First report of Alternaria alternata causing leaf spot on Stevia rebaudiana

Stevia rebaudiana is a herbaceous perennial plant of the Asteraceae, native to Paraguay where it grows in sandy soils near streams (Katayama et al., 1976). It has been recognized world wide for its excellent sweetening property. In India the plant has been introduced in the states of Maharastra, Tamil Nadu, Karanataka, Rajasthan and West Bengal.

For the last three years, during February when temperature ranges from 20-25°C, severe foliar infections were observed in the region of South Bengal. Symptoms initially appeared as small circular spots, light brown in colour. Later, many became irregular and dark brown to grey, while others remained circular with concentric rings or zones. On severely infected leaves several spots coalesced to form large necrotic areas. On older leaves concentric spots were more common at the tips. Leaf spots varied from 2-18 mm (Fig. 1) in diameter. Conidial dimensions varied from 10-40 × 6-12 µm, mid to dark brown or olive-brown in colour, short beaked, borne in long chains, oval and bean shaped with 3-5 transverse septa (Fig. 2).

The pathogen was isolated as a pure culture on potato dextrose agar media (Fig. 3). The fungus produced abundant branched septate, brownish mycelia; conidiophores simple, olive-brown, septate, variable in length with terminal conidia, which were solitary or in short chains. Conidial characteristics from culture were similar to the conidia isolated from infected plants. Based on the morphological characters, the organism was identified as Alternaria alternata and the identification was confirmed by the Agharkar Research Institute, Pune, India.

Pathogenicity test was performed by spraying leaves of 10 healthy, 3-month-old potted S. rebaudiana plants with a spore suspension of 10⁵ conidia per ml. Control plants were sprayed with sterile water. Plants were covered with plastic bags for 10 days and kept in the laboratory garden at 30 ± 20°C. The pathogenicity tests were repeated three times. The first lesions appeared after a period of 12 ± 2 days. The pathogen was consistently reisolated from the lesions.

A survey of the literature reports the occurrence of only a few fungal diseases on S. rebaudiana. These include Erysiphe cichoracearum, Rhizoctonia solani, Sclerotium dephinii, Septoria steviae, Sclerotinia sclerotiorum, Sclerotium rolfsii and A. steviae (Thomas, 2000; Lovering & Reeleeder, 1996; Kamalakannan et al., 2006; Ishiba et al., 1982). This is the first report of A. alternata on stevia in India.