First Report of Stem Rot Caused by Athelia rolfsii on Curry Leaf Tree (Murraya koenigii) in Tripura, India.

Abstract

Murraya koenigii is an important medicinal plant of India and commonly known as curry leaf tree grown in tropical and subtropical regions. The leaves of curry tree are used as a herb due to the presence of following important active constituent bismahanine, murrayanine, murrayafoline-A, bi-koeniquinone-A, murrayazolidine etc. (Jain et al. 2017). During mid-July 2019, stem rot disease symptoms were observed on curry leaf trees at the College of Agriculture, Lembucherra, Tripura (India). The disease symptoms consisted of rotting, wilting and blighting with disease incidence ranging from 8 to 10%. Initially, infected plants gradually withered and white mycelia mats appeared on the surface of the lower stem at the soil line. Infected stem samples were collected and surface was sterilized with 0.25% sodium hypochlorite for 1 min, washed thrice with sterilized distilled water and placed in Petri plates containing 2% water agar. After three days of incubation at 26°C, hyphae produced from plant bits were transferred into Petri plates containing potato dextrose agar. Ten isolates were collected from the diseases samples. Pure cultures were obtained as abundant, aerial and white mycelia with round to irregular sclerotia of 0.8 to 1.5 mm in diam. The sclerotia were initially white in color but later turned into brown color. The pathogen was identified as Athelia rolfsii based on morphology (Aycock 1966). To confirm the identification, the genomic DNA was extracted from a mycelia mat of the isolates using ZR fungal/Bacterial DNA miniprep kit (Irvine, CA) and the internal transcribed spacer (ITS) region of rDNA was amplified using the universal primers, ITS1 and ITS4 (White et al. 1990). A 550 bp PCR product was sequenced and showed 99% similarity with Athelia rolfsii isolate (GenBank accession MH854711).The generated sequence was submitted to GenBank (Accession MT535585). After identification of the pathogen a pot experiment was conducted to confirm the pathogenicity. Earthen pots (29 cm. diam.) were filled with sterilized soil and kept in a green house. Ten curry leaf plants (50 days old) were grown from seed in the separate pot were inoculated with 15-day old mycelia mats prepared in potato dextrose broth. The stem of each curry plant were artificially injured with the help of sterile blade and wrapped with moistened sterilized cotton containing the mycelial mat. Five curry leaf plants artificially injured and inoculated with sterilized distilled water were used as control. The Earthen pots with plants were individually covered with plastic bags and kept in the green house at 26°C for approximately 15 days. The inoculated plants started showing symptoms of stem rot six days after inoculation and started drying onward. The symptoms of stem rot on the inoculated plants were similar to those observed in the field. The fungus was re-isolated from the inoculated plants and A. rolfsii identification was confirmed based on morphology. No symptoms were observed on the control plants. The obtained culture was deposited in the Indian Type Culture Collection, Division of Plant Pathology, ICAR - Indian Agricultural Research Institute, New Delhi, India (ITC-8666). To the best of our knowledge this is the first report of stem rot disease of curry leaf plant caused by A. rolfsii in India and worldwide. Due to medicinal, flavour and aroma properties, it is regularly used in India. Curry leaf plant is regularly used as a medical herb in India and therefore this disease poses a significant risk to production.