared as circular to irregular white patches, which subsequently coalesced to develop into abundant hyphal growth on both sides of the leaves and also on flower stalks and calyxes. As the disease developed, affected leaves became wilted. Two voucher specimens have been deposited in the Korea University Herbarium (KUS-F28526 and F31142). Hyphal appressoria were nipple shaped or indistinct. Conidiophores were cylindrical, 110 to 240 × 10 to 12 µm, and had cylindrical foot cells, somewhat curved at the base, and 50 to 100 μm long, followed by two short cells and two to six immature conidia in chains with a sinuate outline. Conidia were hyaline, ellipsoid to barrel shaped, 26 to 38 × 16 to 22 μm (length/width ratio = 1.4 to 2.0), and devoid of distinct fibrosin bodies. Germ tubes were perihilar on the conidia. These morphological characteristics are consistent with those of Golovinomyces bolayi (Schwein.) U. Braun & R.T.A. Cook (Braun and Cook 2012). No chasmothecia were found during the growing season. To confirm identification, the internal transcribed spacer region of rDNA of KUS-F28526 was amplified with primers ITS1/ITS4 and sequenced. The resulting sequence was deposited in GenBank (accession no. MN420661). A GenBank BLAST search revealed a sequence similarity of 99.7% to G. bolayi ex Torenia fournieri (DQ538345). A pathogenicity test was performed by gently dusting conidia onto leaves of five healthy potted plants bought in a flower market. Five noninoculated plants served as controls. Powdery mildew colonies developed on all inoculated leaves after 7 to 10 days, whereas the control plants remained symptomless. The fungus present on the inoculated leaves was identical morphologically to that observed on the originally diseased leaves. The pathogenicity test was performed twice with similar results. Previously, G. bolayi on T. fournieri was known from Hungary (Braun et al. 2019). Two previous records of Oidium sp. on T. fournieri from Japan (Wada and Hirata 1977, figure plates uploaded on November 2008 and shown at www.naro.affrc.go.jp) and the United States (Holcomb 1999) are likely to be Golovinomyces, probably G. bolayi. To our knowledge, this is the first report of powdery mildew caused by G. bolayi on T. fournieri in Korea. Because powdery mildew infection detracts from the beauty and health of green leaves, control measures should be developed.