It is expected during this time of year to see lots of leaf spotting on corn that may be confused with diseases.  I expect that 99% of the time it is due to paraquat drift and the paraquat may come from your neighbor across the woods and down the road.  Yes, herbicides can drift a long way.  Growers using paraquat as a burn down are certainly conscientious of drift and try to reduce it as much as possible.   The winds of April and early May will pick up the fine liquid particles during spraying and lift them up off target much further than you realize.  It certainly can be severe if a grower is not paying attention to whats growing in the next field over but in most cases it’s not a severe problem. You will see it in 24 hours. Unfortunately, if you happen to have corn in the V11 to V13 stage, it will damage the ear leaf.  If it is just a few spots like the pictures included in this entry then no yield loss is measurable.  It will take much more damage to register any lasting affects.

If you are uncertain as to the cause of the spots, certainly take a sample to your local county extension office and ask for a quick diagnosis.