New Report of Black Leaf Spot Mold (Pseudocercospora fuligena) on Withania somnifera from India.

Abstract

Withania somnifera (family solanaceae) commonly known as ashwagandha and Indian ginseng, originated in India is one of the most powerful medicinal plants for more than 3,000 years (1). It is commercially cultivated for its roots, a natural rich source of glycowithanolides, tannins, potassium nitrate, etc., which are an anti-inflammatory, anti-tumor, anti-oxidant, anti-ulcer, and regulator of the nervous system and sleep (2). During the monsoon of July 2011, black spots on the leaves of infected plants were observed in the ashwagandha growing Lucknow, Raibareilly, and adjoining areas of Uttar Pradesh province with 10 to 20% disease incidence. Early stage of disease were characterized by the presence of light chlorotic spots on both sides of old leaves that later turned into dark black spots resulting in early defoliation. About 27 samples were collected from different locations of the fields for isolation of the causal organism and microscopic studies. Infected leaves were cut into small pieces, surface sterilized with 1% sodium hypochlorite for 1 min, rinsed thrice with sterilized distilled water, and placed onto potato dextrose agar (PDA) plates. After 21 days of dark incubation at 25°C, 8- to 10-mm grayish-brown colonies were observed. Microscopic studies at early and mature stages of infection showed production of conidia in conidiophores. Conidiophores were mostly 5 to 9, few dense pale brown, simple unbranched, septate, geniculate and 14 to 55 × 3 to 5.5 μm. Conidia were subhyline, obclavate to cylindrical, some were straight to slightly curved, multiseptate, base long obconic to long obconically truncate, and 12 to 85 × 3.5 to 5 μm. On the basis of cultural and morphological studies, the pathogen was identified as Pseudocercospora fuligena (3). The pathogen identity was further confirmed at molecular level using universal primers ITS1/ITS4 through PCR (4). An amplification of the expected size (~550 bp) was generated, eluted from agarose gel by QIAquick gel extraction kit (Qiagen), cloned into pGEM-T Easy vector (Promega), sequenced, and deposited in GenBank (Accession No. KF881898). NCBI BLASTn showed 99% identity with P. fuligena (GU214675) strain CPC 12296, isolated from Lycopersicon sp. Pathogenicity test was carried out on 10 plants of W. somnifera cv. Poshita through two approaches, one using mycelia from culture and another using spore suspension from naturally infected leaves. In the first approach, fungal mycelia were applied onto the healthy ashwagandha leaves, whereas in the second approach, infected leaves were washed with distilled water and spore suspension of 106 spores/ml was sprayed on healthy plants. Plants sprayed with sterilized distilled water served as controls. Inoculated plants were placed in a growth chamber at 28°C under 90% humidity for 3 days. After, pots were placed in the glasshouse at 27 ± 2°C with 70 to 80% humidity for 21 days. Initial symptoms appeared on the 7th day while typical symptoms appeared on all the inoculated plants after 12 to 17 days. Control plants remained free of infection. Re-isolation of the pathogen on PDA fulfilled Koch's postulates. Black leaf mold caused by P. fuligena has been reported on tomato (5). This is the first report of black leaf mould caused by P. fuligena on W. somnifera from India. P. fuligena has the potential to reduce yield of W. somnifera.