Tuber Necrosis Following the Desiccation of Potato Foliage With Diquat

Summary. Necrosis at the stem‐end of potato tubers following the application of diquat to the foliage is rare and has occurred mainly in seasons of low rainfall. Damage in the field has been associated with tuber residues of diquat of about ten times the normal level and the extent of damage has shown a linear relationship with the concentration of chemical in the tubers. Normally an increase in the dose of diquat applied causes only a slight increase in amount of chemical in the tubers, but under conditions where damage occurred, tuber residues increased markedly with increase in application rate. The factors influencing diquat residues in potato tubers were examined in pot experiments. Highest residues were found when plants were subjected to drought and to a period of darkness following spraying at a time when senescence of the foliage had commenced and the tubers were still increasing in fresh weight. Further experiments at a similar stage of growth showed that high residues of diquat caused stem‐end rot only when plants were subjected both to a soil moisture stress sufficient to induce wilting before spraying and to reduced light intensity and a high atmospheric humidity after spraying. It is suggested that these conditions promote the rapid downward movement of a high concentration of diquat in the xylem. In a preliminary experiment with a range of chemical desiccants, only sodium arsenite gave appreciable tuber damage when applied to the foliage.