COMPARISON OF PSEUDOMONAS SPECIES AND APPLICATION TECHNIQUES FOR BIOCONTROL OF RHIZOCTONIA STEM ROT OF POINSETTIA

Pseudomonas cepacia (strain 5.5B) was compared with other strains of P . cepacia, P. fluorescens, P. chlororaphis, and P. aureofaciens for bi ocontrol of Rhizoctonia stem rot of poinsettia caused by Rhizoctonia s olani 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%. Infe ction ranged from 63 to 97% with ail 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 bacter ial spray over-the-top of cuttings (at day 0) were the most effective application methods. Cubes soaked with water followed by a bacterial s pray 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 pri or to taking cuttings, or dipping cutting stems in a bacterial suspens ion prior to placing cuttings in cubes, provided no stem rot control.