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Dynamics of Sugar Beet Seed Colonization by Pythium ultimum and Pseudomonas Species: Effects on Seed Rot and Damping-off. R. M. Osburn, Graduate research assistant, Department of Plant Pathology, University of California, Berkeley 94720, Present address: Gustafson, Inc., P. O. Box 6600065, Dallas, TX 75266-0065; M. N. Schroth, J. G. Hancock, and M. Hendson. Professor, chair, and postdoctoral researcher, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 79:709-716. Accepted for publication 10 February 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-709.

Pythium ultimum colonized the seed pericarps of sugar beet within 4 hr under favorable conditions in soil. Strain ML5 of Pseudomonas fluorescens-putida and strain R20 of P. putida, when inoculated onto seed, resulted in a markedly lower incidence of colonization by P. ultimum. The incidence of fungal colonization of seeds treated with ML5 or R20 was 6.7 and 36.7%, respectively, compared with 90% of untreated seeds 24 hr after planting. The tripartite interaction continued even when the fungus invaded the pericarp because the bacteria also colonized the tissues. Interaction between R20 and P. ultimum resulted in a reduction of active or viable fungal mycelium in the pericarp over time. ML5 inhibited both mycelial growth and sporangial germination, whereas R20 inhibited only mycelial growth. Only a small percentage of true seed became infected by P. ultimum regardless of the incidence of pericarp colonization. The amount of damping-off was related directly to the incidence of pericarp colonization (r2 = 0.98). The effectiveness of R20 and ML5 in limiting pericarp colonization by P. ultimum was dosage dependent and independent, respectively. Both attained population densities in the endospermosphere similar to those in the ectospermosphere. Sugar beet seed supported relatively similar ectospermosphere and endospermosphere population densities of R20, ML5, or bacteria in general (106107 colony-forming units/seed). Spermosphere population sizes of R20 and ML5 were similar regardless of the size of the initial inoculum dosage. The doubling time of ML5 in the spermosphere was 3 hr. Species of Pseudomonas comprised 12% of the detectable ectospermosphere bacteria and 71% of those that colonized the pericarp, showing the affinity of this group of organisms for the pericarp. Both ML5 and R20 were comparable to fungicides in suppressing damping-off by P. ultimum in greenhouse experiments.

Additional keywords: biological control, soilborne pathogens.