Survival and possibilities for eradication of Ralstonia solanacearum in cool climates
So far most studies on the ecology of Ralstonia solanacearum (EPPO A2 quarantine pest) have been conducted in warm climates with tropical strains. But recently, due to the outbreaks of potato brown rot in some European countries (see EPPO RS 97/111 for the current status of these outbreaks), data are needed on the survival of the bacterium in cool climates, means of dissemination, detection and possibilities for eradication.
Concerning the spread of R. solanacearum race 3 biovar 2 in Europe, the author reviews the various possibilities. It seems that the outbreaks observed in 1995 in the Netherlands, Italy and Portugal could be related to the circulation of latently infected seed potato tubers. Outbreaks observed in Belgium, France, Sweden, United Kingdom and also in the Netherlands could be connected the dissemination of the bacterium in infested waters used for irrigation of potatoes. In addition, industrial or domestic wastes could have also played a role in the dissemination.
The author pointed out that under temperate conditions in Australia, Kenya, Sweden, and United Kingdom, R. solanacearum could be detected in field soils for no longer than 2 years after harvest of potato crops affected by brown rot. In cool conditions, infection of perennial weed hosts seems to be the most important element in the overwintering and long-term persistence of the pathogen. In several European countries, the bacterium was detected in watercourses used for irrigation and in weeds growing along these courses (in particular Solanum dulcamara). The following weed species could be potential hosts, as infection was found after artificial inoculation, but this has not been verified in natural conditions: Solanum nigrum, Eupatorium cannabinum, Tussilago farfara, Portulaca oleracea, Cerastium glomeratum, Ranunculus sceleratus.
Several detection methods are available. The EPPO phytosanitary procedure no. 26 involve the use of IF and a bioassay on tomato seedlings. More recently, indirect ELISA, culture on semi-selective media and PCR have been proposed. It is generally felt that a combination of different methods is necessary to overcome the disadvantages presented by each method when used alone. The author also stressed that the detection is also highly dependent on the sampling strategy used. The present recommended sampling rate is 200 tubers per 25 t of potatoes.
The author reviews the current quarantine measures which have been set up to prevent the spread of R. solanacearum through infected plant material and stressed that their success will depend on the efficiency of detection methods and national surveys, and also on international cooperation. He also explained the measures which have been applied in European countries to contain the disease in areas where it has been found (see also EPPO RS 97/111). It can be added that strategies of integrated control of race 3 biovar 2 in temperate highland conditions of the tropics included: control of weeds and potato groundkeepers, growing of non-hosts crops (particularly cereal and grass). Soil fumigation is generally not recommended as bacteria in the lower layers are not affected and antagonistic rhizosphere organisms situated in the upper layers are destroyed.
Finally, prospects for eradication are reviewed. It appears that of all biovars of R. solanacearum, biovar 2 is probably the most easy to eradicate, and it can be noted that its eradication from Sweden was successful. In cool conditions, it seems that R. solanacearum does not survive very well in soil but it is able to persist in aquatic S. dulcamara for several years. Further research is needed to determine the origin of the bacterium in waters, in S. dulcamara, and the role of other weed hosts, to be sure of the effectiveness of eradication measures.
Sources
Elphinstone, J.G. (1996) Survival and possibilities for extinction of Pseudomonas solanacearum (Smith) Smith in cool climates.
Potato Research, 39 (extra edition), 403-410.