Resistance to ALS inhibitors in weeds of rice in north-western Italy

In Europe, 10 weed species have developed resistance to acetolactate syntha se (ALS) inhibiting herbicides. Currently, the most serious problems with A LS resistance are found in paddy rice. The situation in Italy is presented: two weed species (Alisma plantago-aquatica and Scirpus mucronatus) in rice , that are among the most sensitive to ALS inhibitors, have developed resis tance. The first cases were reported in 1995 and it is now estimated that a bout 15,000 ha are affected. In greenhouse experiments a total of 53 popula tions, collected from rice fields where weed control by ALS inhibitors was unsatisfactory, were screened with five ALS inhibitors (four sulfonylureas: azimsulfuron, bensulfuron-methyl, cinosulfuron. ethoxysulfuron; one triazo lopyrimidine: metosulam) sprayed at three times the recommended field dose. Only three populations of A. plantago-aquatica and six of S. mucronatus st ill appeared to be susceptible to all herbicides. Three populations (one su sceptible and two resistant) of each species were then used in two dose-res ponse experiments with two herbicides (bensulfuron-methyl and metosulam) an d eight doses ranging from 0 to 64 times the normal field dose. The results indicate that the resistance situation for the two species is similar, wit h a generalised cross-resistance to all the ALS inhibitors used in rice cro ps in Italy. The resistance level to the triazolopyrimidine herbicide appea rs to be lower than that found for the four sulfonylurea herbicides. The av ailable information indicates that an insensitive target site is the resist ance mechanism in both species for all the herbicides tested.