Resistance to ALS inhibitors in rice in north-western Italy

In Europe, 10 weed species have developed resistance to acetolactate synthase (ALS) inhibiting herbicides. Currently, the most serious problems with ALS 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 resistance. The first cases were reported in 1995 and it is now estimated that about 15,000 ha are affected. In greenhouse experiments a total of 53 populations, 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 triazolopyrimidine: metosulam) sprayed at three times the recommended field dose. Only three populations of A. plantago-aquatica and six of S. mucronatus still appeared to be susceptible to all herbicides. Three populations (one susceptible and two resistant) of each species were then used in two dose-response experiments with two herbicides (bensulfuron-methyl and metosulam) and 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, with a generalised cross-resistance to all the ALS inhibitors used in rice crops in Italy. The resistance level to the triazolopyrimidine herbicide appears to be lower than that found for the four sulfonylurea herbicides. The available information indicates that an insensitive target site is the resistance mechanism in both species for all the herbicides tested.