A major justification for taxonomic research is its assumed ability to predict the presence of traits in a group for which the trait has been observed in a representative subset of the group. Taxonomy is regularly used by breeders interested in choosing potential sources of disease resistant germplasm for cultivar improvement. We have testing predictivity in wild potatoes (Solanum section Petota) through our own data (the fungal pathogen Sclerotinia sclerotiorum) and through a database of diverse disease resistances in sect. Petota from the literature. Disease resistances in sect. Petota are characterized by great variation within and among species. No consistent association was observed white mold resistance or other diseases and taxonomic series (based on a phenetic concept), clades (based on a cladistic concept), ploidy, breeding system, geographic distance, or climate parameters. Species and individual accessions with high proportions resistant plants can be identified, but both often exhibit extensive variation and designation of either as resistant or susceptible must take this variation into account. To date, we have not found taxonomic relationships and ecogeographic data to be useful to predict where additional sources of disease resistance genes will be found.