Direct Effects of Naphthaleneacetic Acid on Fungi, Bacteria and Microalgae: An Evidence Map with Dose–Response Synthesis and Its Implications for Plant-Associated Microbiology

1-Naphthaleneacetic acid (NAA) is a synthetic auxin used repeatedly in apple production and routinely in plant propagation, yet its direct effects on microorganisms have not been reviewed. We compiled the observations we could locate and verify of a measured NAA effect on a named fungus, bacterium or microalga, from a screened literature corpus, a taxon-directed sweep, targeted searches and, at revision, Scopus and AGRICOLA, and two authors independently assigned each an evidence tier. The result is a structured evidence map of a defined corpus. Effects were organism-specific. Plant-pathogenic fungi were inhibited by about 40% or more only at 100 mg L−1 and above, at which Trichoderma harzianum lost at most 13% of its radial growth, whereas several cultivated fungi were stimulated at 1–8 mg L−1. NAA altered fungal and bacterial secondary metabolism and bacterial virulence gene expression, a rhizobial inoculant degraded it, and microalgae responded biphasically between 0.037 and 18.6 mg L−1. Among the studies retrieved, one field trial, in tobacco, characterised a soil microbial community after NAA application; none did so in an orchard, and no field study measured a named beneficial microorganism. We outline implications for biocontrol compatibility, inoculant use and phyllosphere exposure.