Comparison of Pseudomonas Species and Application Techniques for Biocontrol of Rhizoctonia Stem Rot of Poinsettia. D. KELLY CARTWRIGHT, former Graduate Research Assistant. D. M. BENSON, Professor, Department of Plant Pathology, Box 7616, North Carolina State University, Raleigh 27695. Plant Dis. 79:309-313. Accepted for publication 14 November 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0309.

Pseudomonas cepacia (strain 5.5B) was compared with other strains of P. cepacia. P. fluores-cens, P. chlororaphis, and P. aureofaciens for biocontrol of Rhizoctonia stem rot of poinsettia caused by Rhizoctonia solani in polyfoam rooting cubes. Over a 2-wk period, complete control (0% infection) of stem rot was achieved with P. cepacia, strain 5.5B. With other strains of P. cepacia, infection ranged from 0 to 93%. Infection ranged from 63 to 97% with all other strains of Pseudomonas spp. Different application methods for delivering strain 5.5B for stem rot control were tested. Rooting cubes soaked with a suspension of strain 5.5B or cubes soaked with a bacterial suspension followed by a bacterial spray over-the-top of cuttings (at day 0) were the most effective application methods. Cubes soaked with water followed by a bacterial spray over-the-top of the cuttings in cubes controlled (P – 0.05) stem rot compared with the control. Three spray applications of P. cepacia strain 5.5B to cuttings during a 2-wk period were more effective than either one or two bacterial sprays in control of Rhizoctonia stem rot. Spraying poinsettia stock plants with a suspension of strain 5.5B prior to taking cuttings, or dipping cutting stems in a bacterial suspension prior to placing cuttings in cubes, provided no stem rot control.