Potato Blackleg
Potato blackleg, Erwinia carotovora, is a common bacterial disease of potatoes, especially in wet seasons. It can be a particular problem on potatoes grown through a thick straw or hay mulch. It can occur at any stage of plant development.
Typical symptoms
Symptoms appear early in the season, before the potato haulms meet across the rows. Affected plants will be stunted and scattered throughout the crop.
- Foliage: Leaves turn pale green or yellowish. Margins of upper leaves roll inwards. Later, leaves turn brown and the haulm dies. Early attacks may result in non-emergence of foliage.
- Stems: Turn brown or black approximately 10cm above and below soil level. During wet periods, or as the season progresses, this rot darkens, becoming wet and soft. Stems can be easily pulled out. Black streaks may be found higher up the stems; the plants wilt and collapse rapidly. Not all the stems on one plant are necessarily affected.
- Tubers: A black rot can extend from the heel end or from lenticels (the breathing pores of the tuber skin). In bad attacks the tubers may all decay in the ground. More commonly, the tubers appear sound on lifting, then rot in storage. Infection will spread between tubers in store.
Life cycle
The main source of infection is the planting of infected tubers; even certified seed can contain a blackleg. Infected tubers can be free of any symptoms. Healthy tubers can be infected from diseased ones that have rotted in the soil, or in store. The bacteria can migrate through moist soil and can be found in potato debris, but these are unlikely to be important sources of infection. The disease is unlikely to spread from plant to plant during the life of the crop.
Prevention and control
There is no way of controlling this disease once a plant is infected. Some approaches can be taken to reduce risk of infection, but this is not easy.
- Storage: Do not store any tubers which show signs of rot or damage, or that have come into contact with rotting tubers. Check through sacks of stored tubers regularly and remove any that have rotted.
- Cultural control: Never save tubers for replanting from an infected crop. Avoid waterlogged and wet sites. Remove ‘volunteer’ potatoes that have survived from the previous season. Do not cut seed potatoes up prior to planting as this increases the likelihood of tubers becoming infected
- Resistant varieties: Choose varieties that are less susceptible. These include Osprey, Kestrel, Merlin, Saxon and Stroma. Avoid very susceptible ones such as Estima, Wilja, Epicure and Maris Bard. No varieties show complete resistance to the disease.
- Harvesting: Lift crops during dry weather if possible. If not, allow the tubers to dry out before storing. For more information on storage conditions, see Garden Organic factsheet, GG8 Storing the Harvest.
Related Garden Organic factsheets: GG5 Growing potatoes; GG16 Growing potatoes in containers; PC24 Potato pests and diseases; GG2 No-dig gardening
A useful website is the British Potato Council at www.potato.org.uk.
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