SASA has a dedicated team of bacteriologists providing a range of services and conducting research and development projects on a wide range of bacterial plant pathogens.

Most of the Bacteriology Unit's work is focused on bacterial pathogens of potato, though pathogens of strawberry are also covered.

The provision of services are either in support of the Scottish Seed Potato Classification Scheme or as part of Scotland's statutory obligation to ensure it remains free of range of quarantine organisms, specifically Clavibacter michiganensis subsp. sepedonicus and Ralstonia solanacearum.

Services include:

  • Provisions of annual surveys of Scottish potatoes for Clavibacter michiganensis subsp. sepedonicus, Ralstonia solanacearum, and Dickeya spp..
  • Provision of annual surveys of Scottish rivers for Ralstonia solanacearum and Dickeya spp..
  • Scientific and technical advice to the Scottish Government on the control of bacterial plant pathogens.
  • Testing of nuclear stock material for a wide range of bacterial pathogens.
  • Diagnosis of diseased samples submitted by Agriculture and Rural Economy Directorate inspectors for the presence of bacterial diseases.
  • Advice and diagnostic support on bacterial pathogens to the potato industry and stakeholder bodies.

For further details please contact Karen Fraser on 0131 244 8894 or email Bacteriology.

In addition, a range of research and development projects are currently underway into bacterial pathogens of potato or have recently been concluded.

Additional information on the main bacterial pathogens we work on (Clavibacter michiganensis subsp. sepedonicus - potato ring rot; Ralstonia solanacearum - potato brown rot; Pectobacterium - Blackleg/soft rot; Dickeya - Top wilt/blackleg/soft rot) can be found using the links below.

Ring Rot

Ring rot is caused by the quarantine bacterium, Clavibacter michiganensis subsp. sepedonicus (Cms). It is a serious plant health threat to potato production in Northern Europe. Ring rot has never been found in Scottish potatoes.

Infected seed lots are the main source of disease spread and Cms can persist for long periods (> 2 years) on surfaces, which raises the prospect of contaminated machinery and storage containers serving as a means of infection.

Symptoms of Ring rotCms causes wilting and chlorosis, which starts at the leaf margins. As the disease progresses the plant can collapse. Tubers develop characteristic symptoms, similar to brown rot, where the vascular tissue becomes discolured eventually leading to collapse and rotting of the tuber.

Although in Europe, the economic damage caused by the disease directly, is low. The costs associated with rejection of the infected crop, clean-up measures on farm, loss of reputation etc. for the grower concerned can be very high.

Brown Rot

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). Symptoms of Brown rotIn 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.


Pectobacterium atrosepticum (previously known as Erwinia carotovora subsp. atroseptica) is the cause of potato blackleg in Scotland. Although blackleg and the associated soft-rot of tubers in store can cause severe economic losses, it's occurrence and severity are dependant on environmental factors in the field (temperature, rainfall, poor drainage etc.), timing of harvest and seed storage conditions.

Blackleg affects the lower part of the stem, producing a brown/black rot accompanied by a distinctive smell, often accompanied by yellowing/curling of the leaves. In early and severe cases, plants may not emerge or are stunted, yellow and die quickly post-emergence. The disease is spread by infected seed tubers, which when planted can infect daughter tubers through the vascular system and can infect neighbouring plants through groundwater as infected tuber begin to decay. Decaying tubers can also infect healthy tubers at harvest or in store by direct and indirect contact.

Propensity of the top 30 varieties to Blackleg

Variety has a very important effect on the incidence of disease symptoms observed at classification inspections. The term ‘varietal propensity’ has been adopted to describe whether symptoms observed within a variety are above or below the average across the whole Scottish seed crop.  Propensity is expressed as (% of disease within crops of a specific variety) / (% of disease in crops of all varieties).  The propensity score also indicates by how much a variety is above or below average, and can therefore be used as an indication of how one variety may compare with another. 

The table below summarises varietal propensity information collected over two periods 2009-2017, as well as just for season 2017, using data on the incidence of blackleg symptoms observed at crop inspection  (see also attached spreadsheet). Data are presented for the top 30 varieties entered for classification within the SPCS in 2017; these top 30 varieties making up c. 75% of total SPCS area in 2017.  By taking the maximum value for blackleg incidence across all inspections for each crop and multiplying this by the area of the respective crop, a quantitative estimate of the infection can be made.  Thus a crop of 4ha with a 3% incidence of blackleg contains 0.12 ha of blackleg infection.  Values greater than 1 indicate that blackleg is more likely to be observed in that variety and values less than 1 indicate that symptoms are less likely to be observed.  The table also shows values which are significantly greater or less than 1 at the 0.05 confidence level.  The cells in the table are shaded in red when the propensity is significantly higher than expected (i.e. 1) and shaded in green when values are significantly lower than expected.  Values that are not significantly different from 1 are left clear.  Sample size has a marked effect on the likelihood of significant departures from 1, hence the 2009-17 column shows a greater number of statistically significant departures from 1 than does the column for 2017 by itself. As the variance associated with the data for varieties with a high propensity is far greater than for those with a low propensity, fewer varieties have a propensity significantly greater than 1 and more varieties have a propensity significantly less than 1.

The interpretation of these data should take into account that the management of blackleg, e.g. heavy roguing, will have an effect on the disease incidence observed during inspections.  However, with approximately 4,000 seed crops entered for classification each year, similar husbandry practices would need to be extended across a high proportion of crops to have a marked influence on propensity. 


Top wilt/ blackleg/ soft rot – Dickeya spp.

The genus Dickeya was previously known as Erwinia chrysanthemi. There are two significant potato pathogens within this genus currently affecting Europe; Dickeya dianthicola and ‘Dickeya solani’. D. dianthicola has never been found on Scottish potatoes. ‘D. solani’ emerged in Europe around 2005-2006. It is highly aggressive on potato, causing rapid wilting and blackleg-like symptoms across wide environmental conditions.

Preliminary research indicates that ‘D. solani’ may be significantly more aggressive than D. dianthicola and Pectobacterium atrosepticum.  The new species appears to be able to rapidly induce blackleg symptoms and also to rot developing progeny tubers, even when inoculum levels are low. Aggressiveness of the new Dickeya pathogen appears to further increase at higher temperatures so there are implications for increased importance of this pathogen in response to global warming.  As yet, there is little substantiated practical information on the biology of this strain in relation to its host range, its ability to survive, establish and spread in the environment or its behaviour on stored potato tubers.

'D. solani’ has only ever been found on small number of Scottish ware crops (15 in total, from 2009 & 2010), all of which were grown from non-Scottish seed.

For further information see the Dickeya solani - a threat to our potatoes page on the Scottish Government website.

Information on a range of measures to protect Scotland's ware growers against Dickeya, and a range of threats, can be found in the Defending your potato crop against disease leaflet.

View recent surveys.

Xylella fastidiosa

Xylella fastidiosa is a bacterial disease not known to occur in the UK. The bacteria colonise the xylem vessels of plants, eventually blocking these, which deprives the plant or tree of water and nutrients. The symptoms vary depending on the host plant species and its degree of susceptibility but can include marginal leaf scorch, wilting of foliage and withering of branches. Severe infections can result in dieback, stunting and death of the plant.

Graphic depicting Xylella fastidiosa within the xylem of an infected tree

Xylella originated in South America but has now been detected in mainland Europe where outbreaks have had significant impact on commercially grown plants and the wider environment. Although the disease is regulated under law, there is concern about accidental introduction to the UK via imported plants. All Xylella fastidiosa species are of concern due to their wide host range which includes trees such as oak, maple and elm.

Common spittlebug (Philaenus spumarius)The bacteria are spread from plant to plant by infected insects of the infraorder Cicadmorpha whilst feeding on the sap; the bacteria can live in the foregut of the vector. There are a number of these insects native to the UK and the most notable of these is the meadow spittlebug Philaenus spumarius. They can be recognised by the foam nests the nymphs (immature spittlebugs) produce commonly known as ‘cuckoo spit’. Spittlebugs do not cause any damage to plants themselves; they simply carry the bacteria.

In order to prevent the introduction of Xylella to the UK, routine surveillance for the disease is carried out on plants in trade and SASA is also involved in projects with EUPHRESCO, Plant Health Centre of Expertise and BRIGIT project collecting spittlebugs and recording information on what plants they live and feed on across Scotland- see posters [1] and [2].