CRITICAL PERIOD OF QUACKGRASS (ELYTRIGIA-REPENS) REMOVAL IN POTATOES (SOLANUM-TUBEROSUM)

Field trials were carried out in 1989 and 1990 at St-Augustin, near Qu ebec City, Canada, to determine the critical periods of quackgrass con trol in potato submitted to three levels of infestation. Potato yield losses due to quackgrass interference increased with quackgrass infest ation and length of duration of interference. Quackgrass interference influenced marketable tuber yields more than total tuber yields. Durat ion of the critical period varied depending on the level of quackgrass infestation and year. Based on an arbitrary 5% level of marketable yi eld loss, the critical period started at ca. 15 days after emergence ( DAE) of potato at low level of infestation, and at ca. 3 DAE at medium level of infestation. At high level of infestation, the critical peri od began prior to the emergence of potato. The end of the critical per iod of quackgrass removal was extremely variable across quackgrass inf estation level and year and ranged from 23 to 68 DAE of potato at a 5% yield loss level. It appears that onset of interference varied less t han the end of it, indicating that early quackgrass control is necessa ry to prevent yield loss.