Persistence and latency of Clavibacter michiganensis subsp. sepedonicus in field-grown seed potatoes

Whole seed tubers were inoculated with different inoculum doses of Clavibacter michiganensis subsp. sepedonicus and planted in the field. Ring rot symptom development and the persistence of C. michiganensis subsp. sepedonicus, in the absence of symptoms, was determined for four generations following introduction of inoculum. Seed tubers were not cut during propagation to preclude redistribution of inoculum. Detection of C. michiganensis subsp. sepedonicus was positively related to inoculum dose (r = 0.97) and rapidly declined with increasing generations. Although the frequency of detection was much less following inoculation with less than or equal to 10(2) cells versus inoculation with greater than or equal to 10(4) cells, all inoculum doses established latent infections that were undetected for at least one, and up to three, generations. Latency for three generations, characterized by C. michiganensis subsp. sepedonicus detection or ring rot symptom development during the fourth generation, was the longest time interval tested. The occurrence of latent C. michiganensis subsp. sepedonicus infections and the persistence of inadvertent inoculum sources may, at least partially, explain our inability to eradicate ring rot based on visible symptom expression.