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.
In addition, a range of research and development projects are currently underway into bacterial pathogens of potato or have recently been concluded.
R&D work includes:
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 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.
Cms 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.
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). In 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.
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 2009-2016, as well as just for season 2016, 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 2016; these top 30 varieties making up 77% of total SPCS area in 2016. 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 2009-16 column shows a greater number of statistically significant departures from 1 than does the column for 2016 by itself. It should be noted for the variety Cara that there is a very high level of variation in the levels of blackleg observed in different crops, so although the propensity for this variety is high, it is not significantly so.
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.
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.
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