ECONOMIC AND INTERFERENCE THRESHOLD DENSITIES OF QUACKGRASS (ELYTRIGIA REPENS) IN POTATO (SOLANUM-TUBEROSUM)

Field studies were conducted in 1989 and 1990 at St-Augustin, Quebec, Canada, to determine the economic threshold density of quackgrass in p otato. Potato yield losses due to quackgrass interference increased wi th quackgrass population density Potato yield ranged from 33 to 73% in 1989, and from 19 to 44% in 1990. The relationship between potato yie ld losses and quackgrass densities was described by a rectangular hype rbolic function. Dry weight of quackgrass proved to be the best predic tor of potato yield loss compared to shoot number. The interference th resholds for 10% potato yield loss amounted to 25 shoots m(-2) or 20 g total dry biomass m(-2). The economic threshold varied between 0.04 a nd 2 shoots m(-2) or 0.0165 and 1.5 g total dry biomass m(-2), dependi ng on the variables considered.