Infection of alternative host plant species by Ustilago maydis.

•  Here, the host specificity of the corn smut fungus Ustilago maydis was analyzed, with the long-term objective of understanding the different aspects of its pathogenic behavior. •  Axenic plantlets obtained in vitro, including one gymnosperm, monocotyledons and dicotyledons, were inoculated with a diploid strain of U. maydis, incubated in a growth chamber, and observed periodically. •  All plants were susceptible to infection. The most common symptoms were growth of fungal mycelium on stems and leaves, increase in root number in monocots, or development of adventitious roots in dicots. Other symptoms - chlorosis, increased anthocyanins, necrosis and stunting - varied among the different plant species. Ustilago penetrated and grew into the plant tissues in the form of pleomorphic mycelium, but no teliospores were formed. Noticeably, the fungus induced formation of lateral buds and tumors in papaya. •  The results provide evidence that U. maydis is able to infect a variety of phylogenetically unrelated plants grown under axenic conditions. These results may be useful in the analysis of different phenomena associated with the complex pathogenic behavior of U. maydis.