Fluorescence microscope studies of Ustilago maydis and Penicillium italicum after treatment with imazalil or fenpropimorph

Imazalil and fenpropimorph caused morphological changes in sporidia of Ustilago maydis and in germinating conidia of Penicillium italicum, as observed by fluorescence microscopy using an optical brightener. Sporidia of U. maydis appeared swollen, distorted, multicellular and, sometimes, branched; conidia of P. italicum swelled in size, and extension of the germ tubes was strongly inhibited. Mycelium of P. italicum, treated with fenpropimorph, showed much enlarged hyphal diameters and relatively short distances between septa. Imazalil and fenpropimorph also caused an irregular deposition of β–1,3 and β‐1,4 polysaccharides, probably chitin, in U. maydis and P. italicum. The latter phenomenon is discussed in relation to the following observed effects of fungicides that inhibit ergosterol biosynthesis: differences in effect on the morphology of budding and filamentous fungi; preferential inhibition of yeast‐hypha conversion in dimorphic fungi; disorganisation of septum formation in budding fungi; and inhibition of spheroplast formation from budding fungi.