More often a wet mount of the sample needs to be made and viewed using the compound microscope. When viewing diseased tissue with the stereo microscope, focus on the affected plant part, usually this is a leaf spot. Fungi produce certain structures within the affected necrotic portion of the leaf that aid in identifying it. Using the stereo microscope you should be able to narrow the fungal identification based on the fungal fruiting structures produced within the leaf spot. Acervuli and pycnidia contain the conidiophores and spores of the fungus.
Conidiophores are the structures that produce the fungal spores.
While focusing on the fruiting structures, cut out a small section of the affected spot containing the structures for viewing with the compound microscope. If too large of a section of tissue is used to make a wet mount you will not be able to view the sample clearly.