Multiple herbicide resistance in littleseed canarygrass (Phalaris minor): A threat to wheat production in India

Littleseed canarygrass (Phalaris minor Retz.), a troublesome weed of wheat in India, has evolved multiple herbicide resistance across three modes of action: photosynthesis at the photosystem II site A, acetyl-coA carboxylase (ACCase), and acetolactate synthase inhibition. The multiple herbicide-resistant (MHR) populations had a low level of sulfosulfuron resistance but a high level of resistance to clodinafop and fenoxaprop (ACCase inhibitors). Some of the populations had GR₅₀ (50% growth reduction) values for clodinafop that were 11.7-fold greater than that of the most susceptible population. The clodinafop-resistant populations also showed a higher level of cross-resistance to fenoxaprop (fop group) but a low level of cross-resistance to pinoxaden (den group). Although clodinafop and pinoxaden are from two different chemical families (fop and den groups), their same site of action is responsible for cross-resistance behavior. The populations that were resistant to four groups of herbicides (phenylureas, sulfonylurea, aryloxyphenoxypropionate, and phenylpyrazolin) were susceptible to the triazine (metribuzin and terbutryn) and dinitroaniline (pendimethalin) herbicides. The P. minor populations that were resistant to the aryloxyphenoxypropionate and phenylurea herbicides were effectively controlled by the sulfonylurea herbicide, sulfosulfuron. In the fields infested with P. minor that was resistant to clodinafop, a sulfosulfuron application (25 g ha⁻¹) increased the wheat yield by 99.2% over that achieved using the recommended rate of clodinafop (60 g ha⁻¹). However, the evolution of multiple resistance against the four groups is a threat to wheat production. To prevent the spread of MHR P. minor populations, as well as the extension of multiple resistance to new chemicals, concerted efforts in developing and implementing a sound, integrated weed management program are needed. The integrated approach, consisting of crop and herbicide rotation with cultural and mechanical weed control tactics, should be considered as a long-term resistance management strategy that will help to sustain wheat productivity and farmers' income.