Potato brown rot is caused by the quarantine bacterium, Ralstonia solanacearum. R. solanacearum is a genetically diverse and geographically widespread plant pathogen. It has a wide host range and is a significant pathogen of potato, causing brown rot. Brown rot is caused by a distinct, closely-related, intraspecific group: race 3/biovar 2A, more recently referred to as phylotype IIB, sequevar 1 (IIB1). In Europe, infection of potato crops with brown rot primarily occurs through the movement of infected seed, though irrigation with contaminated surface water is also important. Brown rot has never been found in Scottish potatoes though the bacterium has been found previously in one Scottish river system, the River Tay, both in water samples and on its secondary host, bittersweet (Solanum dulcamara), growing on the river banks.
Although brown rot can cause wilting of the potato plant, it is unlikley under cooler Scottish conditions. Symptoms are more likley to manifest themselves in the tuber, where bacterial ooze is evident in the vascular ring, and as symptoms develop can lead to a brown discolouration, hence the name ‘brown rot’. In Europe the vast majority of postive findings are made by laboratory tests of asymptomatically infected tubers.