First Report of Root Rot of Nepeta cataria Caused by Macrophomina phaseolina in India

Nepeta cataria (catnip) family Lamiaceae, is a perennial shrub distributed in Europe, Asia, and North America that is used as medicinal herb, as well as for flavor in confectionaries and beverages. It contains 4a-α,7-α,7a-β-nepetalactone, which is a rich essential oil that has proven antimicrobial activity against some foodborne pathogens (Zomorodian et al. 2012). In May 2017, symptoms of wilting were observed in the experimental fields at the Central Institute of Medicinal and Aromatic Plants in Lucknow, Uttar Pradesh, India. Roots of infected plants had dark brown necrotic lesions with black microsclerotia, and sloughing of the cortex led to total vascular crumple. Disease incidence at the field scale ranged from 20 to 30% (Mohanapriya et al. 2017). To isolate the pathogen, root samples from 24 symptomatic plants were cut into small pieces, surface sterilized with 1% sodium hypochlorite, rinsed three times with sterile distilled water, blotted with sterile filter paper, placed on potato dextrose agar (PDA) plates, and incubated at 25 ± 2°C for 5 days in the dark. Similar fungal colonies emerged from each root sample. The fungal colony was purified via single hyphae subculturing. The colonies appeared grayish-black after 4 days and then became darker with the production of microsclerotia after 10 days. For microscopic studies, fungal colonies were grown on PDA and water agar (WA) plates amended with or without 1% aniline/cotton blue. The hyphae were subhyaline to light brown and septate with microsclerotia ranging in size from 50 to 150 μm. Based on symptoms, colony morphology, size, and color of the microsclerotia, the pathogen was tentatively identified as Macrophomina phaseolina (Tassi) Goidanich (Kaur et al. 2012). The identification of the pathogen was confirmed by sequence analysis of internal transcribed spacer (ITS) region of rDNA using the universal primers ITS1 and ITS4 (White et al. 1990). Sequence data of the ITS region was submitted to NCBI GenBank (accessions no. MG965903), and BLASTn results showed 99% similarity with M. phaseolina isolated from Phaseolus vulgaris (accession no. KR012878). The pathogenicity testing was conducted under glasshouse conditions on 15, 3-week-old healthy plants. Ten healthy plants were inoculated with a 7-day-old, 5-mm fungal disc on WA, buried 1.5 cm deep, approximately 2 cm from the base. Five uninoculated plants served as the controls. Symptoms of wilting were observed after 2 to 3 weeks on all inoculated plants in the glass house, whereas uninoculated plants remained symptomless. The fungus was reisolated and identified as described above, thus fulfilling Koch’s postulate; no pathogen was isolated from control plants. To the best of our knowledge, this is the first report of root rot of N. cataria caused by M. phaseolina in India as well as globally. M. phaseolina (Tassi) Goidanich is a common pathogen reported to infect over 500 plant species (Moye et al. 2017). The disease appears to favor long periods of drought and hot temperatures, interspersed with rain showers. The screening of N. cataria germplasm for disease-resistant varieties is recommended.