Markers of Maullinia infection vary across host species, but all exhibit macroscopic signs of hypertrophy. In filamentous hosts (i.e., Ectocarpus), or hosts in gametophytic stages with microthalli (i.e., Macrocystis), swollen appendages can be found. These appendages are oblong to sub-spherical in shape, extending predominantly from the terminal cell on each main filament axis, but are also seen on the lateral branches. Appendages are unpigmented, with apical openings from which Maullinia zoospores are released. These appendages often occur in proximity to host sporangia, increasing the host’s sterility rate and number of abortive sporangia.
