Pseudomonas chlororaphis PA23 inhibits several root pathogens in both the greenhouse and field. A LysR-type transcriptional regulator (LTTR) called PtrA (Pseudomonas transcriptional regulator A) that is essential for Sclerotinia sclerotiorum antifungal activity was discovered through transposon mutagenesis. P. chlororaphis PA23 produces the antibiotics phenazine 1-carboxylic acid, 2-hydroxyphenazine and pyrrolnitrin, and several additional products that contribute to biocontrol. Phenotypic assays and proteomic analysis have revealed that production of these secondary metabolites are markedly reduced in a ptrA mutant. Most LTTRs regulate genes that are upstream of and divergently transcribed from the LTTR locus. A short chain dehydrogenase (scd) gene lies immediately upstream of ptrA in the opposite orientation. Characterization of an scd mutant, however, has revealed no significant changes in antifungal activity compared to wild-type PA23. Gene expression analysis of the ptrA mutant indicates that ptrA may exert its regulatory effects through the Gac-Rsm network, and may be controlling expression of the scd gene. Collectively these findings indicate that PtrA is an essential regulator of PA23 biocontrol and is connected to other regulators involved in fungal antagonism.