HERBICIDE CROSS-RESISTANCE IN RANUNCULUS-ACRIS L SUBSP ACRIS

The responses of seed-derived progenies of an MCPA-resistant (R) and a n MCPA-susceptible (S) population of Ranunculus acris to a range of do ses of MCPA, 2,4-D, MCPB, chlorsulfuron, and thifensulfuron were compa red under glasshouse conditions. In a second experiment, also under.gl asshouse conditions, the responses of R and S to the usual field doses of tribenuron-methyl, bentazone, glyphosate, clopyralid, and asulam w ere compared. R was cross-resistant to both 2,4-D and MCPB. There was temporary cross-resistance to chlorsulfuron and thifensulfuron manifes t in slower development of apical mortality in R plants but all R and S plants ultimately died at the usual field dose rates of these two he rbicides. There was weak evidence of cross-resistance to bentazone but no evidence of resistance to either tribenuron-methyl or glyphosate s ince both resulted in 100% mortality of R and S at usual field dose ra tes. Clopyralid and asulam had no effect on either R or S. The results indicate that neither 2,4-D nor MCPB will be useful for killing MCPA- resistant R. acris, except at very high dose rates but that glyphosate and the sulfonylureas tribenuron-methyl, chlorsulfuron, and thifensul furon should provide a high level of kill at usual dose rates. The cro ss-resistances imply that the MCPA-resistance mechanism in R. acris al so confers resistance to MCPB and 2,4-D and a lower, temporary resista nce to the sulfonylureas chlorsulfuron and thifensulfuron. The nature of the resistance mechanism is not revealed by this study.