Ok I was way off, ignore what I said. Tar spot is a fungus, Phyllachora maydis . The complex happens when a second fungus, Monographella maydis, also attacks the plant. I’m sure P. maydis is still around, but it hasn’t become complex in our area since 2018.

According to a bulletin from Purdue it needs low temps, high humidity, and leaf wetness for extended times.

Here’s an excerpt as well as some links:

“P. maydis is not widely
considered to cause economic damage when present
alone, although there were isolated reports of damage
in old literature. However, significant yield losses were
reported from the tar spot complex, consisting of P.
maydis and another fungus (Monographella maydis)
associated with tar spot.
Monographella maydis was not detected in any U.S.
tar spot samples from 2015 to 2018. However, 2018
observations indicate that the secondary fungus is not
required to cause damage. Here in Indiana, P. maydis
alone can cause yield loss under favorable environmental
conditions.

https://www.extension.purdue.edu/extmedia/bp/bp-90-w.pdf

https://www.pioneer.com/CMRoot/Pioneer/US/Non_Searchable/agronomy/cropfocus_pdf/Tar-Spot-Complex-in-Corn.pdf

I find it interesting that it notes that the fungus is present on the leaf surface without causing harm.

From Pioneer, “M. maydis is commonly found on the surface of corn leaves, and
usually is asymptomatic, because it is only in association with P.
maydis that M. maydis can develop signs of tar spot. P. maydis in
association with M. maydis causes the fungus to become
pathogenic and highly virulent.”

I don’t know plant physiology well enough, but I wonder if the corn plant processes nutrition differently at high humidity and low temps resulting in some imbalance? Or something in the micro-biome results in a trigger for P maydis to begin growing?