Two New Orchid Hosts of Sclerotium rolfsii Sacc. from India

Phaius flavus (Bl.) Lindl. and Paphiopedilum venustum (Wall.) Pfitz. ex Stein. are two important native orchids of the Sikkim Himalayas, being grown in Sikkim, the Darjeeling District of West Bengal, Arunachal Pradesh and other north eastern Indian states. In June-July 2002, a severe and fatal rotting disease was observed on these orchid species growing in community pots in the orchid house at the National Research Centre for Orchid (NRCO), Pakyong 737106, Sikkim.

On Phaius flavus the disease caused a basal rot of the pseudobulbs during the initial stages of infection. Later leaves became yellow and detached from the pseudobulbs. Gradually the disease spread upward, until the entire plant turned brown to black and died. On Paphiopedilum venustum the disease appeared at the collar region as a soft rot, which gradually moved upward resulting in leaf collapse (Fig 1). Numerous small brown spherical sclerotia (0.5-1.99 mm in diameter) and white mycelial growth were observed on the pseudobulbs, leaf base and along the leaf sheaths.

A fungus was isolated, purified and identified in the Plant Pathology laboratory at NRCO. On potato dextrose agarose (PDA) the isolated fungus produced abundant white septate mycelia with clamp connections at each septation, aerial hyphae and also numerous spherical or ellipsoidal, white sclerotia, which turned brown as they matured (Fig. 2). Similarly white mycelial growth and sclerotia were abundantly produced on host inoculated test plants. Based on the morphological and cultural characters, the disease-causing organism was identified as Sclerotium rolfsii Sacc. Pathogenicity was also confirmed under natural conditions outside the greenhouse on potted plants of Phaius flavus and Paphiopedilum venustum.

Sclerotium rolfsii has been reported to cause stem rot and southern blight on several plants in India (Anahosur, 2001) and elsewhere (Punja, 1985). A survey of the literature confirmed that there were no reports of disease caused by Sclerotium rolfsii on Phaius flavus and Paphiopedilum venustum in India (Bilgrami et al., 1991; Sohi, 1992).