Seeds are surrounded by many microorganisms mostly feeding on organic matter that is mostly dead. The photo, copied from our publication of Stalk rot of Corn power point, shows the multiple colonies grown from a particle of soil in the spring of a corn field. These organisms are competing for the carbs locked up in the dead debris in the soil. Fusarium species are among these that can also penetrate some living corn roots.
Fusarium is a fungus genus composed of many species. This group of fungi asexually produce spores called conidia that move in the wind. Most of these species also have a sexual stage in which the haploid hyphae fuse to form a diploid structure (ascus) in which the nuclei undergo meiosis, producing haploid ascospores, to produce more hyphae with new genetic combinations. The fact that the asexual and sexual stages are separate in time and, sometimes, location adds to the difficulty for specialists to associate the two. The ‘rules’ of naming these fungi declares that the sexual stage is primary. Consequently, on fungus commonly infecting corn seedlings, stalks and ears is known as Fusarium graminearum but also by its sexual stage name of Gibberella zeae, Another common Fusarium infecting seedlings, stalks and ears is Fusarium verticillioides also known for the sexual stage name of Gibberella fujikuroi. Adding to the naming confusion, the previous name for F. verticillioides was Fusarium moniliforme.
The names are confusing but biology of these fungi is not much better. They tend to mostly be weak pathogens. They can enter weakened root and mesocotyl tissue of the germinating seed. Host resistance seems to limit the fungus ability to destroy much tissue but it appears to simply live in the corn tissue without causing visible damage. It is not unusual to find at least a few seed germinating on a paper towel that look normal but have some hyphae of Fusarium verticillioides growth. This species frequently can be isolated from corn leaf tissue with no apparent damage. Likewise, Fusarium species are nearly always isolated from stalks of corn plants. Often, if no symptom associated with another pathogen, such as the black streaks in outer rind caused by the anthracnose fungus (Colletotrichum graminicola), we tend to call it Fusarium stalk rot. The death of the stalk mostly was a biological problem of the corn plant and fungi like Fusarium was there, among others, to assist in the digestion of dead stalk tissue.
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.