Biological Control of Rhizoctonia Stem Rot of Poinsettia in Polyfoam Rooting Cubes with Pseudomonas cepacia and Paecilomyces lilacinus

A strain of Pseudomonas cepacia (ex Burkholder) Pallerni and Holmes, strain 5.5B, and an isolate of Paecilomyces lilacinus (Thom) Samson, isolate 6.2F, were evaluated for biocontrol of Rhizoctonia stem rot of poinsettia in polyfoam rooting cubes after preliminary screenings of collected bacteria, fungi, and actinomycetes. In repeated experiments conducted over 2-week periods, no disease developed on cuttings in cubes treated with P. cepacia as opposed to the nontreated, pathogen-infested controls (73 to 100% infection). Control with P. lilacinus was less effective but remained significant compared to the pathogen-infested controls. During 3-week trials, some infection (18%) occurred on cuttings in cubes treated with P. cepacia but the level of infection was significantly less than that in the pathogen-infested controls (100% infection). The P. lilacinus isolate provided similar levels of stem rot control over 2- and 3-week periods. Root development on poinsettia cuttings typically was not affected by P. cepacia but, in some experiments, was less on cuttings in cubes treated with P. lilacinus. These experiments demonstrate the significant potential of these antagonists as biocontrol agents for Rhizoctonia stem rot of poinsettia. P. lilacinus has not been previously reported as an antagonist of R. solani.