Species richness in different groups of microfungi causing disease symptoms on above-ground plant organs is analyzed based on checklists and databases of five tropical regions in the Americas and two regions outside the tropics in North America. In addition, primary data on plant parasitic microfungi collected in natural habitats in western Panama are considered. According to these data, Asterinaceae, Meliolales, Phyllachorales, and rust fungi are highly species rich groups in the Neotropics, while smut fungi appear to be less diverse in the tropics than in extratropical regions. Species of Albuginales, Erysiphales, Peronosporales, and Taphrinales are rarely found in the tropics, and are especially scarce in tropical forests. Morphologically, many tropical plant parasitic microfungi are characterized by melanized walls of fruiting bodies and spores, apparently an adaptation to tropical conditions with strong solar radiation. Gelatinous substances may slow down the process of drying out. The absence of a winter season in the tropics is probably the reason for the lack of spore dormancy and the dominance of asexual developmental stages in the Erysiphales and rust fungi. Accepted 9 December 2010.
