What are they? Plant pathogenic bacteria cause a number of important potato diseases: Potato brown disease is caused by Ralstonia solanacearum. Initial symptoms in the field are wilting of the upper leaves followed by permanent wilting and death of the plant. Brown discolouration of the vascular tissues extends upwards from the stem base. In tubers, initial browning of the vascular ring at the stolon end eventually extends and becomes necrotic with secondary rot development. Cut stems and tubers exude a creamy white bacterial slime from the vascular tissues. Bacterial ooze may exude from the eyes and heel end of affected tubers. Ring rot disease is caused by Clavibacter michiganensis subsp. sepedonicus .I t initially appears in the field as wilting of the margins of lower leaves. Sectors of wilted and chlorotic tissue usually remain delimited by leaf veins and the plant is rarely killed. Vascular bundles of tubers become watersoaked and semi-translucent starting from the stolon end and a cheese-like rot of the tissue develops. Severe rotting and cracking of the tubers can develop. Enterobacterial pathogens Pectobacterium atrosepticum (syn. Erwinia carotovora subsp. atroseptica), P. carotovorum subsp. carotovorum (syn. Erwinia carotovora subsp. carotovora) and Dickeya spp. (syn. Erwinia chrysanthemi) cause soft rotting and/or vascular wilting of potato stems and soft rotting of tubers. Under wet field conditions in temperate climates, a blackened soft rot of the stem base extending from the rotting seed tuber, known as blackleg disease, is usually caused by P. atrosepticum. Several Dickeya spp. can cause disease on potato, D. dadantii and D. zeae are important in warmer regions of the world, whereas D. dianthicola appears to be more important in cooler European climates.