First report of Rhizoctonia solani causing blight on yam bean (Pachyrrhizus erosus) in India

Abstract

Yam bean (Pachyrrhizus erosus) is a native of Mexico and Central America and is widely cultivated throughout the tropics, in both hemispheres. In India, it is popularly grown in parts of West Bengal, Bihar, Orissa and Assam. The plant has many uses: the young tubers are edible and rich in ascorbic acid, while mature tubers yield high-quality starch and the seeds can be used as insecticides (CSIR, 1966). Central Tuber Crops Research Institute, Kerala, maintains 63 germplasm accessions collected from different parts of India. From 2002 onwards, 80% of the yam bean accessions were found to be infected with severe blight spots on leaves, stems and pods. The symptoms first appeared as small, water-soaked lesions with a dark margin and grey centre on leaves. As the disease progressed, spots would expand and coalesce, resulting in severe leaf blight and wilt. Spots could also be seen on both pods and seeds. The pathogen was isolated from infected seeds onto potato dextrose agar medium. After 3 days’ incubation at room temperature(28 ± 2°C), abundant pale brown mycelium with small white sclerotia were evident. The hyphae became dark brown to black with age. Hyphae were superficial, short-celled and abundantly branched, measuring 6–9 µm in diameter. Lateral branches were constricted at their point of origin with a septum in the branch near its junction. Brown-to-black sclerotia were uniform in texture and highly variable in shape and size. Based on the morphological and cultural characters, the pathogen was identified as Rhizoctonia solani. Pathogenicity of the fungus was proved by inoculating healthy leaves with both fungal mycelium and sclerotia using a pin-prick method. Inoculated leaves were covered with a small piece of moist cotton. Moist cotton without inoculum served as a negative control. The fungus was successfully reisolated from leaves showing symptoms after 7 days at room temperature (28 ± 2°C), but not from the negative controls. A culture was deposited in Indian Type Culture Collection of the Indian Agricultural Research Institute (ITCC.5790.04). Although R. solani has been reported previously on various crops from India (Bilgrami & Jamaluddin Rizwi, 1991; Butler, 1997), and R. solani has been found affecting P. erosus in Hawaii (Raabe et al., 1981), this is the first report of R. solani on P. erosus in India. The authors would like to thank Dr Prameeladevi and Ms Nita Mathur, ITCC, Indian Agricultural Research Institute, New Delhi, India for their help in identification.