Exobasidium

Exobasidium is a genus of fungi belonging to the order Ustilaginales. These fungi are plant pathogens that cause a variety of diseases, most notably leaf and stem galls on a wide range of host plants. The genus is characterized by its hyaline, unicellular basidiospores which are produced on a smooth, colorless basidium. The teliospores are absent, and the basidia develop directly from the intercellular mycelium.

The most common and well-known species is Exobasidium vexans, which causes the devastating tea blight disease. This pathogen infects tea plants, leading to leaf spots that enlarge and coalesce, eventually causing leaf death and significant yield losses. Other species of Exobasidium affect various herbaceous plants, shrubs, and trees, causing symptoms such as hypertrophied leaves, distorted stems, and abnormal flower development.

These fungi typically overwinter as mycelium within the host plant tissues. Infection often occurs through wind-borne basidiospores, which germinate on susceptible plant surfaces. Environmental conditions such as high humidity and moderate temperatures often favor disease development. Control measures for Exobasidium diseases can include the use of resistant cultivars, cultural practices to reduce inoculum, and the application of fungicides. The exact taxonomic placement of Exobasidium has been subject to revision with the advent of molecular phylogenetic studies, but it remains a significant group of plant pathogenic fungi.