First report of Neodeightonia subglobosa causing leaf rot disease of betel leaf (Piper betle) in India

Abstract

Betel vine (Piper betle) is one of the oldest agricultural crops that is still farmed commercially in India, Malaysia, Sri Lanka, Thailand, Taiwan and other southeast Asian nations (Punuri et al., 2012). Its leaves are used for chewing and for flavouring in cooking. In June to September 2022 betel leaves harvested in the districts of East Khasi Hills and Ri Bhoi in the state of Meghalaya, India were affected by a leaf rot disease. The leaf rot was more severe during rainy season than the dry season. Infection began with circular, dark brown spots which expanded in size (Figure 1), becoming wet and rotten under continuous high humid conditions causing defoliation. Significant losses (20-100%) of crops occurred in the field and during storage and transportation. Diseased leaves were surface sterilised for 60 seconds with sodium hypochlorite (0.5% v/v) and then rinsed three times with sterile distilled water. After placing the diseased leaves on potato dextrose agar, they were incubated at 28°C for 5 to 15 days. The colony morphology of the fungal isolates initially produced white mycelium, developing into an olivaceous-black colour after 10 to 15 days (Figure 2). Isolate NS1 had pseudoparaphyses that were hyphae-like, septate constricted at the septa (Figure 3a). Conidiogenous cells holoblastic, simple, hyaline (Figure 3b). Conidia spherical to ellipsoidal, pale (9−12 μm) (Figure 3c) as described by Punithalinga (1969). The identification of the isolated fungus was confirmed by sequence analysis using ITS1 and ITS4 primers (White et al., 1990) (GenBank Accession No. OP363241). BLAST analysis showed that the sequence had >99% identity with Neodeightonia subglobosa (KF66199.3). To confirm pathogenicity, the isolate was grown on potato dextrose broth at 28°C for five days, then inoculated onto healthy betel leaves using pin-prick inoculation with 10 μl spore suspensions (1 × 105 spores/ml) of the pathogen. Three days after inoculation, leaf spot symptoms were observed on wounded betel leaves., A fungus was re-isolated from each of the inoculated betel leaves which was morphologically identical to the inolculated isolate (Sang et al., 2014) the fulfilling Koch's postulates (Figure 4). No symptoms were observed on the control plant. Neodeightonia subglobosa has been reported as a pathogen on bamboo (Bengyella et al., 2015) and other species of Neodeightonia have been recorded as plant pathogens, for example N. phoenicum on Phoenix spp. in Greece (Zhang & Song, 2022). To the best of our knowledge, this is the first report of Neodeightonia subglobosa as a fungal pathogen of betel vine (Piper betle). The authors thank the Head, Department of Botany, North-Eastern Hill University for providing laboratory facilities. The first author is also grateful to the Non-Net Fellowship, New Delhi, for financial support in the form of a research fellowship.