The species richness of rust fungi (Pucciniales or Uredinales) in the neotropics is reviewed. Species numbers are presented for all neotropical countries and rust-plant- ratios calculated. It is discussed whether the ratio for a given region can be explained by the species richness of vascular plants alone or whether it is caused by additional factors. In the first case, ratios should apply globally and vary only slightly; in the second case, more diverging ratios are expected. Observed ratios ranged between 1:16 and 1:124 in the neotropics. The large differences are certainly influenced by unequal levels of investiga- tion, rendering interpretation difficult. Differences seem also to be influenced by the tax- onomic composition of floras regarding the percentage of host families or genera bearing different numbers of rust species. This indicates that rust species richness is not driven solely by plant species richness. Ratios calculated for Switzerland, Austria and Japan are distinctly higher than for the neotropics indicating that certain temperate regions are proportionally richer in rust fungi than the neotropics. Uredinial states and short-cycled rust species prevail in the neotropics. The preponderance of uredinial states may be due to the heterogeneous spatial composition of certain vegetation types in the wet tropics. Short- cycled rusts may be adapted to a pronounced seasonality that can be encountered in many drier neotropical biomes. Future research needs to fill our knowledge gaps on the taxonomy and ecology of neotropical rust fungi are discussed.
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