Pyrrolnitrin production by Burkholderia cepacia and biocontrol of Rhizoctonia stem rot of poinsettia

Pyrrolnitrin production by Burkholderia cepacia strain 5.5B was closely related to suppression of stem rot of poinsettia caused by Rhizoctonia solani. Selected strains RR 13-1 and UV 19-4, which produced significantly less pyrrolnitrin than the parent strain 5.5B, lost the ability to control stem rot completely. Selected strain RR 21-2 produced the same level of pyrrolnitrin as strain 5.5B and was as effective as strain 5.5B in stem rot control. The effect of medium type and initial pH of medium on pyrrolnitrin production by B. cepacia was investigated to improve efficacy of biocontrol. More pyrrolnitrin was accumulated when the initial pH was adjusted to 5.8 than 6.8. With an initial pH of 5.8, strains of B. cepacia produced more pyrrolnitrin in a nutrient broth medium than in a minimum salts medium. Pyrrolnitrin was the major factor in biocontrol of Rhizoctonia stem rot of poinsettia by B. cepacia. Improvement in biocontrol efficacy may be possible by adjusting culture conditions to optimize pyrrolnitrin production.