Nearly 150 years of research has accumulated large amounts of data on mycorrhizal association types in plants. However, this important resource includes unreliable allocated traits for some species. An audit of six commonly used data sources revealed a high degree of consistency in the mycorrhizal status of most species, genera and families of vascular plants, but there were some records that contradict the majority of other data (~ 10% of data overall). Careful analysis of contradictory records using rigorous definitions of association types revealed that the majority were diagnosis errors, which often stem from references predating modern knowledge of mycorrhiza types. Other errors are linked to inadequate microscopic examinations of roots or plants with complex root anatomy, such as phi thickenings or beaded roots. Errors consistently occurred at much lower frequencies than correct records but have accumulated in uncorrected databases. This results in less accurate knowledge about dominant plants in some ecosystems because they were sampled more often. Errors have also propagated from one database to another over decades when data were amalgamated without checking their suitability. Due to these errors, it is often incorrect to designate plants reported to have inconsistent mycorrhizas as “facultatively mycorrhizal”. Updated protocols for resolving conflicting mycorrhizal data are provided here. These are based on standard morphological definitions of association types, which are the foundations of mycorrhizal science. This analysis also identifies the need for adequate training and mentoring of researchers to maintain the quality of mycorrhizal research.