U. bromivora spores germinate (A) and form a promycelium (B). Under high nutrient conditions, haploid yeast-like progeny (sporidia) are released and proliferate via budding (C). Under low nutrient conditions intratetrad mating occurs between two adjacent cells of a promycelium by formation of a loop-like mating structure that connects both cells (D). After plant penetration, fungal filaments grow mainly along the stem without triggering macroscopic symptoms (E) until flower development occurs. Upon flowering, macroscopic symptoms are detectable as black, smutted spikelets filled with fungal spores (F). Fungal cell walls and nuclei were stained with WGA-Alexa Fluor 488 and DAPI, respectively (A–D). Plant membranes were stained with FM4-64, fungal hyphae with WGA-Alexa Fluor 488 (E). Scale bars: 5 µm (A–D), 10 µm (E, right panel), or 1 cm (E, left panel) and (F).