Evaluation of an agar immunodiffusion procedure for confirming bacterial ring rot diagnoses

Bacterial ring rot-infected tuber and stem tissue reacted withCorynebacterium sepedonicum antiserum in agar immunodiffusion (AID). The AID test was as effective as immunofluorescence (IF) in confirming ring rot in potato tubers submitted for diagnosis. Precipitin bands developed in AID with tuber preparations that contained, in a 1:10 dilution of the preparation, at least 100 IF positive cells/microscope field with a 100 X oil objective. Only a low percentage of preparations with less than 100 IF positive cells were positive with AID. Samples inoculated withC. sepedonicum strains which produced non-mucoid colony types on YGM medium did not consistently develop precipitin bands in AID which suggests that the usefulness of the test may be limited where non-mucoid strains occur naturally.