Schoenoplectus mucronatus (L.) Palla and Cyperus difformis L. accessions resistant to ALS-inhibitors in italian rice fields

Sulfonylureas and other ALS-inhibitor herbicides have been used in Italian paddy fields for more than 10 years. Since 1995, lack of control has been reported for Alisma plantago-aquatica L. and Schoenoplectus mucronatus (L.) Palla. Starting from 1999, insufficient efficacy of ALS-inhibitors herbicides have been reported also against Cyperus difformis L. A dose-response study has been carried out in the greenhouse to estimate the sensitivity of different S. mucronatus and C. difformis accessions to bensulfuron-methyl and cinosulfuron. Two S. mucronatus (RS1 and RS2) and one C. difformis (RC1)accessions that escaped field treatments were sprayed either with bensulfuron-methyl or cinosulfuron at 8 doses, from 0 to 4x the recommended field rate (100 g a.i.ha-1 and 80 g a.i.ha-1 for bensulfuron and cinosulfuron, respectively). Two additional accessions were also included as sensitive references (SS and SC, for S. mucronatus and C. difformis, respectively). Seeds of both species were placed on 36 cm2 pots filled with commercial potting substrate. Prior to seeding, S. mucronatus seeds were scarified with sulphuric acid for 5’. Treatments were applied at 4-leaf stage. Aboveground fresh weight per pot was determined 15-20 days after spraying. In S. mucronatus, the rate determining 50% of growth reduction (GR50) referred to bensulfuron-methyl and cinosulfuron was more than 9,000 and about 100 times higher, respectively, than that of SS, both in RS1 and RS2. In C. difformis, the GR50 of RC1 referred to bensulfuron-methyl was higher than 4x. In the case of cinosulfuron, GR50 of RC1 was more than 7000 times higher than that of SC. The accessions of both species show a variable degree of resistance both to bensulfuron-methyl and cinosulfuron, which are widely used in Italian rice fields. Further investigations are needed to understand the mechanism of resistance.