Cross-and Multiple Herbicide Resistant Lolium rigidum Guad. (Rigid Ryegrass) Biotypes in Iran

Weed competition, especially from grass species, is estimated to cause 23% reduction in yield in the wheat fields of Iran. During the years 2013 to 2016, a study was conducted to evaluate the resistance to herbicides of 30 rigid ryegrass (Lolium rigidum) biotypes that had been collected from wheat fields of Khuzestan Province. The screening of these biotypes was conducted with clodinafop-propargyl in the greenhouse and revealed biotypes with a survival rate of greater than 20% in response to this herbicide. These biotypes were further studied for the evaluation of cross and multiple resistance. A total of 94 and 75% of the rigid ryegrass biotypes showed resistance to ACCase- and ALS-inhibitors, respectively. Approximately 69% of the rigid ryegrass biotypes included individuals with resistance to at least two herbicide mechanisms of action. This is the first report of cross and multiple resistance in rigid ryegrass biotypes from Iran. The leaves of the rigid ryegrass biotypes cross-resistance to ACCase-inhibitors were analyzed using CAPS and dCAPS markers to identify probable amino acid substitutions at 2,041, 2,088, 1,781, and 2,078 positions on the ACCase gene. In two and nine biotypes, mutations were observed in the 1,781 and 2,041 positions, respectively. These results indicated that there is a serious problem with herbicide resistance in rigid ryegrass, including cross and multiple resistance, and a need to implement long-term integrated management strategies.