A new host for Pithomyces chartarum, the cause of a leaf spot disease on Withania somnifera

Withania somnifera, popularly known in India as ashwagandha, is an important medicinal plant. Root and leaves are used alone or in combination with other drugs for treatment of sexual and general weakness, rheumatism, scabies and sore eyes. Withaferin-A, an important withanolide has been reported to show anticancer properties (Janardhanan, 2002). During a survey for diseases of medicinal plants, a leaf spot disease was found on W. somnifera growing in field and nurseries near Jaipur.

Symptoms of the disease were small circular to irregular necrotic spots of light brown colour which were scattered over the abaxial surface of the leaf, their size varied from 5-12 mm in diameter (Fig.1). In some cases, spots coalesced together, forming large necrotic patches. Individual spots had bright yellow halos. On the adaxial surface, spots were circular, grey to blackish in appearance and scattered over a major portion of the leaf lamina. Severely infected plants defoliated prematurely.

A single spore culture was produced on potato dextrose agar. Colonies were fast growing, dark grey to black. Conidiophores formed laterally on hyphae, 2.4-10 x 1.5-3 µm in size. Conidia were multicellular, darkly pigmented, formed on small peg like branches of the hyphae. They were broadly elliptical, pyriform, oblong and echinulate, 16-22 x 10-14 µm in size. The morphological characters including size of conidia (18-19 x 10-12 µm) fit the original description of Pithomyces chartarum (Ellis, 1971). A culture was deposited with the Agharkar Research Institute, Pune, India (accession number ARIFCC-1023).

For pathogenicity testing, healthy leaves of potted plants of W. somnifera were spray inoculated with an aqueous suspension (10⁵ conidia/ ml) of a 10-day-old sporulating culture of P. chartarum. Inoculated plants and water sprayed controls were covered with polythene bags for 48 hours and kept under natural light conditions and mean weekly temperature of 26.3°C. Typical symptoms started developing on the inoculated plants after 7-8 days. The fungus was re-isolated fulfilling Koch's postulates.

Review of the pertinent literature revealed that P. chartarum is known to cause leaf spot diseases of various plants including medicinal plants (Ponappa, 1977). However, for W. somnifera, it is a new disease report for India and worldwide. The field symptoms showed that the disease was responsible for reducing W. somnifera bio-mass substantially and constitutes a potential threat to an important medicinal plant species.