Production of the phytotoxin thaxtomin A by pathogenic Streptomyces spp. is essential for induction of common scab disease in potato. Prior studies have shown that foliar application of sublethal concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) and other auxin or auxin-like compounds significantly reduced severity and occurrence of common scab in subsequently produced tubers. However, the means of disease suppression by these compounds was not known. We confirm the disease suppressive activity of 2,4-D. Detailed tuber physiological examination showed that lenticel numbers, lenticel external dimensions, and periderm thickness and structure, physiological features believed to be critical to Streptomyces scabiei infection, were not substantially changed by 2,4-D treatments, negating a possible mechanism for disease suppression through alteration of these structures. In contrast, our studies show accumulation of 2,4-D in tubers of treated plants occurs and is associated with an enhanced tolerance to thaxtomin A. Applying 2,4-D to cultures of S. scabiei did not significantly alter in vitro growth of the pathogen. Thaxtomin A production by the pathogen was inhibited by 2,4-D, but only at the highest rate tested (1.0 mM), which is at least 200-fold more than is found in 2,4-D treated tubers. These data suggest 2,4-D has no direct effect on the pathogen or its virulence. Confirmatory evidence from studies with Arabidopsis thaliana seedlings demonstrated that the auxins 2,4-D and IAA ameliorate thaxtomin A toxicity. The evidence presented whereby auxin treatment inhibits toxicity of thaxtomin A secreted by the pathogen suggests a novel indirect means of disease suppression.

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.

No abstract

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.

Prior studies have shown that applications of the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) to the foliage of potato plants can reduce common scab. Here field and glasshouse trials suggest that 2,4-D foliar treatments may also reduce the biologically distinct tuber disease, powdery scab. Significant correlations between suppression of common and powdery scab from the field trials suggested an interaction between the two diseases or possible additional broad spectrum mechanisms of enhanced defence against pathogen invasion provided by 2,4-D treatment.