White mustard (Sinapis alba) resistance to ALS-inhibiting herbicides and alternative herbicides for control in Spain

Sinapis alba is a competitive weed in cropping systems in southwest Spain. Over the past 10 years, reports of erratic control of S. alba with ALS-inhibiting herbicides have increased. Sixteen S. alba field accessions (AR(1) to AR(16)) were collected from Malaga, in southwest Spain, and cultivated under greenhouse conditions for screening tests. AR(8) and AR(1) accessions were selected for in vivo and in vitro assays, while the site for the AR(8) population was also used for field experiments. Results demonstrated that tribenuron-methyl reduced plant fresh weight by over 90% in the AR(1), AR(2) and AR(3) populations; AR(4) biomass was reduced 30.7%; biomass of the AR(5) to AR(16) populations were only reduced from 5.7 to 13.1%. In vivo, the rate to reduce above ground plant fresh weight by 50% (ED50) was greater for AR(8) (1.76) than AR(1) (0.18), indicating a resistance factor of 9.8 for tribenuron-methyl. In vitro, a comparison of ALS enzyme activity resulted in a resistance factor of 4128 for tribenuron-methyl, and 884 (mesosulfuron) and 839 (iodosulfuron) for the other two sulfonylurea herbicides. In vivo, the AR(8) biotype was cross-resistant to four of the five ALS-inhibiting herbicide groups (sulfonylurea, imidazolinone, triazolopyrimidine and sulfonylaminocarboniltriazolinone), but not to pyrimidinyloxybenzoic acid (bispyribac-sodium). The leaf contact angle for interception of tribenuron-methyl was 55.6 degrees on the adaxial surface of AR(8) plants compared to 17.6 degrees on the leaves of AR(1) plants. This resulted in significantly less tribenuron-methyl retention on R versus S (125.6 versus 166.1 mu L g(-1) dry shoot weight) plants. In field trials, tribenuron-methyl resulted in minimal control of AR(8) plants (28.7%). However, tribenuron-methyl + MCPA (92.3%) and tribenuron-methyl + mecoprop-P (88.7), resulted in the highest white mustard control. This research confirms resistance of S. alba to tribenuron-methyl. In vitro assays suggest resistance is due to the inability of tribenuron-methyl to interact with the target site (ALS). Foliar retention and contact angle might contribute to the evolved resistance of S. alba to tribenuron-methyl. For improved control of white mustard, tribenuron-methyl combined with herbicides exhibiting different modes of action, such as MCPA or mecoprop-P, should be considered. Crown Copyright (C) 2011 Published by Elsevier By. All rights reserved.