Optimization of Biological Control of Rhizoctonia Stem Rot of Poinsettia by Paecilomyces lilacinus and Pseudomonas cepacia . D. KELLY CARTWRIGHT, Former Graduate Research Assistant; Department of Plant Pathology, North Carolina State University, Raleigh 27695. D. M. BENSON, Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695. Plant Dis. 79:301 -308. Accepted for publication 19 December 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0301.

Effects of antagonist concentration, nutrient status, and antagonist-free preparations (culture filtrate) of Paecilomyces lilacinus (isolate 6.2F) and Pseudomonas cepacia (strain 5.5B) on control of Rhizoctonia stem rot of poinsettia in polyfoam rooting cubes were investigated. Infection of cuttings ranged from 27 to 83% in cubes soaked in log 7.4 to log 3.4 conidia per milliliter of P. lilacinus. Percent infection of cuttings ranged from 0 to 56% in rooting cubes treated with P. cepacia applied in concentrations from log 9.4 to log 5.4 cfu per milliliter. Infection of cuttings in infested controls ranged from 87 to 93%. No significant differences in biocontrol occurred when P. lilacinus or P. cepacia were applied with or without a dilute potato-dextrose solution. There was no control (100% mortality) of stem rot with sterile culture filtrate from cultures of P. cepacia, but 70% control was achieved with filtrate from cultures of P. lilacinus. Both antagonists reduced the severity of Rhizoctonia stem rot on rooted cuttings in soilless medium. After 15-18 days, treatments with P. lilacinus had disease severity ratings (1 = healthy, 5 = dead) from 1.0 to 4.6 compared with infested or noninfested controls with ratings of 5.0 or 1.0, while ratings of poinsettia plants with all treatments of P. cepacia ranged from 2.3 to 3.8.