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A preliminary report on the potential resistance of a soapbush (Clidemia hirta (L.) D. Don) biotype to metsulfuron-methyl in an oil palm plantation in Jerantut, Malaysia

The present study was undertaken to confirm the presence of a metsulfuronmethyl resistant (R) biotype of Clidemia hirta (L.) D. Don in an oil palm plantation in Jerantut, compared to a susceptible (S) biotype found in Jengka, Pahang, Malaysia. The first study was conducted as an on-site field spraying experiment using several concentrations of metsulfuron- methyl, whereas the second was carried out in the laboratory using 10 mm diameter leaf discs immersed in several metsulfuron-methyl concentrations. The third study was done in a greenhouse where the growth of the R and S biotypes were observed under two different light intensities. For the on-site field experiment, spraying at the recommended dosage [30 g active ingredient (a.i.) ha-1] gave 100% control of the S biotype, whereas the R biotype showed full recovery (0% control) 56 days after treatment with metsulfuronmethyl at levels as high as eight times (240 g a.i. ha-1) the recommended rate. From the comparison of the ED50 of the leaf discs, the laboratory test showed that the resistant biotype was 16-times more resistant to metsulfuron-methyl than the susceptible biotype. The greenhouse experiment indicated that, relatively, the R biotype expressed slower growth in the upper part growth measurements in plant height, total leaf area, stem mass ratio and relative biomass growth ratio but in contrast, showed greater allocation to root mass compared to S biotype. Both biotypes of C. hirta planted under high light intensity showed slower growth in plant height, relative stem length growth rate, total leaf area, leaf area ratio, leaf mass ratio and stem mass ratio, but allocated more mass to root and relative biomass growth rate.