First Report of Sclerotium rolfsii Causing Southern Blight and Leaf Spot on Common Bean (Phaseolus vulgaris) in India

Abstract

Common bean (Phaseolus vulgaris L.) is an economically important crop grown worldwide. In India, it is grown for both green bean and dry pods (seeds). During 2012-2014, sudden wilt and leaf spots were observed during early growth to preflowering stage in the Mysore and Mandya districts of Karnataka State. The disease incidence was 30 to 35% in nearly 45 to 48 ha. Symptoms include wilting of foliage, tan lesions on stems at ground level, and gradually drying of the whole plant. Leaf spot symptoms on 30- to 45-day-old plants were water-soaked lesions with concentric necrotic spots (5 to 12 mm). White mycelial strands with spherical dark brown sclerotial bodies at the stem-soil interface were observed. Infected tissues were disinfected with 2% sodium hypochlorite solution for 3 min, and plated on potato dextrose agar and incubated at 28 ± 2°C. A single associated fungal pathogen was consistently isolated. It was characterized by dense, aerial, whitish cottony mycelium. Uniformly globoid sclerotia were observed after 10 to 12 days of incubation. Whitish sclerotia (1 to 3 mm) turned to dark brown at maturation. Based on etiology, and morphological and cultural characteristics, the pathogen was identified as Sclerotium rolfsii Sacc. (syn. Athelia rolfsii (Curzi) Tu & Kimbrough) (Mordue, 1974). Identity was further confirmed through PCR amplification of internally transcribed spacer (ITS) region using ITS1/ITS4 universal primer. Amplified PCR product (550 to 570 bp) was sequenced and BLASTn search comparison revealed 99% homology to Sclerotium rolfsii (GenBank Accession Nos. DQ093668 and JX914480). Representative sequence of S. rolfsii was deposited in GenBank as KP412468.1 and KP412469.1. Pathogenicity tests were conducted on 15-day-old common bean plants (cv. S-9) by soil infestation and foliar inoculation of sclerotial bodies obtained from a 12-day-old culture of S. rolfsii. The experiment was conducted on 30 plants along with control plants. Soil infestation with sclerotia resulted in wilting and crown rot symptoms with 90% disease and 100% leaf spot incidence and no such symptoms were observed on control plants. The fungal pathogen was reisolated from inoculated plants and the identity was confirmed through morphological and cultural characteristics. Southern blight and leaf spot diseases of common bean are a major constraint for its production in India. To our knowledge, this is the first report on the occurrence of S. rolfsii causing southern blight and leaf spot of common bean in India.