Biotypes of smallflower umbrella sedge and ricefield bulrush resistant to acetolactate synthase (ALS)-inhibiting herbicides have been reported in several rice areas of the world. Here, we present results of a study conducted on whole plants of seven smallflower umbrella sedge and four ricefield bulrush biotypes collected in Italian, Spanish, and Californian rice fields to evaluate cross-resistance to ALS herbicides in these important weeds of temperate rice. The following herbicides were tested: bensulfuron-methyl, halosulfuron, cinosulfuron, imazamox, and bispyribac-sodium. The smallflower umbrella sedge and ricefield bulrush biotypes studied exhibited different cross-resistance patterns, some of which have not been previously reported. The Italian smallflower umbrella sedge biotype was cross-resistant to bensulfuron-methyl, cinosulfuron, imazamox, and bispyribac-sodium, but was susceptible to halosulfuron. One smallflower umbrella sedge biotype from California was also resistant to bensulfuron-methyl, imazamox, and bispyribac-sodium, but had a lower level of resistance to halosulfuron. In contrast, the second smallflower umbrella sedge biotype from California was strongly resistant to halosulfuron and was also resistant to bensulfuron-methyl and bispyribac-sodium, but moderately resistant to imazamox. The Spanish smallflower biotype was resistant to the sulfonylurea herbicides bensulfuron-methyl and halosulfuron. Different responses were observed in ricefield bulrush. The Italian biotype was resistant to the sulfonylureas only, whereas the biotype from California exhibited broad cross-resistance to all the ALS herbicides tested. Knowledge on cross-resistance is needed to formulate herbicide use and weed management strategies for delaying the evolution of resistance to ALS herbicides in rice systems.
Nomenclature: Bensulfuron-methyl, halosulfuron, cinosulfuron, imazamox, bispyribac-sodium, smallflower umbrella sedge, Cyperus difformis L. #3 CYPDI, ricefield bulrush, Schoenoplectus mucronatus (L.) Palla (syn. Scirpus mucronatus L.) # SCMPU.
Additional index words: ALS herbicides, herbicide resistance, cross-resistance, weed management, temperate rice, rice production systems.
Abbreviations: SU, sulfonylureas; IMI, imidazolinones; TP, triazolopyrimidines; PB, pyrimidinylbenzoates; SCT, sulfonylaminocarbonyltriazolinones; FW, fresh weight; GR50, herbicide dose necessary to cause 50% growth reduction; I50, herbicide dose necessary to cause 50% inhibition; LD50, herbicide dose necessary to cause 50% mortality; R, resistant; RI, resistance index; S, susceptible.
