Tuber soft rot and concentrations of Erwinia spp. in potato washing plants in South Australia

Abstract

Potato tubers and water samples from four washing plants in South Australia were collected on multiple occasions between 1998 and 2002 to determine the main source of bacterial soft rot infection on tubers and the level of Erwinia spp. in wash water. Incubating tubers in an environment conducive to soft rot showed that an average of 26% of tubers collected from the field developed soft rot. This increased to 64% or more when tubers were immersed in water. The incidence and severity of soft rot increased further after tubers were placed in a rotating tumbler to remove adhering soil. Concentrations of Erwinia spp. around 10⁴ colony forming units (cfu)/mL and occasionally 10⁶ cfu/mL were found in wash water collected at various sites in the washing plant and from ponded recycled water. The most severe amount of rotting developed when tubers were immersed in Erwinia spp. concentrations of 10⁴ cfu/mL or greater, suggesting that frequent replacing of potato wash water with clean water to reduce the levels of bacteria may be a useful method of managing the problem.