The mechanisms governing traffic across the Golgi are incompletely understood. We studied, by live-cell microscopy, the consequences of disorganizing theAspergillus nidulansGolgi, using an extended set of fluorescent protein markers to resolve early from late cisternae. The early Golgi syntaxin SedVSed5 and the RabORab1 regulatory GTPase play essential roles in secretion, cooperating in the ER–Golgi interface. Following a temperature shift-up ‘on-the-stage’, hyphae carrying engineeredsedVR258GandrabOA136Dtsmutations arrest polarized growth. This arrest correlates with overall Golgi disorganization and characteristic hyphal tip swelling. Using v-SNARE SynA as reporter, we show that thesedVR258Gphenotypes correlate with arrested secretion. Both the morphogenetic defect and the secretory deficit are reversible. Thus downregulation of secretion, like that of endocytosis, has morphogenetic consequences, implying that mechanisms tuning the secretory pathway might be involved in developmental processes. According to the cisternal maturation model, acute impairment of traffic in the ER–Golgi interface should lead to disorganization of both the early and the late Golgi cisternae. Thus, the relatively rapid late Golgi disorganization observed upon shifting ER–Golgi interface mutants to the restrictive temperature seems incompatible with anA. nidulansGolgi network organized on the basis of stable early and late compartments, supporting instead cisternal maturation.