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Increased Potato Yields by Treatment of Seedpieces with Specific Strains of Pseudomonas fluorescens and P. putida. T. J. Burr, Former Graduate Student, University of California, Berkeley, CA 94720, Present address of senior author: Department of Plant Pathology, New York State Agricultural Experiment Station, Geneva, NY 14456; M. N. Schroth(2), and T. Suslow(3). (2)(3)Professor of Plant Pathology and Research Assistant, respectively, University of California, Berkeley, CA 94720. Phytopathology 68:1377-1383. Accepted for publication 20 March 1978. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1377.

Significant increases in growth and yield of potato plants were achieved by treating seedpieces with suspensions of two Pseudomonas spp. at ~ 109 colony-forming units (cfu)/ml prior to planting. The pseudomonads were selected from over 100 strains that were isolated from the surface of potato tubers and also exhibited antibiosis against Erwinia carotovora var. carotovora in vitro. The isolates were identified as strains of Pseudomonas fluorescens and P. putida. These strains survived for at least 1 mo on treated seedpieces planted in loamy sand field soil at populations of ~109 cfu/0.785 cm2. Also, they colonized developing potato roots and were the predominant bacteria in the rhizospheres up to 2 mo after planting. Bacterization of seedpieces planted in field soils in the greenhouse resulted in up to 100% increase in fresh weight of shoot and root systems in a 4-wk period. Statistically significant increases in yield ranged from 14 to 33% in five of nine field plots in California and Idaho. The pseudomonads had no effect on plant growth or tuber yield when seedpieces were planted in peat soil, or in soil that was relatively dry. Both Pseudomonas spp. were compatible with fungicides that were commonly used to treat seedpieces, except for manganese ethylenebisdithio-carbamate zinc salt (mancozeb). The mechanism by which these bacteria enhance plant growth and tuber yield may be associated with changes in the composition of rhizosphere bacterial flora.