Fusarium

​Corn seed is planted in complex environment with a mix of soil particle density, moisture levels and other organisms, some of which capable of digesting any carbohydrates they can reach. When seed does not emerge as expected we often look for a cause including fungi associated with the poorly emerging seed. This often is not simple.

Among the perplexing interactions in corn is that with corn and the fungus Fusarium verticilloides. This fungus was formerly known as Fusarium moniliforme and is the asexual stage of the fungus Gibberella fujikuroi. Many, (most?) seed samples germinated by paper methods will show at least a few seeds with this fungus growing from them, even with the appearance of normal seed germination. The fungus does produce a toxin called fumonisin that can cause rejection of grain by some livestock and grain elevators. This can occur in kernels showing not symptoms of infection.

The fungus can be found in corn roots, stalks and leaves as well, often without symptoms. It is not unusual to find this species growing from dead corn leaf tissue when moistened. It is as if it is an inhabitant of corn. Does it become transmitted to the next generation from infected seed? It is acknowledged that seed can be infected via growth of the fungus in the silk. Or does it become from infected debris in the soil? Or, perhaps entering thru injuries to plant tissue.

A study published at Appl Environ Microbiol. 2003 Mar; 69(3): 1695–1701 followed the spread of F. verticilloides from infected seed and from inoculated soil through corn plants using a carefully designed group of experiments using fluorescent strain of the fungus detected by fluorescent microscopy. They confirmed that this fungus can infect through the root to the mesophyll, often growing between cells. This can result in stunted seedlings, especially if grown under low light conditions. This growth can advance into the stalk tissue, in leaves and even into the seeds, sometimes without showing symptoms. More of the fungus was found in the plant if the soil inoculum load was increased. There was spread to the roots and to the rest of the plant from infected seed but the soil inoculum load appeared to be more significant.

Low light association with increased symptoms suggests that the metabolic health of the plant affects the defense against this fungal invasion. That also is consistent with the presence of this fungus in nearly all rotted stalks. If other stalk rotting fungi, such as those associated with Diplodia stalk rot, Gibberella stalk rot or Anthracnose stalk rot are not found, we tend to call it Fusarium stalk rot because it is always there. The low physiological state of stressed corn plants as they approach completion of grain fill increase the vulnerability the expansion of this fungus into the dead stalk tissue.

Microbes inhabiting the corn plant reflect the complexity that actually is affecting the biology of the corn plant.