Resistance of Alisma plantago aquatica to sulfonylurea herbicides in Portuguese rice fields

The first known case of herbicide resistance found in Portugal occurred in the aquatic environment: populations of Alisma plantago-aquatica have shown resistance to bensulfuron-methyl. A large survey was carried out in 1996, covering 81% of the river valleys of Sado and Caia, 41% of Tagus and Sorrai a and 26% of Mondego. In this survey, 164 fields were selected for sampling (77, 55 and 32 for each region). The present paper presents the first resu lts from single dose bioassays for resistance confirmation, using 98 sample s (55 soil samples and 43 seed samples). Thirty six new resistant populatio ns were confirmed. The Tagus and Sorraia valleys were the most affected are as with 23.8% of the total fields having Alisma plantago-aquatica populatio ns resistant to bensulfuron-methyl, followed by the Sado and Caia, and Mond ego valleys with 7.3% and 1.2%, respectively. A new set of bioassays with t he remaining samples is in progress.