Spread of new corn diseases

​We are often surprised when a new (to us) race of an established corn pathogen is found or when a pathogen is found in a new area.  It becomes reported as new, but it is highly probable that it had occurred in past seasons but was not found and identified by someone to realize that it is distinct.  
 
Having inoculated disease nurseries with common pathogens near other nursery plants for multiple years, it was always notable to me that the fungal blight was usually not seen elsewhere in the same field.  The northern leaf blight fungus, Exserohilum turcicum, produces spores in 2-3 weeks after first infecting the leaf.   Those spores from those lesions blown to damp leaf tissue to allow new infection will not produce more spores for another 2-3 weeks. Most corn environments do not provide the needed moisture for continual new infections when the beginning one started at the V8 stage of corn growth.  The discovery of the new race in a seed field in Indiana probably was aided by frequent movement in the field by machinery and people plus a susceptible inbred. The fact that after first identified it was found by several specialists scattered across many areas of USA the same year suggest that it had existed for a several seasons slowly building in intensity.
 
I recall seeing a field in Southern Minnesota that was heavily infected with the fungus Kabatiella zeae causing eyespot that was mostly limited to the area of the field that was in corn the previous year.  Apparently, the minimal tillage for the previous season had resulted in large loads of inoculum to infect the new crop but spores had not spread sufficiently to adjacent fields to be noticeable.  
 
A major factor in spread of new fungal pathogens is initial infection, probably not noticed by humans for a few years but eventual increase with minimal tillage and continuous corn growing in the same field.  Continuous crop growing is associated with root worm vectored Maize Chlorotic Mottle Virus, one of the components of the corn lethal necrosis in Nebraska and Kansas.
 
Some pathogens are spread over long distances by wind. Rust fungi, Puccinia sorghiand Puccinia polysoraspread north into the central corn belt by winds.  Vectors of viruses are spread by wind as well.
 
Genetic diversity available within Zea mayshas always provided adequate resistance available to corn breeders within a few years after a new pathogen is identified. Early detection is important to preventing significant damage to the crop and altering the cropping system can be a significant immediate control.