Fusarium Seedling Disease

​When a poor stand is apparent as the seedlings emerge, we search for a cause. Seed treatments include control of Pythium. After eliminating soil compaction, freezing and insect damage, seedlings can be submitted to a pathology lab for microscopic examination. It is probably that the fungus Fusarium will be found.  But that is not a simple answer to the problem. 
 
The fungal species of the genus Fusarium have a complicated relationship with germinating corn seedlings. The most studied species, Fusarium verticilloides (formerly known as Fusarium moniiforme and its sexual stage as Gibberella fujikuroi commonly is found in germinating corn seeds.  It often is found in corn plants without symptoms of damage and therefore is characterized as an endophyte because it appears to live within the plant tissue but does not always cause symptoms.  It is not uncommon to see growth of this fungus from germinating seed in paper germination test. A study published in 1997 (Plant Dis. 81:723-728) compared seedling growth from seed artificially infected with this fungal species with those that were not infected.  There was no difference in germination percentage between infected vs uninfected seed. There was a slight size difference favoring the uninfected seedlings at 7 days after planting but at 28 days those growing from the infected seedlings were slightly bigger and with more lignin in cell walls. Is this because of a hormone (gibberellin?) produced by the fungus or because of some defense compound produced by the plant? The fungus was easily recovered from the seedlings but less from the older leaves. Infected plants showed no symptoms of disease. 
 
It is clear that Fusarium verticilloides can be damaging to germinating seed sometimes but I don’t think all the factors are clearly understood.  Is the difference caused by the strain of the fungus, the host plant or the environment?  I know from experience that it is so common to find Fusarium growing from a dead leaf sample that one tends to ignore it.  It seems to live in much of corn plant’s tissue.  It often leads to confusion with diagnosis of problems including stalk rot, almost as if one cannot find other fungi usually associated with rotting stalks, there is always Fusarium. 
 
It’s complicated!