Cross-resistance pattern to four AHAS-inhibiting herbicides of tribenuron-methyl-resistant flixweed(Descurainia sophia) conferred by Asp-376-Glu mutation in AHAS

Flixweed（Descurainia sophia L.） is a problematic and widespread weed in winter wheat fields and has been controlled by tribenuron-methyl for more than twenty years in China. In this study, a flixweed accession（Hebei 25, HB25） with an Asp-376-Glu mutation in acetohydroxy acid synthase（AHAS） was identified and purified. The purified HB25 accession（p HB25） developed 758.1-fold resistance to tribenuron-methyl and exhibited obvious cross-resistance to four AHAS-inhibiting herbicides. The resistant/susceptible（R/S） ratios of 50% plant growth reduction（GR_（50）） to herbicides of halosulfuron-methyl, flumetsulam, imazethapyr and pyribenzoxim were 346.1, 15.7, 8.1 and 7.1, respectively. The reduced AHAS sensitivities to four different AHAS-inhibiting herbicides, which were caused by the Asp-376-Glu mutation, were responsible for the resistance and cross-resistance to AHAS-inhibiting herbicides. The R/S ratios of 50% inhibition of AHAS activity（I50） to tribenuron-methyl, halosulfuron-methyl, flumetsulam, imazethapyr and pyribenzoxim were 844.5, 532.9, 74.5, 13.3 and 5.5, respectively. The results of AHAS activity in vitro were highly correlated with that of whole-plant response experiments.

Broadleaf Weed Control and Crop Safety with Premixed Pyrasulfotole and Bromoxynil in Winter Wheat

For more than two decades acetolactate synthase (ALS) inhibiting herbicides have been the major weed control tools in winter wheat which resulted in selection of resistant weeds to those herbicides. Premix of pyrasulfotole & bromoxynil (Huskie?), a relatively new herbicide, was registered for use in wheat in 2008. Pyrasulfotole inhibits 4-hydoxyphenylpyruvate dioxygenase (HPPD) enzyme in susceptible plants and is the first significant new mode of action for use in cereals for more than two decades. Field experiments were conducted from 2007 to 2010 at two locations in Kansas, USA to test the efficacy of pyrasulfotole & bromoxynil for broadleaf weed control and crop safety in winter wheat. Treatments included pyrasulfotole & bromoxynil alone at 253 g·ai·ha-1 and tank mixtures of pyrasulfotole & bromoxynil at 207 g·ai·ha-1 with MCPA at 280 g·ai·ha-1, dicamba at 140 g·ai·ha-1 or metsulfuron-methyl at 4.2 g·ai·ha-1. Herbicides were applied postemergence in fall and spring seasons. Pyrasulfotole & bromoxynil alone or in combination with tank-mix partners, regardless of application time, controlled flixweed, blue mustard, bushy wallflower and field pennycress 98% or more. Henbit control was better when pyrasulfotole & bromoxynil treatments were applied in fall than spring (≥98% vs ≥67%). Pyrasulfotole & bromoxynil alone applied in spring was not effective on wild buckwheat, but tank mixing with dicamba or metsulfuron-methyl controlled it 84% or more. Pyrasulfotole & bromoxynil alone or as tank mixture caused little (≤7%) or no injury to wheat and the injury did not influence wheat grain yields. Based on excellent control of broadleaf weeds evaluated, pyrasulfotole & bromoxynil is an alternative tool to control ALS-inhibitor resistant weeds in winter wheat. Fall season application and tank mixing with other herbicides are desirable for effective and broad spectrum weed control.