<jats:p><jats:italic>Pseudomonas cannabina</jats:italic> pv. <jats:italic>alisalensis</jats:italic> (<jats:italic>Pcal</jats:italic>) causes bacterial leaf spot and blight of Brassicaceae and Poaceae. We previously identified several potential <jats:italic>Pcal</jats:italic> virulence factors with transposon mutagenesis. Among these a <jats:italic>trpA</jats:italic> mutant disrupted the tryptophan synthase alpha chain, and had an effect on disease symptom development and bacterial multiplication. To assess the importance of TrpA in <jats:italic>Pcal</jats:italic> virulence, we characterized the <jats:italic>trpA</jats:italic> mutant based on inoculation test and <jats:italic>Pcal</jats:italic> gene expression profiles. The <jats:italic>trpA</jats:italic> mutant showed reduced virulence when dip- and syringe-inoculated on cabbage and oat. Moreover, epiphytic bacterial populations of the <jats:italic>trpA</jats:italic> mutant were also reduced compared to the wild-type (WT). These results suggest that TrpA contributes to bacterial multiplication on the leaf surface and in the apoplast, and disease development. Additionally, several Brassicaceae (including Japanese radish, broccoli, and Chinese cabbage) also exhibited reduced symptom development when inoculated with the <jats:italic>trpA</jats:italic> mutant. Moreover, <jats:italic>trpA</jats:italic> disruption led to downregulation of bacterial virulence genes, including type three effectors (T3Es) and the phytotoxin coronatine (COR), and to upregulation of tryptophan biosynthesis genes. These results indicate that a trade-off between virulence factor production and <jats:italic>Pcal</jats:italic> multiplication with tryptophan might be regulated in the infection processes.</jats:p>