Fátima Rocha

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 Sorraia 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 results from single dose bioassays for resistance confirmation, using 98 samples (55 soil samples and 43 seed samples). Thirty six new resistant populations were confirmed. The Tagus and Sorraia valleys were the most affected areas with 23.8% of the total fields having Alisma plantago-aquatica populations resistant to bensulfuron-methyl, followed by the Sado and Caia, and Mondego valleys with 7.3% and 1.2%, respectively. A new set of bioassays with the remaining samples is in progress.