Y First Report of Powdery Mildew Caused by Golovinomyces monardae on Scarlet Beebalm (Monarda didyma) in China

Scarlet Beebalm (Monarda didyma) is a perennial ornamental flowering plant in the mint family, Labiatae. Due to low-maintenance, and a long blooming period, it is commonly cultivated in gardens as an ornamental plant in China. In May 2021, a disease was observed on the leaves of a capitals beebalm plant in a Ten Mile Flower Sea in Xiayi county (116°13'8″E, 34°14'45″N), Henan province of China. Symptoms first appeared as nearly circular, small, white, powdery mildew-like spots on the leaves which gradually expand, covering the entire leaves. The lesions spread from the lower leaves to the upper canopy, and the stems were also covered by white mycelia. In severe cases, early defoliation occured. About 30% plants were affected. Representative voucher specimens (SQNUMd04, SQNUDn4) were deposited in the herbarium of Shangqiu Normal University (SQNU), Shangqiu, China. Conidiophores (n = 30) were cylindrical, 92.0 to 142.2 µm long and 10.8 to 14.2 µm wide, and produced 5 to 7 immature conidia in a chain. Foot-cells of conidiophores were mostly curved at the base. Conidia (n = 30) were hyaline, ellipsoid, 23.3 to 29.8 μm (avg. 26.6 μm) long, and 11.2 to 16.9 μm (avg. 14.5μm) width, a length/width ratio of 1.5 to 2.1, and germ tubes were produced at the perihilar position. No chasmothecia were observed. Based on morphological comparison using the description by Scholler et al. (2016) description the fungus was tentatively identified as Golovinomyces monardae (G.S. Nagy) M. Scholler, U. Braun & Anke Schmidt. For molecular identification, DNA was extracted from mycelia and conidia, which were collected by scraping symptomatic leaves.The ITS regions and LSU were amplified using primers ITS1/ITS4 (White et al. 1990) and NL1/NL4 (Horisawa et al. 2013). BLASTn analysis of the (MZ303741) and LSU (MZ305434) sequences showed 100% identity with those of G. monardae (AB307667, LC076800, LC076802, LC076808, and AB077691) reported on Monarda species (Matsuda et al. 2003; Takamatsu et al. 2013; Scholler et al. 2016). Pathogenicity tests were carried out by gently dusting conidia from infected leaves onto healthy leaves of five M. didyma plants and five non-inoculated plants used as controls. After 9 days, typical powdery mildew colonies started to appear on the inoculated leaves while control plants remained disease free. All plants were placed in a greenhouse without temperature and humidity control. Based on morphology, fungus was identified as the same as that used for inoculum, fulfilling Koch's postulates. Although G. monardae has been reported on various genera in the Labiatae and Verbenaceae (Farr and Rossman 2021), to our knowledge, this is the first report of G. monardae causing powdery mildew of Scarlet Beebalm(M. didyma) in China.