Alternaria alternata – a new foliar fungal pathogen of tea in North Bengal, India

Between July-September 2003, severe foliar infection was observed on nursery-grown tea plants (Camellia sinensis) in five tea estates in the region of Dooars, North Bengal, India. Disease symptoms first appeared as greyish brown patches around tips and margins of young leaves (Fig. 1A). These lesions extend towards the midrib resulting in leaf curl, death and defoliation. In severe infections, over 70 % of the tea crop in affected nurseries was unusable. Older leaves appear to be less susceptible. The plants (T-17) in the nurseries where disease was found were growing under optimal conditions (i.e. not stressed): temperature (28 ±2°C), humidity (60 - 70 %), and were not affected by any other pathogens or physiological conditions. Similar symptoms have also been observed in field grown plants. Field symptoms were observed one month following transfer of plants from the nursery. The disease was also noticed in the tea nursery the following year.

A fungus was consistently isolated from the margins of these lesions onto potato dextrose agar (PDA) medium amended with streptomycin sulphate. Cultures were maintained at 28 ±2°C for 15 days. Microscopic examination revealed brown and septate hyphae and conidiophores (17-28 x 3-6 µm). Muriform conidia (23 -34 x 7-10 µm) were usually solitary but occasionally in short chains. Based on the morphological characters, the fungus was identified as Alternaria alternata (W8053) (Fig. 1B) and this identification was confirmed by the Global Plant Clinic, CABI Bioscience UK Centre.

Pathogenicity tests were conducted on twenty three tea varieties using unwounded, detached leaves, cut shoots and whole plants inoculated (5 x 10⁵ spores per ml of the original A. alternata isolate) and tested in the laboratory, glass house and field respectively (Chakraborty et al., 1995). Distilled water was used as controls. Among these twenty-three varieties, twelve were from the Tocklai Experimental Station, Jorhat, Assam (TV), eight from UPASI Tea Research Center Valparai, Tamilnadu (UPASI), while three were from Darjeeling Tea Research Center, Kurseong (T). Inoculated plants were covered with a polythene bag and incubated at 28 ± 2°C, under 16 h photoperiod. Disease symptoms developed within 3 days on inoculated detached and attached leaves and shoots Control plants remained unaffected. The pathogen was re-isolated from lesions, thereby completing Koch's postulates.

All varieties tested showed variable degrees of susceptibility to the pathogen apart from TV-28, UPASI-2 and UPASI-8 that were resistant. Tea varieties T-17, T-78 and TV-22 were found to be highly susceptible. A polyclonal antibody has been raised against the virulent isolate of A. alternata (W8053) and an immunodiagnostic kit has been developed for early detection of the pathogen from tea leaf tissues.

This is the first report of a pathogenic A. alternata affecting tea in India.