Sweet basil (Ocimum basilicum), a member of the Lamiaceae, is used as an ornamental as well as a culinary herb. It is a rich source of the phenolic compound methyl chavicol and is used as a traditional medicinal plant in India, where the crop is grown on ~2,500 ha annually (4). The species is native to India, where it has been cultivated for >5,000 years. During the rainy season, August of 2013, a severe leaf blight was observed on 30- to 45-day-old sweet basil plants in experimental fields (approximately 5 ha) at the CSIR-CIMAP and adjoining areas in Lucknow. Initial symptoms comprised small, irregular, necrotic lesions that coalesced into a leaf blight. Infected parts of the leaves turned black during wet and humid conditions. The incidence of symptoms ranged from 20 to 30%. Infected leaf samples were cut into small pieces and surface-sterilized with 1% sodium hypochlorite for 1 min, followed by two rinses in sterilized, distilled water. The leaf pieces were then blotted dry with sterilized filter paper, placed onto potato dextrose agar (PDA), and incubated at 28°C for 3 to 5 days. Blackish-brown fungal colonies developed. Microscopic examination revealed the presence of brown conidiophores that were cylindrical, septate, unbranched, and straight or geniculate near the apex. Conidia were three-septate, mostly curved at the third cell from the base, which was usually larger and darker than the other cells; intermediate cells were brown or dark brown; terminal cells were subhyaline or pale brown and 16 to 23.5 × 8.5 to 11.5 μm (the average size of 100 conidia was 19.9 × 10.18 μm). On the basis of these characteristics, the fungus was identified as Cochliobolus lunatus (anamorph Curvularia lunata (Wakk.) Boedijin) (1,2). The identification was confirmed by sequencing the internal spacer (ITS) region of ribosomal DNA (rDNA). Genomic DNA was extracted from five fungal isolates, using the 5 Prime Archive Pure DNA Cell/Tissue kit, and subjected to a polymerase chain reaction (PCR) assay with the universal primers ITS1 and ITS4 (5). The amplified product was cloned and sequenced. An NCBI-BLASTn search showed greatest homology (98% similarity) with the ITS sequence of C. lunatus (GenBank Accession No. DQ836800). The sequence was deposited in Genbank (KM272001). A pathogenicity test was carried out using 10, 30-day-old sweet basil (cv. CIM Soumya) plants in pots, by spraying a spore suspension (105 spores/ml) onto the leaves of each plant. Five plants treated similarly with sterilized, distilled water served as a control treatment. The plants were kept at 27 ± 2°C and 85 ± 3% RH for 8 to 10 days. Small, irregular, necrotic lesions appeared after 4 days on all inoculated leaves, while leaves of control plants remained asymptomatic. Fungi re-isolated from inoculated leaves resembled C. lunatus on the basis of microscopic and sequence data, fulfilling Koch's postulates. The fungus was not re-isolated from the control plants. C. guatemalensis has been reported to cause a leaf spot on sweet basil in Korea (3). To our knowledge, this is the first report of a sweet basil leaf blight caused by C. lunatus in India. C. lunatus has the potential to reduce the yield of sweet basil.
References: (1) L. M. Liu et al. Plant Dis. 98:686, 2014. (2) D. S. Manamgoda et al. Fungal Divers. 56:131, 2012. (3) J. H. Park et al. Plant Dis. 96:580, 2012. (4) H. A. A. Taie et al. Not. Bot. Hort. Agrobot. Cluj. Napoca 38:119, 2010. (5) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, San Diego, 1990.