First report of Macrophomina root rot (Macrophomina phaseolina) on medicinal coleus (Coleus forskohlii) in India

Medicinal coleus (Coleus forskohlii) is a medium sized, herbaceous plant native to India, which is intensively cultivated in South India for the extraction of 'Forskolin': a labdane diterpene. In June-July 2003, coleus grown in Vilupuram, Salem and Erode districts of Tamil Nadu suffered heavy losses due to a disease. The symptoms observed were yellowing and drooping of the leaves (Fig. 1), blackening of the stem (Fig. 2), rotting of the root, basal stem and bark peeling (Fig. 3).

The presence of black sclerotia was observed on the rotted portion. A fungus was isolated aseptically from diseased parts on Potato Dextrose Agar (PDA) medium. The mycelium was initially hyaline and later became grey in colour (Fig. 4). Sclerotia were minute, black, round to oblong or irregular in shape with mycelial attachment (Fig. 5). Based on the symptoms and mycelial characters, the fungus was identified as Macrophomina phaseolina (Dhingra & Sinclair, 1973). The pathogen was found to cause root rot in many legumes, cereals, oilseeds and fibre crops.

To confirm pathogenicity, 25-30 days old seedlings were planted in pots containing 100 parts of sterilized soil and 1 part of M. phaseolina inoculum. The inoculum was multiplied in a sand maize medium (Riker & Riker, 1933), with the sand and ground maize grains mixed at a ratio of 19:1. The soil moisture content in the pots was maintained at 40%. The pots were kept at 35°C (Jha & Dubey, 1998). The plants were assessed 30 days after planting. Inoculated plants produced typical symptoms on leaves, stem and roots. The fungus was consistently re-isolated from infected plant. In contrast, the control plants did not show any symptoms.

Recently, a wilt disease of C. forskohlii caused by Fusarium chlamydosporum was reported for the first time from Karnataka, India (Shyla, 1998). To the best of our knowledge, this is a new host for M. phaseolina which is reported in India.