Soil and Seed Tubers as Sources of Inoculum ofErwinia carotovorapv.carotovorafor Stem Soft Rot of Potatoes

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Abstract
To determine the relative importance of soil strains and seed tuber strains of E. carotovora pv. carotovora (Ecc) as primary inoculum for potato plant infection in the field, a soil enrichment technique and a modification of the tuber incubation method were used to isolate Ecc strains from soil samples and seed tubers, respectively. The strains were characterized by the double diffusion method. Tubers from 3 ''Russet Burbank'' seed lots were naturally contaminated with Ecc, and all strains recovered from the seed tubers were serologically different from those isolated from soil. Ecc strains recovered from potato stems during the growing season were identified serologically with antisera produced against seed tuber strains and soil strains. In 1980, seed tuber serogroups were obtained most frequently in isolations from symptomless plants in 2 of 3 experiments. In 2 experiments during 1981, 11 and 15% of the strains isolated from symptomatic stems were serogroups characteristic of soil strains, whereas 14 and 4%, respectively, were serogroups characteristic of seed tuber strains. A majority of the strains isolated from stems in 3 of 5 experiments were different from the known soil or seed tuber serogroups. Other sources of inoculum, such as irrigation water, may be involved in the stem soft rot disease.

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