First report of Rhizoctonia solani causing leaf spot of Adhatoda vasica

Adhatoda vasica (syn. Justicia adhatoda), popularly known in India as malabar nut, is an important medicinal plant in traditional as well as modern systems of medicines. The leaves are used, either alone or in combination with other drugs, for preparation of expectorants (Singh et al., 1996; Jain & DeFilipps, 1991).

During October-November 2005, a leaf spot disease was found in A. vasica. The disease was present on all A. vasica plantations located in the foothills on the outskirts of Jaipur, covering an area of 8–10 km². The disease was present on leaves of all ages but was more severe on older (lower) leaves. Symptoms start as minute, round, light brown spots. Within 8-10 days the spots grow and acquire a round to irregular shape. Some spots coalesce. Fully developed spots were water soaked, dark brown to blackish, scattered all over the leaf lamina. Margins of the spots were diffuse and each spot had a light coloured dot in the centre. The spots were equally visible on lower and upper leaf surfaces. Severe infection resulted in defoliation.

Repeated isolations from infected leaves yielded a similar fungal growth on Potato Dextrose Agar. Five-day-old cultures (26 ± 1°C) produced profuse mycelium and dirty white sclerotia. Older hyphae were abundantly branched and dark brown. Lateral mycelium branches were constricted at the point of origin. Mature sclerotia were brown to black and highly variable in shape and size. Based on the cultural characters and morphology of mycelium and sclerotia, the fungus was identified as Rhizoctonia solani. The identity of the fungus was confirmed by Mycology Section, Agharkar Research Institute, Pune, India.

For pathogenicity testing, healthy leaves of potted plants of A. vasica were spray-inoculated with aqueous suspension of mycelium and sclerotia taken from 7-day-old culture on PDA. Inoculated plants, vis-à-vis water-sprayed control plants were covered with polythene bags for 36 hours. Typical leaf spot symptoms started developing after 10 days. The fungus was successfully reisolated of from artificially developed symptoms.

Although R. solani is known to cause foliar diseases of many plants including medicinal plants (Janardhanan, 2002), this is the first report of R. solani causing leaf spot of A. vasica in India and elsewhere. The field symptoms show that the disease is able to reduce A. vasica bio-mass substantially. Rhizoctonia leaf spot constitutes a potential threat to the exploitation of an important medicinal plant species.