Use of Two Sulfonyl Urea Herbicides in Lowbush Blueberry

SUMMARY The continuous use of the broad spectrum herbicide hexazinone since 1981 has resulted in many changes to the weedy flora of lowbush blueberry fields, including shifts to hexazinone-tolerant species. Many of these occur in patches and could best be controlled by selective, foliar herbicide treatments. Preliminary assessments of several sufonyl urea (SU) herbicides indicated that tribenuron (Spartan or Express 75% DF) was effective against some important weed species with little risk to the crop. Long-term control (> 95%) of eastern bracken [Pteridium aquilinum (L.) Kuhn], yellow loosestrife [Lysimachia terrestris (L.) BSP], speckled alder [Alnus incana (L.) Moench], common wild rose (Rosa virginiana Mill.) and several others was obtained with mid summer, prune-year foliar applications of tribenuron at 0.2 g L_1 water with 0.2% Agral 90. Screening trials targeting black bulrush (Scirpus atrovirens Willd.), currently the most serious weed problem in Nova Scotia blueberry fields, indicated that SU's with activity against grasses were also effective against this weed, and a 1:1 commercial mixture of nicosulfuron/rimsulfuron (Ultim DF) was selected for further work. Summer and fall prune-year applications of nicosulfuron/rimsulfuron at 0.031 g L_1 water with 0.2% Agral 90 generally provided > 90% control, except when plants were stressed by drought. Although blueberry plants were injured (< 20% injury) when sprayed directly with these SU herbicides, there was little effect on plants in the understory following applications to weeds. Both herbicides have become registered for use in lowbush blueberry in Canada for control of the above species.