First Report of Sclerotium rolfsii Associated with Boll Rot of Cotton in India

Abstract

Cotton (Gossypium hirsutum) is the most important fiber-yielding crop grown in India and worldwide. In India, during 2012-2013, 1.16 million ha were under cotton cultivation, with an average production of 480 kg/ha. As production of cotton has increased in areas, so has the occurrence of various pests and diseases. During a field survey conducted in the cotton-growing areas of Devalapura Village of the Mysore District during August and September 2013, a boll rot disease was observed with distinctive symptoms and signs of a fungal pathogen observed in 27% of fields. Incidence of the boll rot within fields ranged from 8 to 12%, while 3 to 5% stem rot was observed on boll rot-affected plants in about 15 to 18 ha of total cotton fields surveyed. White mycelial strands with spherical, dark-brown sclerotial bodies were found on the surface of infected bolls near the ground and the affected bolls were completely decayed. The disease-affected bolls were disinfested with 2% sodium hypochlorite solution for 3 min, and 5-mm samples were placed on potato dextrose agar (PDA) medium and incubated at 28 ± 2°C. The fungal colonies that grew from infected tissues were whitish with cottony aerial mycelium and abundant globoid sclerotia observed after 10 to 12 days of incubation. Sclerotial bodies were initially pale whitish, becoming dark brown over time. Based on morphological and cultural characteristics, the associated fungal pathogen was identified as Sclerotium rolfsii Sacc. The identity of the pathogen was further confirmed through PCR amplification of internally transcribed spacer (ITS) region using ITS1/ITS4 universal primer. The amplified PCR product (550 to 570 bp) was sequenced and BLASTn search comparison revealed 100% homology with Sclerotium rolfsii (Accession Nos. JF966208 and JX914480). A representative sequence of S. rolfsii was deposited in GenBank (KP412471.1 and KP412472.1). Further, pathogenicity tests were conducted on 30 healthy cotton bolls in vivo by inoculating the bolls with sclerotia obtained from 12-day-old cultures; noninoculated bolls served as controls. Characteristic boll rot symptoms were observed on 60% of total inoculated cotton bolls 7 days after inoculation and S. rolfsii was reisolated from the infected bolls and the identity of the pathogen was confirmed. S. rolfsii is a serious soil-borne fungal pathogen with a wide host range (Mullen 2001) and prevalent in tropical and subtropical regions, where high temperature and moisture are sufficient to permit growth and survival of the fungal pathogen (Punja 1985). The association of S. rolfsii with boll rot of cotton has been reported from Bangladesh (Shamsi et al. 2008). In India, major diseases of cotton include wilt (Fusarium oxysporum f. vasinfectum), anthracnose (Colletotrichum capsici), and areolate mildew (Ramularia areola) (Rangaswamy and Mahadevan 2002). S. rolfsii infect bolls on branches near ground level, causing boll decay. Numerous sclerotia produced on the affected bolls are the source of secondary inoculum and are dispersed by rain splash during the late monsoon, thus resulting in new infections on the basal stem and leaves of the cotton plant. The disease has resulted in reduced yield and market value of the crop. To the best of our knowledge, this is the first report on the occurrence of S. rolfsii causing boll rot of cotton in India.