Reaction of Bhutanese wheat cultivars and differential lines to rust diseases at mid and low altitudes in Bhutan
Wheat rusts are the important fungal diseases that limit the production and downgrade wheat quality. There are three types of wheat rusts; stem rust caused by Puccinia graminis f. sp. tritici, stripe rust caused by Puccinia striiformis f. sp. tritici and leaf rust caused by Puccinia triticina. The study was conducted to assess the susceptibility of Bhutanese wheat cultivars and differential lines to rust diseases at mid and low altitudes. The study was conducted in Mendagang for mid altitude (1,332 masl) and Samtenling for low altitude (378 masl) from December 2016 to March 2017. The experiment followed Randomized Complete Block design (RCBD) with 15 treatments (wheat lines) and three replications. However, only 11 lines germinated in both locations. Disease assessments were performed approximately at 60, 90 and 120 days after sowing following the modified Cobb?s disease rating scale. Only leaf rust was observed and the disease incidence 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 to moderately resistant was observed at mid altitude; while 5 to 100% with immune to susceptible field response 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 study also revealed that disease severity was positively correlated (r=.359); (r=.034); with mean minimum and maximum temperature respectively and (r=.361) with mean minimum relative humidity. All the correlations were highly significant (p=.003). The study found that ICARDA line 1 with severity (100%) was highly susceptible (100S) to leaf rust at low altitude while Bajosokha kaa remain an immune in both the locations. The results indicate that leaf rust can occur in both low and mid altitudes; however selection of suitable lines requires more extensive studies.