This application relates to the production and characterization of a mutant strain of Aspergillus fumigatus. The application also relates to a method for inhibiting siderophore biosynthesis in Aspergillus fumigatus and an assay for identifying drug candidates or other agents having potential inhibitory activity. The method may comprise, for example, the step of inhibiting an enzyme catalyzing siderophore biosynthesis, such as L-ornithine N.sup.5-oxygenase. In one embodiment the siderophore is a hydroxamate siderophore, such as N'N''N'''-triacetylfusarinine C (TAF) or ferricrocin. A method of preventing or treating fungal infections in a patient is also described comprising administering to the patient an agent suitable for inhibiting fungal secretion of siderophores. The method is particularly useful for immunocompromised patients susceptible to fungal infections caused by Aspergillus fumigatus, such as pulmonary aspergillosis. The invention also relates to diagnostic methods for detecting a biomarker indicative of likely A fumigatus infection in vivo, such as serum presence of TAF.