Optimal rates of 2,4-dichlophenoxyacetic acid foliar application for control of common scab in potato
Applications of the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) to the foliage of potato plants can reduce common scab, a tuber disease. However, in prior research effective applications at 200 mg L−1 2,4-D resulted in phytotoxic side effects with reduced tuber yield and quality. This study showed that minimal significant threshold rates from 8.3 to 23.6 mg L−1 2,4-D reduced disease incidence in pot trials, and from 10.8 to 41.0 mg L−1 minimised disease severity in both pot and field trials. In only one pot trial, significant phytotoxicity was found with rates of 100 mg L−1 or greater, reducing mean total tuber mass per plot and 38 mg L−1 or greater, reducing mean mass per tuber. Notably, within the field trial, a more reliable plant growth system for estimation of yield, no significant impacts were observed. Disease control was associated with decreased sensitivity of tubers to thaxtomin A, the phytotoxin produced by the common scab pathogen essential for disease induction. The amount of residual 2,4-D in tubers at harvest varied with cultivar, Russet Burbank accumulating more 2,4-D than Desiree. Application rates less than 100 mg L−1 resulted in levels of 2,4-D below the Australian standard maximum residue limit. These data suggest that applications of 2,4-D at low rates could provide a commercially suitable control strategy for common scab.