Tan spot of wheat, caused by Pyrenophora tritici-repentis (Died.) Drechs. (anamorph Drechslera tritici-repentis), is an important disease of wheat especially in cooler countries of the world. The fungal pathogen produces necrotic and chlorotic symptoms on different wheat genotypes. A survey was done to detect this disease in India during seven crop seasons (1999-2006). Blighted leaf samples (1,600) from 257 genotypes were collected from 46 locations in six agroclimatic zones of India. The tan spot pathogen was identified from scrapings of necrotic spots placed on glass slides and examined and from fungal growth on 2-cm pieces of infected leaves incubated on water-soaked Whatman No. 1 filter paper in presterilized transparent plastic petri dishes that were incubated for 3 days with a regimen of 21°C and 24 h of light and 16°C and 24 h of dark. On incubated leaf pieces, the pathogen produced erect, single, dark yellow-brown conidiophores with single, light yellow-brown conidia, 5 to 9 distoseptae, 75 to 200 μm, and a basal cell with a snake-like head typical of D. tritici-repentis(2). Disease incidence data revealed that the tan spot was most common in the northern hills zone (comprising the states of Uttaranchal, Himachal Pradesh, Jammu, and Kashmir) as well as in the Kalimpong locality in West Bengal. The pathogen was isolated from 26 to 83% of 374 leaf samples from the northern hills zone and 5 to 11% of 560 leaf samples from the northwestern plains zone (comprising the states of Punjab, Haryana, Western Uttar Pradesh, and Rajasthan). Suspensions of single-spored cultures grown on V8 potato dextrose agar with 3,000 conidia/ml until runoff were inoculated with a fine atomizer on 2-week-old seedlings of A-9-30-1 and Katepwa wheat genotypes in a polyhouse at the Directorate of Wheat Research, Karnal, India. Tween 20 (one drop per 100 ml of suspension) was used before inoculation. Necrotic and chlorotic symptoms that developed on the plants confirmed pathogenicity of the cultures. The pathogen has previously been reported from the northern plains and central zones of India during 1934 and 1972, respectively, (3,4) and from Pakistan during 2000 (1). The currently used practices of zero tillage and incorporation of crop residue back into fields may promote tan spot in the future. With the possibility that tan spot may become a major problem, particularly in the cooler parts of India, we recommend that the future wheat varieties to be planted in the northern hills and plains zones be screened for susceptibility to tan spot and that resistant varieties be developed to minimize possible loss.
References: (1) S. Ali et al. Plant Dis. 85:1031, 2001. (2) C. Drechsler. J. Agric. Res. 24:641, 1923. (3) A. P. Misra and R. A. Singh. Indian Phytopathol. 25:350, 1972. (4) M. Mitra. Indian J. Agric. Sci. 4:692, 1934.
