Deciphering the Enigma of Annual Recurrence of Wheat Rusts in India

Abstract

In the absence of alternate hosts, the epidemiology of wheat rusts has remained elusive in many countries including India. This paper synthesises salient information from historical records collected at the Indian Institute of Wheat and Barley Research, from surveys of alternate and alternative hosts, and from observations in off-season wheat disease monitoring nurseries. The primary source of rust inoculum to wheat in the plains of India was previously suggested to originate from off-season wheat, self-sown plants and green bridges in the hills. However, nowadays there is negligible cultivation of off-season wheat in the hills, but still recurrence of wheat rust is a regular phenomenon in India. As observed previously, none of the Berberis spp. or grasses could be related to wheat rusts in this study. Thus, the wheat rust pathogens appear to multiply clonally in India. At present, new virulent pathotypes of Puccinia striiformis f. sp. tritici are identified initially from the plains of Punjab and other areas in the foothills of Himachal Pradesh, and after 6–7 years these pathotypes become predominant in the hills. Moreover, owing to the cultivation of traditional wheat germplasm, less virulent pathotypes of the stripe rust pathogen occur in the higher hills, and most of the present-day wheat varieties are resistant to these pathotypes. The avirulence/virulence profiles of Indian pathotypes of Puccinia spp. on wheat indicate that our pathotype flora is different from that of adjoining countries and continents, which rules out their incursion from the adjoining countries. Wheat rust infection in the plains appears to be independent of the hills and movement of both leaf rust and stripe rust occurs both ways gradually year after year. It appears that the wheat rusts survive on grasses or other non-gramineous hosts occurring locally near water bodies, forests, grasslands or damp areas in the hills and plains of India.