Tar Spot of corn

​This corn disease has become evident in areas of northern Midwest corn states of USA in 2018. Although the disease has been known to occur in specific areas of Central and South America for many years, its first known appearance in the USA was in 2015.  I saw it in our plots in Northern Illinois in 2016 but it was only on a few plants.  Spread and intensity on most of our nursery in 2018 was surprising.  It appears to be in most fields in the region late in the season.  This disease occurrence raises many questions about its significance to effect on yield and stalk breakage vulnerability.  Answers will depend upon understanding the interaction between the fungus (or fungi) biology and that of the corn.
 
The fungus (Phyllachora maydis) is believed to be an obligate parasite of only the corn species.  This implies that it can only grow and reproduce in living corn tissue. It has not been shown to be transmitted in corn seed.  Phyllachora maydisis only known by its sexual reproduction stage.  It belongs to a group of fungi called Ascomycetes identified by production of haploid spores in elongate mycelial tubes called asci after the fusion of spores.  Many corn leaf diseases are caused by Ascomycetes but most of these pathogens reproduce asexually as conidia.  Most also cause initial infection with these conidia being produced on infected dead leaves.  Have we misunderstood this potential in this fungus?
 
It is currently unclear how this pathogen is interacting with corn.  Our PSR corn disease nursery includes many inbred and hybrid entries from US seed companies.  We inoculate these with 5 pathogens to evaluate resistance as a service to the companies.  Most are not identified to us by hybrid or seed parents, but it is assumed they represent a range of genetics and maturities.  I observe plots frequently during the summer but missed the early development of tar spot.  I did not record seeing the disease until at least 2 weeks after most plants had flowered.  Increase of symptoms was dramatic.  It also appeared to be related to rapid increase in gray leaf spot and leaf senescence.  These confusing apparent interactions of senescing leaf tissue increase in gray leaf spot and tar spot has continued.  
 
Many corn researchers are now working to resolve questions brought about by this 2018 observations.  Where did the inoculum originate?  From infected debris of past year, seed or spores blown in from Mexico?  What was the environmental influence that resulted in the disease outbreak? How to identify resistance if artificial culture of the pathogen is impossible?  How to distinguish resistance from maturity differences?  Does Phyllachora maydiscause do significant damage to the plant or is it only significant when combined with other leaf pathogens?  Does the pathogen produce a phytotoxin resulting in death of leaf cells?
 
I suspect these questions will not be easily or quickly resolved.  Biology of this pathogen, interaction with corn plant and environmental influences are all involved in the complex.