First report of root rot of stevia caused by Sclerotium rolfsii in India

Stevia (Stevia rebaudiana) is an important medicinal crop grown in India. The leaves of stevia contain a non-calorific sweetener, stevioside, which is used as an alternative to artificially produced sweeteners. Between June and September 2005, a disease survey was conducted in different stevia-producing villages of Erode district in Tamil Nadu State. A new root rot disease was observed on 2-month old stevia plants in all areas surveyed. Symptoms first appeared as yellowing and drooping of leaves, with wilting of plants and white cottony mycelial growth at the collar region (Fig. 1). The mycelial growth spread to the stem and roots, with associated tissue rotting (Fig. 2). On the diseased areas, brown sclerotia were observed. A fungus was consistently isolated from diseased roots plated on potato dextrose agar medium and incubated at 25 ± 2oC (Fig. 3). The mycelium of the fungus was hyaline, branched at clamp connections and septate, corresponding with published descriptions for Sclerotium rolfsii (Mordue, 1974). The abundant sclerotia were round to oblong, initially white and later brown, with an average diameter of 0.5-2.0 mm.

Figure 1: White mycelial growth at the collar region of stevia

Figure 2: Rotting of the roots and stem base of stevia

To confirm pathogenicity, 25-30 day old seedlings were planted in pots containing 100 parts of sterilised soil and 1 part of mycelial inoculum, using an isolate of S. rolfsii from stevia multiplied in a sand/maize medium. The pots were kept at 25 ± 2oC with soil moisture content maintained at 70%. The plants were assessed 30 days after planting. Inoculated plants developed typical symptoms on leaves, stem and roots, and S. rolfsii was consistently re-isolated. Uninoculated plants did not show any symptoms.

Chang et al., (1997) observed a stem rot disease of stevia for the first time in India and identified the causal agent as Sclerotinia sclerotiorum. Megeji et al. (2005) recorded a stem rot disease on stevia at Palampur, Himachal Pradesh, India by visual observation without confirming the pathogen. To the best of our knowledge, this is the first report of S. rolfsii infecting stevia in India.