Corn pathogen diversity

Northern leaf blight is the name usually given to the corn disease caused by the fungus Setosphaeria turcica.  The original name for this fungus was Helminthosporium turcicum was given in 1876 and was based upon the pigment and shape of the conidia spores.  Taxonomy researchers attempt to group closely related organism into a common genus name.  Fungi have limited morphological characters to use with spores being the most commonly as a stable feature.  It is indicative of the prevalence of asexual reproduction in this fungus that the genus name changed to different names based upon asexual characters from Helminthosporium to Bipolaris, Drechslera, Luttrellia and then Exserohilum in 1974.  The sexual stage of this fungus was not confirmed until the 1950’s, initially named Trichosphaeria turcica and later in 1974 to Setosphaeria turcica.  The time lapse between the recognition of this pathogen based upon it asexual conidial stage and identification of its sexual stage indicates the significance of asexual reproduction of this pathogen.
 
Setosphaeria turcica sexual reproduction occurs when hyphae of two mating types (MAT1 and MAT2) fuse, followed by the combining of the nuclei chromosomes, recombination and eventual segregation into new haploid nuclei.  These form 4-6 individual spores within a sack called an ascus. Sexual reproduction does assure new genetic diversity but the rarity of finding the two mating types perhaps indicates that this is not the most significant source of genetic diversity of this species.
 
Those of us that have grown isolates of this fungus in artificial culture media frequently see differences in growth patterns, pigments and sporulation among the isolates.  This pathogen is widely distributed on corn.  Although it usually prefers cooler environments of 15°-25°C (59°F-77°F) for infection it is adapted to most temperate and semitropical environments.  The higher frequency of both mating types in more tropical environments, especially in Mexico, suggests that it originated along with the early development of corn and perhaps was distributed with the crop. 
 
Distribution to many geographic locations, exposure to multiple corn genotypes, haploid hyphae producing huge numbers of conidia has resulted in diversity within the species, whether we call it Setosphaeria turcica, Exserohilum turcicum or even Helminthosporium turcicum.