Ustilago filiformis  

Sori visible as long dark stripes on living leaves, to ca 1 mm broad and ranging from around 10 mm in length to the entire length of the leaf, rupturing the epidermis at maturity (usually on the upper surface) to release the spores, the spore mass pale to olivaceous brown and powdery. Ustilospores 4-6 (-8) µm x 4-5 µm, globose to subglobose, sometimes rather irregular in form, not formed in clusters, pale brown, thin-walled, aseptate, appearing smooth but actually minutely verrucose when viewed under oil immersion.