Fusarium in maize

Fungi identified as a species of the genus Fusarium are found in many plant species and even humans.  Fusarium species are defined by their characteristic macroconidia shape.  After mating types are identified and resulting sexual stages are seen, most of the Fusarium species are renamed as a Gibberella species. Fusarium verticillioides (previously known as Fusarium moniliforme) is a cause of an ear mold.  The sexual stage of this fungus is Gibberella fujikuroi. 

Gibberella zeae, one of the fungi associated with stalk rot produces small black structures (perithecia) on the surface of the stalk near an internode of a rotting stalk.  These contain the products of meiosis (ascospores) produced after two mating types of the fungus also known as Fusarium graminearum.  Although this dual naming system for the same organism is confusing to us, the fungi seem to know what they are doing!  There are some Fusarium species named because of distinctive characteristics that specialists have yet to associate with a sexual stage.
 
This naming confusion is only part of the difficulty of defining the significance of this group of fungi.  They often appear to be ‘opportunistic’ pathogenic. The fungus is common in dead plant refuge in most soils, obtaining nutrition from the decaying organic matter.  This propensity to feed on dead, or dying, plant cells plus production of an abundance of spores results in it being found on and in corn leaves, seed, roots and stalks, often without clarity of whether it is the prime cause of the plant condition or only an opportunistic occupant taking advantage of weakened tissue that is unable to fend off this invader.
 
The frequent isolation of a Fusarium species from corn plant tissue causes confusion as to its importance in actually causing the corn disease problem.  It is not unusual to see Fusarium growing from what appears to be normal germinating corn seed in germination tests.  The fungus is frequently found in leaf tissue killed by an herbicide or insect feeding.  Seedlings struggling for growth under stress conditions frequently have Fusarium present and therefore the condition is called Fusarium seedling disease.  Fusarium is a common occupant of rotting roots and stalks and often gets implied as the cause of the condition.
 
It is not unreasonable to expect a range of pathogenic capabilities among the isolates of a Fusarium species, making some association of the species with disease condition very appropriate.  Like much of the real world, the actual interactions among the host, environment and pathogen are not easily characterized.