Ultrastructure of the conidia of Helminthosporium maydis

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.