Tracya hydrocharidis  

A smut fungus forming minute spore balls in senescing leaves of Hydrocharis morsus-ranae (frogbit).

Sori occuring in pale spots on leaves of Hydrocharis morsus-ranae (frogbit), inconspicuous, forming minute black or grey balls when held up to the light and examined with a hand lens, or when illuminated from below under a dissecting microscope. Spore balls up to 260 μm diam., spherical to ovoid, pale brown, gregarious or scattered, immersed in senescing leaf tissue; exterior comprising a relatively smooth continous outer single layer of joined prismatic and radially arranged spores; interior a central network of loosely woven, pale brown, septate hyphae.

Description adapted from Spooner (2006). Additions and amendments to the list of British smut fungi. Mycologist, 20(3), 90-96.