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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.

The interactions between soybean plants and two species of mycorrhizal fungi were examined. The number of infection units in young seedlings was found to increase in the presence of increasing quantities of Glomus fasciculatus inoculum, until a maximum was reached. Soybean seedlings remained free of mycorrhizal infection during early stages of development. First infection units appeared 10–12 days after planting which was also the approximate time that rhizobium root nodules and tertiary roots were first observed. The response to colonization by vesicular–arbuscular mycorrhizal fungi was evident as early as 6 weeks after planting.

A technique is described that employs vapor fixation and a simple vapor diffusion dehydration apparatus to minimize the disturbance of delicate fungal structures during preparation for scanning electron microscopy. The technique is applicable to fungi grown on agar media or on natural substrates.

The vesicular–arbuscular mycorrhizal fungus Glomus mosseae was examined by transmission electron microscopy and energy dispersive X-ray analysis. Electron-dense granules, found within vacuoles, were analyzed by X-ray analysis and found to contain high concentrations of phosphorus and calcium. These granules were similar in appearance to the polyphosphate granules described in other organisms. Highly vacuolated intercellular hyphae and vesicles possessed the greatest number of phosphorus granules. The granules were present in vacuolated arbuscular hyphae but absent from completely collapsed arbuscules. It was determined that the active arbuscule was the most probable site of breakdown of phosphorus granules. There were two other types of cellular inclusions, one vacuolar and one cytoplasmic, that could not be identified by X-ray analysis but that could easily be mistaken for phosphorus granules. It was concluded that general morphological appearance and location were not adequate criteria for the identification of phosphorus granules.

The ultrastructure of the interfacial zone which separates the intracellular structures of vesicular-arbuscular mycorrhizal fungi from host cytoplasm has been described in a variety of ways by recent investigators. Evidence is presented here which suggests that previous interpretations of the ultrastructure of the interfacial zone have been based on an artifact of fixation. Using an improved procedure, a dense, granular material was found in the interfacial zone. This material was preserved by simultaneous glutaraldehyde-osmium fixation but not by conventional prefixation and postfixation in glutaraldehyde and osmium, respectively.

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.

Phomopsis longicolla Hobbs, a causal agent of Phomopsis seed decay, colonizes the seed coats and embryos of mature soybean (Glycine max (L.) Merr.) seeds. The mode of infection of immature soybean pods and seeds by P. longicolla was examined using scanning electron microscopy. Phomopsis longicolla penetrated directly through pod walls by appressoria. Penetration through stomata in the pod surface was not observed. Immature seeds were also penetrated directly. Heavily infected immature seeds showed extensive colonization of the seed coats and embryos.

Mature conidia of race O and of race T of Helminthosporium maydis were examined microscopically to characterize the general ultrastructure of the conidia and to determine if the two races could be differentiated on the basis of conidial morphology. At the light-microscope level, the two races could not be separated. Histochemical analyses showed that, in both races, the cell wall was composed of chitin with melanin pigmentation. Electron-microscopic examination of thin-sectioned conidia revealed the presence of two distinct cell walls. Cross-walls were found to have typical ascomycetous septal pores. The types of cell organelles found in mature conidia did not provide any indication of the pathogenic capability of the fungus. No consistent differences were found in the ultrastructure of the two races and it was concluded that race T could not be differentiated from race O on the basis of conidial morphology.