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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.

Item Type:	Article
Authors/Creators:	Tegg, RS and Gill, WM and Thompson, HK and Davies, NW and Ross, JJ and Wilson, CR
Journal or Publication Title:	Plant Disease
ISSN:	0191-2917
DOI / ID Number:	10.1094/PDIS-92-9-1321
Additional Information:	Copyright © 2008 The American Phytopathological Society
Item Statistics:	View statistics for this item

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