Three centuries ago, Linneaus came up with a system of giving names of all living organisms. It included a genus name to include closely related plants and animals and a species name for individuals that were distinct and supposedly would remain distinct by not interbreeding with other species. It is a universally accepted method to allow communication across languages attempting to assure that all are addressing the same organism. Identification of plants generally emphasized the sexual reproduction stage as a distinction, as morphology of the flower often affects genetic isolation of a species. The ‘scientific’ name of an organism is usually in Latin and consequently is italicized when included in English text.
But what to do with those organisms that are known only to reproduce asexually but later discovered to have a sexual stage as well? The name for the genus Helminthosporium was created to include fungal species with elongated, multi-cellular, dark pigmented, asexually-produced spores (conidia). The name was created because of the similarity to a group of parasitic worms called Helminths. Helminthosporium species are largely pathogens of grass species such as corn, oats, rice, wheat and turf grasses. Helminthosporium maydis, causal agent of southern corn leaf blight, H. carbonum causes northern leaf spot and H. turcicum causes northern leaf blight. Each of these species primarily spread via the conidia and the shape of the conidia as it appears under a microscope is diagnostic for the species. Then the sexual stages were discovered as well as a few other conidial distinctions. H. maydis and H. carbonum not only share a similar sexual reproduction structure but also the conidia germinate in a similar manner from each end of the spore. Thus the asexual genus name for these and some others was changed to Bipolaris and the name for the sexual name is now Cochliobolus. So the cause of southern corn leaf blight is now Cochliobolus heterostrophus or Bipolaris maydis. Northern leaf spot is caused by Cochliobolus carbonum or Bipolaris carbonum. As if this is not confusing enough, it is also known that these two species can cross, causing all sorts of confusion in names, as will be discussed in a future blog.
Helminthosporium turcicum conidia have distinctions from some others of the original genus so now are grouped together with similar morphology as the genus Exserohilum. The sexual stage also has been identified making that name as Setosphaeria turcica. To communicate on a practical basis, most corn pathologists refer to the cause of northern leaf blight as simply Exserohilum turcicum and some of us old guys have trouble avoiding the use of Helminthosporium turcicum. Most non-pathologists prefer to simply name the disease and forget all about naming the fungus, and that is ok too.
Visit us at the ASTA in Chicago, Dec 9-12 (booth G207)
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.