Clopyralid and Dicamba Residue Impacts on Potatoes and Weeds

Clopyralid and dicamba are used in Alaska to control certain invasive and agricultural weed species; however they may have an extended soil half-life in interior Alaska resulting in carry-over injury in potatoes. Field studies at experiment stations in Delta Junction, Fairbanks, and Palmer, Alaska were established to determine the dose-response of weeds and above and below ground potato growth to soil-applied clopyralid or dicamba (0, 35, 70, 140, 280, and 560 g ae ha(-1)). Both Norwegian cinquefoil (Potentilla norvegica) and narrowleaf hawksbeard (Crepis tectorum) were susceptible to clopyralid with over 90 % control in Delta Junction; whereas only flixweed (Descurainia sophia) was partially controlled (70 %) with dicamba. In Palmer narrowleaf hawksbeard was controlled (87 %) with clopyralid. At Delta Junction and Fairbanks, clopyralid applied at140 g ae ha(-1) injured potatoes greater than 25 %, whereas at Palmer visual injury was greater than 25 % at 70 g ae ha(-1). Above ground dicamba injury was greater than 25 % at 140, 70, and 35 g ae ha(-1) at Delta Junction, Fairbanks and Palmer, respectively. Potato tuber production was reduced by clopyralid at rates of 35 and 140 g ae ha(-1) at Delta Junction and Palmer, respectively. At Delta Junction, dicamba did not reduce potato tuber production, however in Palmer, dicamba rates at 70 g ae ha(-1) and greater reduced potato tuber production more than 50 %. Sub-samples of potato tubers from Delta Junction and Palmer were grown out to determine if clopyralid and dicamba content in tubers would reduce subsequent growth. Dicamba at rates of 140 g ae ha(-1) or greater injured plants grown from daughter tubers and reduced shoot height, but had no effect on the number of emerged shoots. Clopyralid at all rates injured plants that emerged from daughter tubers and injury increased with increasing rate. At Palmer, clopyralid in daughter tubers rates at 140 g ae ha(-1) or greater reduced shoot height and at 280 g ae ha(-1) or greater reduced shoot number. At Delta, clopyralid in daughter tubers reduced shoot height at 280 and 560 g ae ha(-1), but had no effect on shoot number.