While auxin is involved in virtually every process in plant development and orientation in the environment, plant pathogens have exploited the auxin machinery of the plant to alter plant growth and development in their favor. On the opposite, the plant is able to turn this against invaders and uses the same pathways for defense reactions. Also, plant beneficial microbes can interfere with the auxin metabolism of the host plant to induce growth of the plant for mutual benefits. Here, the role of auxin in disease symptom development will be reviewed, where either the plant or the pathogen contributes to alterations in host auxin synthesis and metabolism. Due to the many interactions known, the focus here will be on bacteria, protists, and fungi. On the one hand, auxin can be rated as pathogenicity factor, but also on the other hand alterations in auxin levels can result in changes of target genes, which then lead to changes in plant defense. In addition, the auxin-signaling pathway is directly utilized in the defense reaction against some pathogens.