Interpretation of the EC method for the detection of latent Corynebacterium sepedonicum infections in potato1

The EC method for the detection of latent ring‐rot infections (Corynebacterium sepedonicum) consists essentially of an indirect immunofluorescence test and a pathogenicity test on eggplant (EP). When interpreting the results of this method, care should be taken that: (a) eggplants are grown at 21°C. At 28–29°C the detection level of the EP was increased 102 to 103‐fold to 105 to 106 cells ml‐1 and latent infections (which may go unnoticed) occurred with 104 to 105 cells ml‐1; (b) reisolations are made from symptomless eggplants grown at the optimum temperature (21°C) as latent infections can also occur in these plants at concentrations of 102 to 103 cells ml‐1; (c) potato tuber extracts are tested without freezing or stored freeze‐dried or in a proper protective agent. Freezing at ‐20°C in maceration buffer of cells of C. sepedonicum and Pseudomonas solanacearum (the latter was included for comparison) reduced their numbers by 90–98% after one day. This could easily cause viable cell numbers to drop below the detection level of the pathogenicity test.