The Influence of Environment on Potato Haulm Kill by Diquat and on Its Residues in the Tubers

Summary. The effect of various environments on the degree of desiccation and of residues in King Edward and Majestic potatoes was studied. The higher residues frequently associated with the variety King Edward are ascribed from glasshouse experiments to greater diquat movement; they could not be accounted for by differences in uptake between the two varieties. In glasshouse experiments, uptake was increased both by high air humidities and reduced light intensities before and after treatment. Since earlier work with cocksfoot and wheat indicated that dry soil conditions markedly increased movement, it is concluded that dull, humid air conditions associated with dry soil would be most conducive to efficient haulm desiccation and also to highest residues in the tubers. Late afternoon or evening treatment often provides thes conditions, and residues were in fact least from morning treatment. In the field, increases in haulm desiccation were associated with reductions in residues. As they were observed only at light intensities below 5000 lux, they are unlikely to occur in normal dull weather. Rain falling soon after spraying did not reduce the extent of haulm desiccation or diquat residues. Influence de I'enviromurtunt sur la destruction des fanes de pomme de terre par le diquat el sur ses