Enumeration of plant pathogenic Streptomyces on postharvest potato tubers under storage conditions

Abstract

To determine the potential for spreading pathogenic Streptomyces on seed tubers into field soil, we enumerated populations of pathogens on diseased and healthy tuber surfaces. Since nonpathogenic Streptomyces strains are indistinguishable from pathogenic ones by colony morphology, we used colony polymerase chain reaction to detect txtA, a gene of the thaxtomin biosynthetic pathway. Of the total Streptomyces spp. isolated from the tubers of potato ‘Yukon Gold’, 99% were pathogenic from scabby lesions, whereas only 12% were pathogenic from visually healthy surfaces of scabby or scab-free tubers. The density of pathogenic populations declined from 10⁹ to 10⁸ colony-forming units per gram of lesion tissue after storage of tubers for 18 weeks. Populations also declined from 10⁵ to 10³ colony-forming units per gram of tissue on healthy surfaces of scabby tubers and scab-free tubers after storage. Scabby tubers obtained from growers, prior to spring planting, also harbored large populations of pathogenic strains.

Keywords:

• potato scab
• Streptomyces
• tuber-borne pathogen
• polymerase chain reaction
• txtA gene