Dioscorea alata, a new host of Sclerotium rolfsii in India

Greater Yam (Dioscorea alata) is an important edible yam of Asia and is mainly used as a subsidiary vegetable. Kerala, West Bengal, Bihar, Orissa, Assam, Gujarat and Maharashtra are the major yam cultivating states in India.

In 2002, the leaves of D. alata growing in farmer's fields in the Pathanamthitta district of Kerala, showed black, circular, concentric spots of between 5 and 10 mm in diameter, in the middle and bottom portion of the vines. As the disease advanced, the centre portion of the leaf spots dried and fell out, resulting in shot hole symptoms (Fig 1). When diseased leaves were placed in a humid chamber, abundant fluffy white mycelia emerged from the leaf spots. The fungus was isolated on Potato Dextrose Agar (PDA) medium. The diameter of hyphae ranged from 1.5 to 2.7 µm. Numerous round to ellipsoidal, dark brown-to-black, smooth sclerotia were observed in the culture (Fig 2). Disease symptoms were duplicated when detached D. alata leaves were inoculated with the isolated culture and incubated in moist Petri dishes for 4 days. The pathogen was reisolated from the inoculated leaves. Based on morphological and cultural characteristics (Singh, 1982), the pathogen was identified as Sclerotium rolfsii. A culture has been deposited in the Indian Type Culture Collection of the Indian Agricultural Research Institute, New Delhi (ITCC 5789.04).

Sclerotium rolfsii associated with the leaf spot of Dioscorea spp. has been reported earlier in Nigeria (Amusa, 1999). A perusal of literature shows that there are no previous records of this disease on D. alata from India (Bilgrami et al., 1991; Butler, 1997).