WEED MANAGEMENT STRATEGIES IN POTATO AND ONION PRODUCTION IN THE PACIFIC NORTHWEST

Diana L Velasco

Integrated weed management (IWM) strategies are essential for maximizing crop productivity while minimizing herbicide-induced crop injury. The study evaluated the efficacy and crop safety of herbicide programs targeting yellow nutsedge (Cyperus esculentus L.) in potato (Solanum tuberosum L.) and onion (Allium cepa L.) production systems in the Pacific Northwest (PNW). In a field trial conducted in Pasco, WA, and Ontario, OR, preemergence (PRE)-only and PRE fb postemergence (POST) programs combining EPTC (S-ethyl dipropylthiocarbamate), S-metolachlor, metribuzin, and rimsulfuron consistently controlled yellow nutsedge over 90% season-long with control levels of 93% in WA and 96% in OR. Crop injury was minimal (0 to 0.5% in WA), and no injury was detected in OR; and in both locations, injury did not reduce yield. Despite comparable levels of weed control, marketable tuber yield was higher in OR (58.5 t/ha) than in WA (47.5 t/ha), suggesting that environmental factors such as soil properties and climate may have had a greater influence on tuber production than herbicide effects. Greenhouse and field studies in onion production evaluated bicyclopyrone-based herbicide programs. Crop injury was rate-dependent and most severe when applications were made at the 75% radicle emergence stage, with injury ranging from 5% to 40% in mixed soils and 18% to 94% in sandy soils. In contrast, onions at the 4-leaf stage showed less susceptibility with no visible crop injury in mixed soils and only 10% in sandy soils, even at the highest bicyclopyrone rates, whereas plants treated at the 1- and 2-leaf stage averaged 20% injury in mixed soils and 43% in sandy soils. Pendimethalin delayed-preemergence (DPRE) fb bicyclopyrone at 38 g a.i./ha plus bromoxynil program achieved 6% - 17% crop injury and produced total and marketable yields of 26 tons/ha; the yields obtained in the study remained lower than yields reported in prior research (up to 100 tons/ha when applied at the 2–4 leaf stage), likely due to differences in organic soil matter and environmental conditions. Overall, the research highlights the importance of site-specific agronomic considerations when designing herbicide programs. Programs composed exclusively of labeled herbicides—such as S-metolachlor, EPTC, metribuzin, and rimsulfuron—were both effective and crop-safe in potatoes, offering practical options for growers. However, the repeated use of these same modes of action may increase the risk of herbicide resistance, particularly in persistent weed species like yellow nutsedge. Therefore, further research is essential to evaluate additional chemistries and integrated strategies that can diversify weed control programs and sustain long-term efficacy in potato systems. In onions, the success of bicyclopyrone integration depends on precise timing, soil conditions, and formulation strategies. Further research should explore POST-only programs, varietal tolerance, and adjuvant effects to improve reliability and expand weed control options in onion systems.

WEED MANAGEMENT STRATEGIES IN POTATO AND ONION PRODUCTION IN THE PACIFIC NORTHWEST

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