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.
Carbohydrates stored in the endosperm of a corn kernel is a potential source of nutrition for fungi and insects. The pericarp can be a major barrier to attack to these potential invaders. Studies concerning the tropical corn storage insect, Maize Weevil (Sitophilus zeamais), showed that the cross-linked structural components of the pericarp cell walls were highly correlated with resistance to this insect. Other factors included phenols (Afr. Crop Sci. J. 9:431–440) produced by the pericarp cell metabolism and even endosperm hardness (flintiness) contributed to reduced susceptibility to this storage insect (Crop Sci. 44:1546–1552 (2004)).
Pericarp tissue also is a barrier to entrance into the seed by multiple kernel rotting fungi. Most enter the ovary through the silk channel immediately before pollination. This becomes most evident when silks are left exposed for several days in an environment favoring the pathogen. After invasion, the fungus can spread cell-to-cell within the pericarp through small holes (pits) in the cell walls that allows movement of metabolites between cells. Integrity of the pericarp is a significant factor in avoiding invasion by many potential fungal species.
The phenomenon known as silk-cut can expose the seed to fungal infection. After the pollen tube grows down the silk channel and dumps the pollen nuclei into the ovule, silk tissue deteriorates and detaches from the ovary. Not all silks are pollenated even under ideal conditions, leaving some attached to their ovary while adjacent pollenated ovaries grow. These remaining silks interfere with normal contiguous growth of the pericarp cells in the adjacent ovary wall (Plant Disease 81 (5):439-444). This can result in a break in pericarp as the kernels enlarge and thus an opening for invasion by fungi. Genetics and environments influence the occurrence of silk-cut. Stresses that delay silking beyond pollen availability can be an important factor but genotypes vary in vulnerability both to reaction to the stress and probably the tendency of this phenomenon.
The corn kernel is a fruit. Grains are fruits with a single seed enclosed. The ovary wall, part of the female plant, grows after pollination results in enlargement of the single embryo it encloses. The ovary wall thus becomes the pericarp. Genetics of the pericarp cells are those of the female plant and therefore the genetics of the female inbred parent in hybrid seed production or both hybrid parents in the grain field.
These genetics influence the important cell wall components that give both strength and resistance against insect and fungal invaders. Cell walls get their strength from various polymers that are cross-linked. Whereas non-grass species tend to have more lignin chemistry in cell walls than those in grasses and especially the pericarp features less lignin and more of a class of complex compounds called xylan. A major component of these xylan compounds is feruloyic acid. Feruloyic acid is associated with resistance to Maize weevil damage to corn kernels (Crop Sci. 44:1546–1552 (2004). Varieties differ in chemical components of pericarp cell walls that probably influence many aspects of corn grain storage. https://www.frontiersin.org/files/Articles/219955/fpls-07-01476-HTML/image_m
The female parent of a corn seed is the sole genetic source for the pericarp, mitochondria and chloroplasts as well as half of each diploid chromosome.
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.