New corn diseases come from where?

Most ‘new’ corn diseases are not really new.  Instead they often are newly acknowledged in a country or a state.  Some are like Goss’s bacterial wilt, which is caused by a variant of pathogen (Clavibacter michiganensis subspecies nebraskensis) that was known to infect wheat and other grasses including foxtail, but,  in 1969 it was found to badly damage corn.  It caused severe losses to specific hybrids that had been damaged by hail previous to flowering.  It is probably significant that these fields were in an area of Nebraska where there was continuous corn cropping, often with the same popular genetics, and frequent early season hailstorms.  The extreme susceptibility seemed to be limited to relatively few corn parent inbreds.  Later the disease was found further east and most recently caused alarm, again with relatively few susceptible hybrids. This is one disease that seems to have originated from the combination of pathogen diversity, inadvertent inclusion of genetic susceptibility in host and perhaps continuous corn planting.
 
Head smut of corn, caused by a variant of the fungus Sphacelotheca reiliana, has showed up sporadically in many scattered areas in many countries.  The species is more common on sorghum and Sudan grass but the variant attacking corn is specific to corn.  Initial infection occurs in seedlings in relatively dry, sandy soils when initially planted at warmer temperatures than usual (70°F).    The fungus then grows within the plant towards the growing points as the plant develops, ultimately completely replacing the ear and the tassel with spores.  These thick-walled spores can remain viable in the soil for many years.  Although only a few plants initially are infected, the numbers increase over seasons, especially with susceptible hybrids, until it noticed and then appropriate changes are made.  How did it first get there? Maybe wind or perhaps carried by a few seed but it probably initially was so insignificant that no one noticed.  Head smut is not the same as common smut although both cause black spores.  Microscopic examination can distinguish between them. A field tip is that when head smut fungus shows up on the tassel there is nearly always an infected ear with no kernels.
 
We move seed and grain around the world and probably potential pathogens in very low rates move also.  Most corn pathogens also infect other grasses near our cornfields.  Genetic variation in the potential pathogens allow adaptation to corn.  Genetic variation in corn, contributing to the continual increase in performance, also allows for undetected genes for susceptibility to unsuspected pathogen.  By the time we see the ‘new’ disease, it probably has been present for a few years.  Takes a few more years to understand the dynamics but ultimately a combination of effort by many gets the disease under control.