Ring rot of potatoes (C. michiganensis)

Ring rot of potatoes is caused by the bacterium Clavibacter michiganensis subsp. sepedonicus.


Ring rot causes a vascular wilt in the above ground parts of the potato plant, with discolouration and loss of texture/consistency noted in infected tubers. Losses of up to 50% of the crop have been noted in severe infections.


Ring rot is a major threat to the potato growers and potato seed producers in Northern Ireland and the rest of Western Europe. It is listed as a notifiable quarantine pathogen in the EU plant health legislation 93/85/EC (as amended), and national legislation is in place in Northern Ireland to prevent its entry (The Plant Health Order (Northern Ireland) 2006 (as amended)). The major yield loss caused by ring rot is through rotting of tubers. If the pathogen was to become established in Northern Ireland, not only would our potato industry suffer direct yield losses but the knock-on effect on exports from our seed-potato industry could be substantial. Once established, the costs of control could be very high.

Control of this disease requires vigilance by all sectors of the industry; from growers through to merchants, packers and retailers.

Northern Ireland Risk register rating

Organism Hosts NI Risk Rating
Clavibacter michiganensis subsp. sepedonicus Solanum 125

Susceptible species

Ring rot mainly only affects potato, although asymptomatic infection of sugarbeet has been recorded. Other subspecies of Clavibacter michiganensis affect other crops, such as Clavibacter michiganensis subsp. michiganensis causing ‘birds eye’ in tomatoes.


Important means of spread are the planting of infected seed potatoes and the use of contaminated containers, equipment and premises. Planters and graders which have been contaminated by the pathogen from a few badly diseased potatoes are also a potent infection source. Spread in the field from plant to plant is usually very low, but there is experimental evidence that some insects can transmit the disease.

Outbreak stage and national plans

Ring rot is not present in Northern Ireland or Ireland. The first finding of Ring rot in Britain was in November, 2003 in a Welsh seed potato crop planted with seed produced in the Netherlands. The pathogen is now considered eradicated and absent from Britain. Ring rot could easily become established under our conditions because it favours cool climates.


The disease can cause plants to wilt, but you are more likely to come across symptoms in tubers. The initial symptom is a soft cheese-like rotting of the vascular ring, hence the name "ring" rot. In severe cases this completely rots and the skin of the potato may crack. Disease spread is largely by vascular infection of daughter tubers derived from infected seed. The bacterium is thought to be unable to overwinter in the soil but will do so in groundkeepers and debris from infected crops.

Infected groundkeepers lifted at the same time as an otherwise clean seed crop, can infect that crop. Bacteria can also survive and remain infectious on potato bags, barn walls, machinery and other equipment that have been contaminated by rotting ooze and, although this is not the main means of disease transmission, it can make eradication of the disease very difficult. The return of contaminated potato waste to fields is a possible source of infection.

Symptoms on potato tubers may be confused with those caused by brown rot (causal agent Ralstonia solanacearum). The two may be distinguished by a bacterial ooze that often emerges from the eyes and stem-end attachment of R. solanacearum-infected tubers. When this bacterial exudate dries, soil may adhere to the tubers at the eyes.

Clavibacter michiganensis subsp. sepedonicus infected potato tuber
Clavibacter michiganensis subsp. sepedonicus infected potato tuber

Reporting suspect cases

If you think you have spotted the disease, please check our symptoms section before reporting it using one of the Further Information contact points below.

Management, grants, treatment

Ring rot is a notifiable plant disease in Northern Ireland under national legislation (Plant Health (Northern Ireland) Order 2006 (as amended)), and as such suspected cases should be reported to DAERA using the Further Information contact points below.

As with the management of all potato diseases, there are a number of steps that can be taken to reduce the chance of infection and spread.

  1. Only plant classified seed
    All classified seed potatoes produced in the EC are derived from material tested and found free from these diseases. Using home-saved seed does not provide the same guarantee of health.
  2. Control groundkeepers
    Groundkeepers are a key factor in the long-term survival of both diseases. Their control removes an important source of disease inoculum.
  3. Practice good hygiene
    Regularly clean and disinfect all machinery, equipment, containers, vehicles and storage facilities used during potato production. Dirty machinery spreads infection between stocks.
  4. Keep potatoes for planting away from ware grading lines
    Do not plant any potatoes that have passed over machinery handling imported ware potatoes. Bacteria from rotten potatoes can easily transfer to the next stock to be graded. Prepackers and processors should never return “seed” to growers.
  5. Don't dump waste on agricultural land
    Discarded potatoes and waste from potato processing could both harbour these diseases and should not be dumped back onto fields. If possible, dispose of all potato waste at an approved tip or by incineration.
  6. Feeding potatoes to livestock
    If feeding raw potatoes to animals, spread the slurry produced on fields for grazing rather than on ground to be planted with potatoes.

The science

Clavibacter michiganensis is a short, non-motile, Gram-positive rod bacterium. The species comprises of five defined subspecies, Clavibacter michiganensis subsp. insidiosus, Clavibacter michiganensis subsp. michiganensis, Clavibacter michiganensis subsp. nebraskensis, Clavibacter michiganensis subsp. sepedonicus and Clavibacter michiganensis subsp. tesselarius. The diagnostic tests for the pathogen include isolation onto selective media, consideration of the type of host infected, and the use of specific PCR primers are used to distinguish the subspecies.


The origin of this pathogen is unknown. Ring rot disease of potato was first described in Germany in 1905. In 1932 the disease was reported in Norway and was known to be present in Sweden prior to 1956 when it was widespread in seed and ware potatoes. It was reported to be present in France by 1934 but may have been present for a longer period of time, having been misidentified as a fungal wilt (i.e. Verticillium wilt). In 1940 it was noted that the disease was present in Russia. It was first detected in Britain in 2003, but has since been eradicated.

In 1931 the disease occurred in Canada, in the province of Quebec, and in 1937-1938 was known to exist in the provinces of Alberta, Manitoba, Nova Scotia, Ontario, Prince Edward Island and Saskatchewan; and in 1943 it was reported in British Columbia. In the USA it was first reported from Maine in 1932, and by 1939 the disease had been reported from 27 states and by 1948 from 45 states of the USA.

Pest risk analysis

There is currently no Pest Risk Analysis (PRA) available for the ring rot pathogen.

Import & movement restrictions

The Department of Agriculture, Environment and Rural Affairs (DAERA) carry out an annual survey of potato stocks from seed producers and from a random selection of ware producers. Stocks grown from both local and imported seed are also surveyed annually to look for latent infection. These samples are tested for both Ring Rot and another bacterial disease of potatoes (Brown Rot) at Agri-Food and Biosciences Institute (AFBI) Potato Diseases Laboratory, Applied Plant Science Division.

Recent legislation requires growers and merchants to notify DAERA in advance of imports of seed potatoes from outside Northern Ireland. This includes imports from Scotland and the Republic of Ireland as well as imports of seed from England and mainland Europe. DAERA will try to sample all imported seed stocks which are from areas with a history of Brown Rot and Ring Rot outbreaks. A test in advance of planting could detect these diseases and prevent growers from planting infected stocks.

Consignments of imported ware potatoes are monitored by DAERA inspectors. Imports at processors premises are inspected and sampled with special attention given to stocks originating in areas known to have a history of these diseases.

Further information

Questions and answers

Plant Health Inspection Branch
DAERA Northern Ireland
Tel:  0300 200 7847
Email: planthealth@daera-ni.gov.uk

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