First report of Plasmopara sphagneticolae causing downy mildew on bhringraj (Eclipta alba) in India

Abstract

Bhringraj (false daisy, family Asteraceae) is a valuable herb commonly used in ayurvedic and traditional medicine in the Indian subcontinent (Jahan et al., 2014); the leaf extract has antibacterial, antifungal and antiviral activity (Dalal et al., 2010). In experimental plots of false daisy at the ICAR-Indian Institute of Horticultural Research in Bengaluru, Karnataka, India, symptoms of severe downy growth were observed in September 2022 (post monsoon season). In the initial stage of infection, leaves showed yellowing and necrosis on adaxial surfaces and sparse mycelia growth was observed on the abaxial surface of older leaves. Later, thick white mycelial growth was observed on the lower side of young leaves (Figure 1). As the disease progressed, leaves dried and turned brown, then blackened, leading to 80–100 % loss in yield. One infected leaf was collected from each of three plants for further examination and pathogen identification. Microscopic observation revealed downy growth on the abaxial surface of leaves with numerous sporangia. The mycelium was hyaline, aseptate and branched, 5–15 μm in diameter. Sporangiophores were slender with monopodial branching at right angles and with a slight bulbous base. The sporangia were hyaline, smooth, globose to ellipsoidal with apical papilla and basal hilum on three sterigma, measuring 18–30 × 14–20 µm (Figure 2). Based on these characteristics, the pathogen was identified as Plasmopara sphagneticolae (Davis et al., 2020; Wang et al., 2021). To confirm the identity of the pathogen, total DNA was isolated from two of the samples (PSB-1 and PSB-2) using the CTAB method and the COX 2 region was amplified using primers Cox2F and Cox2R (Hudspeth et al., 2000). Analysis of the sequences using BLASTn showed the highest identity to P. sphagneticolae isolates BRIP61010 and BPI919166 (Genbank Accession Nos. KM085175 and MT292790, respectively). The consensus sequences were submitted to Genbank (OQ200475 and OQ200476). Phylogenetic analysis showed that the sequences of PSB-1 and PSB-2 clustered closely with isolates of P. sphagneticolae. Both P. sphagneticolae and its sister species P. halstedii cause downy mildew on many crops in the Asteraceae. Plasmopara sphagneticolae was first identified in Australia infecting Sphagenticola trilobata (McTaggart et al., 2015), and was then found on Lipochaeta integrifolia in Hawaii (Davis et al., 2020) and Taiwan (Wang et al., 2021). This is the first report of P. sphagneticolae in India and the first record of this pathogen causing downy mildew on bhringraj. The authors are grateful to the Director, Indian Institute of Horticultural Research, Bangalore. Financial assistance to K. Hima Bindu from NMPB, India (Project No; Z. 18017) is acknowledged.