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Occurrence of Soft-rot Erwinia spp. in Soil and Plant Material. T. J. Burr, Department of Plant Pathology, University of California, Berkeley, CA 94720; M. N. Schroth, Department of Plant Pathology, University of California, Berkeley, CA 94720. Phytopathology 67:1382-1387. Accepted for publication 14 April 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1382.

A selective, differential agar medium (PT) was used to study the occurrence of the soft-rot bacteria, Erwinia carotovora var. carotovora and Erwinia carotovora var. atroseptica (ESR), in soil and plant materials. The efficiency of isolation was greatly enhanced by using an anaerobic enrichment technique prior to plating on the medium. Erwinia soft-rot spp. were isolated from rhizosphere soils of many weeds and crop plants, and from some field soils containing plant residues. Populations at some sites were large enough to be detected without the aid of enrichment. Erwinia soft-rot bacteria were not detected in fallow soils devoid of vegetation or recognizable plant residues. The ESR bacteria were not considered true soil inhabitants although they have a protracted soil phase because of their capacity to perpetuate themselves indefinitely when in association with roots of a diverse number of plants. A direct lenticel isolation technique was developed which enabled the determination of the percent ESR infestations in potato seedlots within a 48-hr period. Infestations ranged from eight to 100%. Many of the strains of ESR isolated in these studies could not be identified as either Erwinia carotovora var. carotovora or E. carotovora var. atroseptica with standard testing schemes.

Additional keywords: potato blackleg, blackleg-free seed.