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Resistance to iodosulfuron, a herbicide targeting acetolactate synthase (ALS), was identified and characterised in a population of Lolium perenne L. from Canterbury, New Zealand. The resistant population was 264 times more resistant to iodosulfuron than a susceptible population, based on a ratio of the rate corresponding to a 50% reduction in resistant and susceptible plant survival (R/S LD50). The resistant population was also 14-times more resistant to nicosulfuron, another ALS-inhibitor of the same class. However, no resistance to imazapyr, an ALS-inhibitor from a different chemical class, was detected in the resistant population. Pre-treatment with an inhibitor of cytochrome P450 reduced the level of iodosulfuron resistance to 46-fold in the resistant population, suggesting enhanced herbicide metabolism, a non-target site mechanism, contributed to resistance of this herbicide. In addition, a proline to leucine substitution at codon 197 was detected, suggesting a target-site-based mechanism was also associated with resistance to iodosulfuron. This is the first report on the resistance mechanisms to ALS-inhibiting herbicides identified in Lolium perenne in New Zealand and highlights the importance of integrated weed management approaches to reduce the likelihood of ALS-inhibitor herbicide resistance becoming more widespread. [ABSTRACT FROM AUTHOR]

To estimate the prevalence of herbicide-resistant weeds, 87 wheat and barley farms were randomly surveyed in the Canterbury region of New Zealand. Over 600 weed seed samples from up to 10 mother plants per taxon depending on abundance, were collected immediately prior to harvest (two fields per farm). Some samples provided by agronomists were tested on an ad-hoc basis. Over 40,000 seedlings were grown to the 2–4 leaf stage in glasshouse conditions and sprayed with high priority herbicides for grasses from the three modes-of-action acetyl-CoA carboxylase (ACCase)-inhibitors haloxyfop, fenoxaprop, clodinafop, pinoxaden, clethodim, acetolactate synthase (ALS)-inhibitors iodosulfuron, pyroxsulam, nicosulfuron, and the 5-enolpyruvyl shikimate 3-phosphate synthase (EPSPS)-inhibitor glyphosate. The highest manufacturer recommended label rates were applied for the products registered for use in New Zealand, often higher than the discriminatory rates used in studies elsewhere. Published studies of resistance were rare in New Zealand but we found weeds survived herbicide applications on 42 of the 87 (48%) randomly surveyed farms, while susceptible reference populations died. Resistance was found for ALS-inhibitors on 35 farms (40%) and to ACCase-inhibitors on 20 (23%) farms. The number of farms with resistant weeds (denominator is 87 farms) are reported for ACCase-inhibitors, ALS-inhibitors, and glyphosate respectively as: Avena fatua (9%, 1%, 0% of farms), Bromus catharticus (0%, 2%, 0%), Lolium spp. (17%, 28%, 0%), Phalaris minor (1%, 6%, 0%), and Vulpia bromoides (0%, not tested, 0%). Not all farms had the weeds present, five had no obvious weeds prior to harvest. This survey revealed New Zealand's first documented cases of resistance in P. minor (fenoxaprop, clodinafop, iodosulfuron) and B. catharticus (pyroxsulam). Twelve of the 87 randomly sampled farms (14%) had ALS-inhibitor chlorsulfuron-resistant sow thistles, mostly Sonchus asper but also S. oleraceus. Resistance was confirmed in industry-supplied samples of the grasses Digitaria sanguinalis (nicosulfuron, two maize farms), P. minor (iodosulfuron, one farm), and Lolium spp. (cases included glyphosate, haloxyfop, pinoxaden, iodosulfuron, and pyroxsulam, 9 farms). Industry also supplied Stellaria media samples that were resistant to chlorsulfuron and flumetsulam (ALS-inhibitors) sourced from clover and ryegrass fields from the North and South Island. [ABSTRACT FROM AUTHOR]