Reaction of Bhutanese wheat cultivars and differential lines to rust diseases at mid and low altitudes in Bhutan
Wheat rusts are one of the important diseases that limit the production and downgrade wheat quality. Three rust diseases of wheat are stem rust caused by Puccinia graminis Pers. f. sp. tritici Eriks., stripe rust caused by Puccinia striiformis Westend. f. sp. tritici Eriks., and leaf rust caused by Puccinia triticina Eriks. This study was conducted to determine the reaction of wheat varieties to wheat rusts at different altitudes. Field experiments were conducted from December 2016 to March 2017 at Mendagang (27.5886°N, 89.8711°E, 1332 masl), Punakha Dzongkhag (district) for mid altitude and at Agriculture Research and Development Center (ARDC), Samtenling (26.9058°N, 90.4308°E, 378 masl), Sarpang Dzongkhag, Bhutan for low altitude. The experiment followed a RCBD with 15 treatments comprising of three Bhutanese released varieties, eight SAARC varieties, and four ICARDA varieties. Each treatment was replicated three times. Assessment of disease incidence and severity were performed three times starting from tillering to ripening stage, approximately at 60, 90 and 120 days after sowing (DAS). Disease severity was determined following the modified Cobb’s disease rating scale. Of the 15 varieties, only 11 germinated in both the sites. Among the three wheat rust diseases, only leaf rust was observed in both sites. Leaf rust incidences ranged from 2.5 to 10% and 2.5 to 16% at mid and low altitudes respectively. Disease severity of 5 to 20%, corresponding to field response of immune (5O) to moderately resistant (20MR), was observed at mid altitude, while 5 to 100%, with immune (5O) to susceptible (100S), was observed at low altitude. There was a significant difference in disease incidence by site (p=.038) but not in disease severity (p=.129). The variety, ICARDA 1, with 100% severity was highly susceptible (100S) to leaf rust at low altitude while Bajosokha Kaa remained immune (5O) in both the sites. The results indicate that leaf rust can occur in both low and mid altitudes; however selection of suitable varieties requires more extensive studies.