Determination of optimal timing of 2,4-dichlorophenoxyacetic acid foliar applications for common scab control in potato

Application of 2,4-dichlorophenoxyacetic acid (2,4-D) has been shown to be an effective control method for common scab of potato. Prior research targeted applications at or shortly after tuber initiation during the period of known susceptibility, with later treatment providing little or no control. This study examined the effect of a range of application dates and the influence of multiple applications at low and high rates on common scab control in both Russet Burbank and Desiree cultivars. We show that applications as early as 5 days after average plant emergence gave the greater disease control for both cultivars. These early treatments provide sufficient material to the tuber to induce resistance that lasts throughout the tuber susceptibility period with no requirement for, or benefit of repeated applications. Agronomic assessments suggested minor effects on total tuber mass produced with 2,4-D treatment but these were not consistent. Effective treatments result in levels of 2,4-D at harvest below the Australian maximum residue limit.