Resistance of Rotala indica Koehne var. uliginosa Koehne to sulfonylurea herbicides

The present paper aims to quantify the degree of resistance development in Rotala indica Koehne var. uliginosa Koehne to sulfonylurea (SU) herbicides based on whole plant responses. Seeds of resistant (from Omagari City, Akita Prefecture, Japan) and susceptible (from Tsukuba City, Ibaraki Prefecture, Japan) R. indica plants were seeded in 200 cm² pots. The plants, at the 1.5 leaf stage, were blanket-applied with different rates of three sulfonylurea herbicides: bensulfuron methyl (BSM), pyrazosulfuron ethyl (PSE) and imazosulfuron (ISN). Of the three herbicides, the weed appears to have developed the highest degree of resistance to BSM. The resistant/susceptible (R/S) values based on mortality, root length and dry weight are consistently highest at 197, 157 and 101, respectively. Responses based on shoot length, however, showed a higher R/S value for PSE. Roots of R. indica are more sensitive to higher rates of SU herbicides than the shoots. Indeed, a high degree of resistance to SU herbicides have evolved in R. indica , for which the recommended field rates are not enough for effective control of the weed.