Humans are compelled to assign names to things, partly for the practicality to communication and partly to bring order to the life we witness. A system called binomial nomenclature was formalized by Carl Linnaeus in 1753 in which living organisms were named by genus and species. These Latin-based names were intended to assign individuals that were morphologically similar to the same genus but separate species if they were sufficiently distinct and did not sexually cross in nature. For most flowering plants, this naming system is clear. Sunflower species in genus Helianthusare morphologically distinct from corn in genus Zea. Most specialists (taxonomists) recognize 6 species of the genus Zea of which Zea mays (corn) and Zea diploperennis (teosinte) are most prominent.
This nomenclature often is dependent on assumption that distinct species do not intermate in nature and thus not produce intermediate types. Consequently, the emphasis in higher plants includes morphological features of the flowers. This principle was also applied to micro-organisms although the difficulty of recognizing morphological features often required microscopes.
Difficulty in identifying multiple distinct characters of fungi is further complicated by rarity of sexual reproduction. Many corn fungal pathogens reproduce asexually producing huge numbers of spores to spread to new host surfaces. Consequently, initial nomenclature for a fungal pathogen is based upon the features of these spores. The fungus causing southern corn leaf blight of corn was name Helminthosporium maydisbecause the asexual spores, called conidia, were long, darkly pigmented and slightly curved. This species was distinguished from Helminthosporium carbonum, cause of northern leaf spot, because the latter had similar, but slightly darker spores without curves. H. carbonumtended to infect corn in cooler areas than H. maydis but the presence of race T of H. maydisallowed massive mingling and sexual reproduction between two species resulting multiple intermediate conidia features and corn lesions.
Northern leaf blight of corn is also caused by a fungus with long dark conidia and thus was assigned to the genus Helminthosporium, called H. turcicum. It was later realized that a major difference in spore shape between other Helminthosporium species what a protrusion on the spore called the hilum and thus the genus name was changed to Exserohilum. H. maydisand H. carbonumshare a conidial feature of germinating at both ends and thus put in the genus Bipolaris. Current accepted names for these pathogens are Bipolaris maydis, Bipolaris zeicolaand Exserohilum turcicum.
We humans try to communicate a complex reality of living organisms with a simple nomenclature that is vulnerable to change as we learn more about these organisms.
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About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.