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Abundant conidiophores and conidia of Helminthosporium maydis were produced within 48 h on diseased corn leaf pieces. Observations of ultrastructure revealed that conidiogenesis in H. maydis is holoblastic; all layers of the conidiophore wall are involved in the initiation of conidia.

Procedures for sectioning fungal fructifications in host tissues or on artificial media are described, which allow observation of internal structures by scanning electron microscopy. Perithecia of Ceratocystis fimbriata and Phyllachora graminis, and telia of Puccinia xanthii showed excellent preservation of exposed structures in sections which were osmium-coated before being dried. While similar preservation was obtained in sectioned acervuli of Lecanosticta acicola and Marssonina juglandis and in pycnidia of Dothiorella ribis and Phomopsis occulta, the mucilaginous substances produced in these fructifications precluded observation of conidiophores. Extraction of mucilage from these sections was accomplished by periodic acid and dilute KOH treatments, followed by an osmium-coating procedure. In such preparations, mucilage was removed, internal structures were preserved, and pertinent characteristics of conidiogenous cells were resolved.

An intercellular matrix was observed to develop in normal cytoplasmic male-fertile corn lines infected by either race O or race T of Helminthosporium maydis. It usually occurred at the junction of necrotic and healthy cells but did not develop in infected Texas cytoplasmic male-sterile corn lines. Cytochemical tests indicated that the matrix was proteinaceous and ultrastructural examination showed that it was fibrous.