Seed potato (Solanum tuberosum) tubers from eight cultivars of 21 fields in Vacaria, Canguçu, Piratini and Ibiraiaras, all in the state of Rio Grande do Sul, were harvested from May to August of 2002. Each tuber was washed in running water, left to dry at environment temperature (23 ºC), stabbed with toothpicks in ten lenticels, covered with a thin layer of soybean oil, put individually on a wet paper towel inside a transparent plastic bag and incubated at 23 ºC for four days. The incidence of soft rot in lenticels ranged from 20-100% among the cultivars. Pectobacterium sp. was found in tubers from every field. Two hundred and twenty three isolates of Pectobacterium sp. were obtained on CPG medium, from lenticels showing soft rot symptoms, and identified by biochemical and physiological tests and PCR, at subspecies levels. One hundred and nineteen and 96 strains were identified as P. carotovorum subsp. brasiliensis and P. carotovorum subsp. carotovorum, respectively. Eight strains did not fit in the biochemical classification. P. carotovorum subspp. were present in seed tubers potato, regardless of the cultivar, seed class or county. Pectobacterium carotovorum subsp. atrosepticum, the major agent associated with blackleg on potato in other countries, was not found.
