First Report of Leaf Spot Disease in Withania coagulans Caused by Alternaria alternata in India
Authors: Sharma A. , Singh V. , Singh G. , Pati P. K. 
Published: September 2012

Withania coagulans (Paneer doddi) is a medicinal plant in the Solanaceae (1) that grows in northwestern India. An unknown disease appeared in the Amritsar district of Punjab, India on W. coagulans starting in 2009, and was noticed annually in the post-monsoon seasons through 2011. The plants were grown in a net house (74.82323 to 74.82332°E, 31.63678 to 31.63688°N, 221 m elevation), Guru Nanak Dev University, Amritsar. Symptoms first appeared as brown to black spots (2 to 10 mm in diameter) that were sometimes surrounded by a yellow halo, starting on the lower leaves and gradually spreading to the upper leaves. Spots were apparent on both the dorsal and ventral sides of the leaves. In the most severe outbreak of the disease, more than 90% of the leaf area was covered with the leaf spot symptoms. The pathogen was isolated by placing pieces of surface-sterilized leaf spots on potato dextrose agar (PDA), and the hyphae that emerged from the leaf pieces were sub-cultured to PDA. Initially, the fungal colony was white, and gradually turned olive-brown as it matured. Conidiophores were mostly simple, usually becoming geniculate by sympodial elongation. Conidiophores were mostly branched and conidia were arranged in acropetal chains of 6 to 10. Conidia were 15 to 25 μm long and 5 to 10 μm wide, with 3 to 4 transverse septa and 2 to 3 longitudinal septa, muriform, ovoid to ellipsoid, with a broadly rounded base and an apical beak. Based on these morphological characters, the pathogen was identified as Alternaria alternata (Fr.) Keissler (4). The identification of the pathogen was also confirmed by MTCC, Chandigarh, India and deposited in their culture collection with Accession No. MTCC-10939 ( http://mtcc.imtech.res.in ). During preliminary studies, the optimum temperature for its growth was 25 ± 2°C. Pathogenicity of the fungus was demonstrated on three potted W. coagulans plants after spray-inoculating leaves with a spore suspension of 3 × 105 spores per ml. A batch of three plants sprayed with sterile distilled water served as controls. Both inoculated and control plants were incubated at 100% relative humidity for 5 days and transferred to a glasshouse at 25 ± 2°C. Initial symptoms developed on a few plants at 5 to 7 days post-inoculation, and the same leaf symptoms described above from field plants became apparent on all inoculated plants by 10 to 14 days. Control plants did not develop symptoms. The pathogen was recovered from leaf spots using the isolation technique described above, and was identified as A. alternata, thus fulfilling Koch's postulates. Although the leaf spot diseases caused by A. alternata and A. dianthicola were reported on W. somnifera (2,3), to the best of our knowledge, there is no report of either pathogen infecting W. coagulans. Thus, the identification of pathogen will facilitate the disease management program and for enhancing the commercial value of this important endangered medicinal plant.

References: (1) S. A. Gilani et al. Afr. J. Biotechnol. 8:2948, 2009. (2) C. K. Maiti et al. Plant Dis. 91:467, 2007. (3) P. K. Pati et al. Indian J Microbiol. 48:432, 2008. (4) E. G. Simmons. Alternaria: An identification manual. American Phytopathological Society, St. Paul, MN, 2007.