Fusarium and corn
Fungi, like the rest of us non-photosynthesizing organisms, are dependent on plants for existence. Species of the fungal genus Fusarium are ubiquitous, often feeding on dead and living plant materials. They are identified by microscopic observation of their asexually produced spores (conidia) abundantly produced from the filaments (mycelium) growing in and on plant tissue. When these fungi are stimulated to sexually reproduce, they develop spores in microscopic ‘sacs’ called asci. This means of sexual reproduction places Fusarium in a class of fungi called ascomycetes. The genus of ascomycetes that have Fusarium as a conidial form is Gibberella. Consequently, a fungus frequently found in corn stalks, leaves and ears may be identified by its conidia shape as Fusarium verticilloides but if stimulated to form sexual bodies it would be called Gibberella fujikuroi. Another species of Fusarium (Fusarium graminearum) more frequently is found on corn stalks and ears as the sexual stage, Gibberella zeae. This dual naming system, tolerated by mycologists and plant pathologists, occurred because the fungus was initially only known and named by the asexually produced conidia.
These fungi on corn are not aggressive pathogens but mostly invade dead or weakened cell tissue. Infection of corn ears by Fusarium species is often through old silk tissue. Fusarium or Gibberella stalk rot occurs after the stalk tissue has been weakened by desiccation due to roots rot. Fusarium mycelium is easily found in leaf tissue, as if it is an occupant, perhaps feeding on weakened or dead cells within living leaves. Its widespread presence in corn seeds, seedlings, stalks, leaves and ears often leads to difficulty in determining its significance. Was it an aggressive pathogen killing the tissue or was it an invader of weakened tissue? Is seedling blight due to a Fusarium species attacking a vigorous, corn seedling or was it simply infecting a corn seedling weakened by environment? Gibberella stalk rot occurs when the plant-environment- genetics interaction results in roots dying from insufficient carbohydrate to sustain metabolism. Stalk cells die because of consequential wilting and shortage of carbohydrates as well. Fusarium graminearum feeds on the weakened and dead tissue, eventually producing the sexual reproduction bodies of Gibberella zeae and thus allowing us mere humans to call it Gibberella Stalk Rot.
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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.