Symptom expression occurs at different rates in different cultivars and is affected by temperature and other environmental conditions. Some cultivars may only rarely express symptoms.
Foliar symptoms are fairly typical of a vascular wilt and generally occur late in the season. They usually start as a wilt of the lower leaves which may involve some leaf rolling. Areas between the leaf veins eventually become chlorotic and the leaf margins necrotic. Symptoms can be difficult to distinguish from those of other diseases and other crop damage, symptoms can also be masked by the natural senescence of the crop. Symptoms are much more likely to be observed in tubers. Symptoms are somewhat similar to those of brown rot, but the initial discoloration of the vascular ring is usually glassy and water-soaked rather than brown, and the ooze from the ring is cheese- or cream-like. Further disease development leads to tuber breakdown with internal hollowing. More rarely, in severe cases external symptoms are observed in which the skin of the potato may show slightly sunken, dry and cracked patches and tubers may mummify. Infected tubers may be invaded by secondary pathogens and this leads to complete breakdown.
Symptomless or latently infected seed tubers are the primary source of disease. Some cultivars are partially resistant and may not show symptoms of the disease, particularly during the first year of infection. Infected tubers of some partially resistant varieties may appear healthy and bacteria can multiply during storage. Infected plants, whether or not they display symptoms, are carriers of the disease.
The bacteria can survive in a dessicated state and remain infectious for a number of years on storage materials, equipment etc.. Infection can spread from diseased to healthy tubers by direct contact particularly if the latter are damaged, from contaminated wash water or by contact with contaminated equipment.
In the UK control is through exclusion of the disease and the disease is treated as a quarantine pest in the European Union.
If you see any of the symptoms described above you must immediately contact your local Defra Plant Health and Seeds Inspector or the PHSI HQ, York:
Tel: 01904 455174
Or, in Scotland, you must contact your local RPID Area Office or AFRC HQ, Edinburgh:
Tel: 0131 244 9692
Fax: 0131244 6777
European and Mediterranean Plant Protection Organization (EPPO):