EXPERIENCES WITH BACTERIAL BROWN ROT RALSTONIA SOLANACEARUM BIOVAR 2, RACE 3 IN THE NETHERLANDS
Janse J.D.
Department of Bacteriology, Plant Protection Service P.O. Box 9102, 6700 HC, Wageningen, The Netherlands
Brown rot (caused by Ralstonia solanacearum biovar 2, race 3) was found for the first time in 1992 in one field of ware potatoes, where uncertified seed of unknown origin was used, in the south of the country. In the same year brown rot infections were found in Belgium and the UK, all in ware potatoes. In all of these cases no clonal links could be established. In September 1995 (after two unusually warm summers) the disease was found again, now in seed, of a local variety. The Plant Protection Service (PPS) immediatly started a survey to trace the origin of the infection and a testing of all traded (national and export) seed, some 55.000 samples. Essentially the agreed EPPO method (Bulletin OEPP/EPPO Bulletin 20, 255-262, 1990) was used. This means sampling of 200 tubers/25 ton, extracting of heel ends, screening with immuno-fluorescence (IF) and confirmation of IF-positive cases with a host test on tomato or plating on selective medium (to isolate the bacterium), identification of the bacterium using IF, fatty acid analysis and a host test with the pure culture. It was found that mainly one seed line (grower number) of the local variety was heavily infected and spread the disease to various locations in the country. In a number of cases no clonal links could be established and contamination by (contaminated) surface water, used for overhead irrigation during the warm summer remained the only explanation. When a small survey was initiated, in the neighbourhood of these farms the bacterium was detected in water as well as in bittersweet (Solanum dulcamara), a solanaceous weed growing with its roots (floating) in water. In total 94 places of production were found to be contaminated, they were excluded from seed production. When the farmer wants to return to seed production, he cannot grow potatoes on the infected field for 5 years and the first three years it is only allowed to have bare fallow with weed and volunteer control or pasture or to plant cereals (excluding corn). All under supervision of the PPS. In the same year France reported also a case of brown rot, where water apparently played a role. In 1996 again all seed to be traded was tested and now only 9 places of production were found to be contaminated. Again these were excluded from seed production. In the same year an extensive survey (c. 15.000 samples) of surface water was performed. The bacterium was detected especially in the north of the country, explaining all cases where no clonal links were established. Zones were determined where irrigation with surface water is forbidden. In this year it became clear that also in France, Belgium and the UK water played a dominant role and that there is a possible link to sewage plants of the potato processing industries and municipalities. Infections in our and other Western-European countries may have been caused at some time in the past were by contamination of water by infected (waste of) early consumption potatoes imported from mediterranean countries where the disease occurs. Some preliminary experiments on the survival of the bacterium in/on different substrates will be reported.
