tar spot biology

​Several aspects of the tar spot fungus, Phyllachora maydis. Appears to be unique corn leaf pathogens.  It belongs to a group of fungi called Ascomycetes, because its sexual reproduction stage involves formation of an ascus after the fusion of two nuclei.  Miosis occurs to produce 4 haploid single cells each of which divides to produce 8 ascospores within a sac called an ascus.  A thick-walled group of cells form black ‘stromata’ on the surface of the leaf enclosing several of these spore-containing asci.  With the right environment, such as a humid warm night, the spores are released from the ascus and easily moved in the wind.  After germinating on the surface of the corn leaf, the hyphae establish specialized structure that grows into the leaf.  Apparently, within susceptible hosts, the fungus absorbs nutrients and reproduces soon, again releasing spores into the air.
 
Many corn leaf pathogens are ascomycetes but most of the fungal spread is done be asexual reproduction, with conidia the pathogen spreading mechanism, with the sexual stage being restricted to end of season reproduction.  Although genetic mutation can occur within the fungus before conidial reproduction, this opportunity for development of new genetic combinations is not as profound as with fusion followed by meiosis such as in meiosis following the sexual union of nuclei.  This may allow variants to continually develop within this pathogen.
 
Being an obligate pathogen, only known to grow on living corn leaves, certainly makes study more difficult but eventually some of the variation will become understood. Currently no other host has been identified but genetic variability would make this a possibility.  Widespread outbreaks like in summer of 2021 seemed to favor this disease.  Perhaps variants will also adapt to less humid weather as well.
 
This disease has potential to cause significant damage to corn and must be studied with urgency.