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Evaluation of Serological Tests for Detection of Clavibacter michiganensis subsp. sepedonicus in Composite Potato Stem and Tuber Samples. S. H. DEBOER, Agriculture Canada, Research Station, Vancouver, B.C., Canada V6T 1X2. D. E. STEAD, Central Science Laboratory, Ministry of Agriculture, Fisheries, and Food, Harpenden, Herts., AL5 2BD, United Kingdom; A. S. ALIVIZATOS, Benaki Phytopathological Institute, GR 145 61 Kifissia, Greece; J. D. JANSE, Plantenziektenkundige Dienst, 6700 HC Wageningen, Netherlands; J. VAN VAERENBERGH, Rijksstation voor Plantenziekten, 9820 Merelbeke, Belgium; T. L. DE HAAN, Agriculture Canada, Seed Potato Certification Laboratory, Charlottetown, P.E.I., Canada CIA 7M8; and J. MAWHINNEY, Agriculture Canada, Seed Potato Certification Laboratory, Fredericton, N.B., Canada E3B 5G4. Plant Dis. 78725-729. Accepted for publication 6 April 1994. Copyright 1994 Department of Agriculture, Government of Canada. DOI: 10.1094/PD-78-0725.

A procedure to index potato seed lots for bacterial ring rot was evaluated by testing composite samples of tissue from 200 potato stems or tubers, in blind experiments, for the presence of Clavibacter michiganensis subsp. sepedonicus by immunofiuorescence and enzyme-linked im-munosorbent assay (ELISA) in six laboratories. Different antibody preparations were used for the two serological tests. Tissue samples were from ring rot-free potato plants, but ring rot-infected tissue was added to 88 of the 385 composite samples to create authentic positive composites. One laboratory achieved 100% efficiency for detection of ring rot infections in stem samples with ELISA, but the overall sensitivity and specificity of ELISA were 97.7 and 90.1%, respectively, for stems and 93.3 and 86.7%, respectively, for tubers. Efficiency of detection by immunofluorescence was 100% in three laboratories for stem and tuber samples. Overall sensitivity and specificity for immunofluorescence were 90.9 and 98.5%, respectively, for stems and 97.3 and 97.8%, respectively, for tubers. When a positive result in both serological tests was set as the requirement to designate a sample as infected with ring rot, specificity was 100% for stem samples in all laboratories and for tuber samples in four of the six laboratories. When a positive result for only one of the two serological tests was set as the requirement for designating a ring rot infection, sensitivity was 100% in all laboratories for stems and in five of the six laboratories for tubers, but specificity was reduced.