Movement of corn pathogens

​The 2019 early corn growing season will probably feature unusual disease movements.  Having personally seen many of the diseases shortly after being first identified, I do often reflect on how they got there. Eyespot was first identified in Japan in 1959 but more than 10 years later it was found in North Central USA, Argentina, Brazil, Europe and New Zealand. Was it a minor pathogen (Kabatiella zeae) so weak on corn that it only became noticed after more susceptible inbred like Wf9 and W64A became widely used?  
 
Goss wilt bacterium, Clavibacter michiganensis sp. nebraskensis, was first identified in Nebraska in 1970 where it was especially virulent on some genotypes. Switching to more resistant hybrids quickly reduced the damage but the disease was later found in several states. These pathogenic bacteria were found to overwinter in diseased corn debris but how did it spread? 
 
Corn Lethal Necrosis (also known as Maize lethal Necrosis caused by a combination of Maize Chlorotic Mottle Virus MCMV and either Maize dwarf Mosaic virus or Wheat Streak Mosaic virus was first identified in 1976 in Nebraska and Kansas.  The virus had been identified in Peru in 1974.  Since then MCMV has been found in Argentina, China, Thailand and central Africa.  This virus is transmitted by beetles and thrips.  Severe symptoms show only when MCMV plants are also infected with the other viruses.  Seed transmission has been shown at an extremely low rate.  Resistance has been identified.
 
Damages races of more common corn pathogens such as race T of Bipolaris maydis, Races 1 and 3 of Bipolaris zeicola, Race 1 of Exserohilum turcicumall were related to use of specific
susceptibility genes. Grey leaf spot became notable as susceptible genotypes became widely used combined with less crop rotation and more previous crop debris.
 
Bacterial Leaf Streak, caused by Xanthomonas campestris pv. zeae, was first identified in Nebraska in 2016. Previously it was only known to occur in South Africa. Is has since been found in 7 USA State and probably in Argentina. Was it always in these areas as a minor pathogen of other grasses but environment and host susceptibility allowed it to increase sufficiently to be identified?  Tar spot has been recently noted in north central states after previous identification in higher altitude South American areas. Caused by synergism of two fungi species, much is still be learned about the disease.
 
There remains much to be learned on mechanisms of susceptibility, environmental factors and movement of pathogens involved in occurrence of ‘new’ corn diseases.  It is a non-ending problem in a widely grown crop.