Authors
Robert S. Tegg,
Warwick M. Gill, and
Hannah K. Thompson, Tasmanian Institute of Agricultural Research, University of Tasmania, New Town Research Laboratories, 13 St. John's Avenue, New Town, Tasmania 7008, Australia;
Noel W. Davies, Central Science Laboratory, University of Tasmania, Private Bag 74, Hobart, Tasmania 7001, Australia;
John J. Ross, School of Plant Science, University of Tasmania, Private Bag 55, Hobart, Tasmania 7001, Australia; and
Calum R. Wilson, Tasmanian Institute of Agricultural Research, University of Tasmania, New Town Research Laboratories, 13 St. John's Avenue, New Town, Tasmania 7008, Australia
ABSTRACT
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.
