Several Fusarium species infect corn at all stages of the host’s life cycle. These fungi often are not aggressive pathogens in the sense of attacking vigorous cell tissue but are ubiquitous in soils and on decaying plant materials allowing opportunity to invade living, weakened plant tissue. Among those are senescing silks still attached to freshly pollinated corn ovules. This can result in obvious kernel rot on the ear but also some internal infection of developing seed. The fungus remains in the seed after drying and revives after seed is moistened. This seed may not be sufficiently damaged by the fungus to inhibit germination. Imbibitional damage to embryo cell organelles may allow more damage by the fungus, especially if cold stresses inhibit membrane repair, but germination, although delayed, may still proceed.
Cool soils during germination also gives advantage to Fusarium species in the soil to invade weakened seminal roots and mesocotyl tissue as the coleoptile is being pushed to the soil surface. If sufficient damage is made to the mesocotyl, cutting off the supply of nutrients from the seed endosperm and seminal roots, coleoptile and enclosed new leaves will die, as the very young seedling wilts shortly after emergence. If the young plant survives this early infection, the Fusarium can continue to spread within the growing plant, perhaps having no visible effect on the plant.
The genus Fusarium is the asexual stage of certain members of fungi known for their sexual stage as Ascomycetes. Asexual stages of these fungi often reproduce and spread through asexually produced spores called conidia. They allow aerial spread of the fungus but also can move within the vascular system of the plant. Sexual reproduction in most ascomycetes requires the combination of two mating types and thus is less frequently found. Consequently, often a fungus is identified by its asexual stage, such as Fusarium, and only associated with its sexual stage after study by specialists. Fusarium graminearum, commonly identified in seeds is the asexual stage of Gibberella zeae and Fusarium verticilloidessexual stage is Gibberella fujikuroi.
Gibberella zeae is associated with Gibberella stalk rot because the sexual stage (perithecia) is expressed at the outside of the dead stalk. On the other hand, fusarium stalk rot is associated only with the asexual stage of a different species, Fusarium verticilloidesbecause that is the stage most frequently found in the dead stalk.
Ubiquitousness of Fusarium fungi in corn often allows them to be associated with dying or dead tissue although they may not be the main cause. This is true of seedling rots and of stalk rots of mature plants often involves environment stresses but these fungi quickly spread to damaged tissue and thus become associated with the problem.
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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.