The Relationship of Pathogenicity in Pseudomonas solanacearum to Colony Appearance on a Tetrazolium Medium

Mutants of Pseudomonas solanacearum differing in colony morphology from the normal or wild type were detected readily when bacterial suspensions from stock cultures were streaked on a medium containing tetrazolium chloride and examined with obliquely transmitted light. The most common mutant formed a round, butyrous, deep red colony with a narrow bluish border. In contrast, the normal or wild type formed an irregularly-round, fluidal, white colony with a pink center. Inoculation tests on tomato seedlings demonstrated that colony appearance on the tetrazolium medium could be related to pathogenicity. Cultures derived from butyrous red colonies were either weakly pathogenic or non-pathogenic, whereas cultures from fluidal white colonies with pink centers were highly pathogenic.