New Disease Reports (2005) 11, 48.

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

A. Kamalakannan 1*, L. Mohan 2, V. Valluvaparidasan 3, P. Mareeswari 3 and R. Karuppiah 3

*kamals2k@yahoo.co.in

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Accepted: 29 Jun 2005

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.

Figure1+
Figure 1: Yellowing and drooping of leaves
Figure 1: Yellowing and drooping of leaves
Figure2+
Figure 2: Blackening of stem
Figure 2: Blackening of stem
Figure3+
Figure 3: Rotting of root and basal stem
Figure 3: Rotting of root and basal stem
Figure4+
Figure 4: Mycelium of Macrophomina phaseolina
Figure 4: Mycelium of Macrophomina phaseolina
Figure5+
Figure 5: Sclerotium with mycelial appendages
Figure 5: Sclerotium with mycelial appendages

References

  1. Dhingra OD, Sinclair JB, 1973. Location of Macrophomina phaseolina on soybean plants related to culture characteristics and virulence. Phytopathology 63, 934-936.
  2. Jha AK, Dubey SC, 1998. Effect of plant age and weather parameters on collar rot of Okra caused by Macrophomina phaseolina. The Journal of Mycology and Plant Pathology 28, 351- 353.
  3. Riker AJ, Riker RS, 1933. Introduction of Research on Plant Disease. St. Louis, USA: John Swift Co.
  4. Shyla M, 1998. Etiology and management of root rot of Coleus forskohlii. Bangalore, India: University of Agricultural Science, MSc thesis.

This report was formally published in Plant Pathology

©2005 The Authors