Biology of one affects biology of others

​Humans are witnessing the affects of genetics of a virus affecting human behavior everywhere.  Dynamics of the COVID-19 and people interactions will have some permanent affects, some of which are not yet clear.  Biology, affected by genetics, in plants and their pathogens have similar long-term interactions.  We witnessed this in the outbreak of the Southern Corn Leaf Blight in 1969 and 1970.  That phenomenon was blogged in Corn Journal issue 129/2019.
 
Race t of Helminthosporium maydis(Bipolaris maydis) (Cochliobolus heterostrophus) spread across most corn growing areas in USA and elsewhere in 1970.  The traditional version, race 0, of this pathogen was common in the Southeastern USA where temperatures and humidity favored the biology of the fungus.  A related fungus Helminthosporium carbonum (Bipolaris zeicola) (Cochliobolus carbonum) was a common pathogen of corn but tended to be more frequent in the northern part of the US corn belt.  The summer of 1970 not only featured epidemics of race T of H. maydis but spread of this pathogen to much of northern corn belt. This allowed the co-mingling of the two species.
 
The two species were distinguished by microscopic examination of their conidia, the asexually produced spores associated with spread of these fungi.  H. maydis spores were consistently curved and appeared to be gray when viewed with a light microscope.  H. carbonum conidia were darker in pigment and mostly straight.  Both species had shown to have similar sexual reproduction structures and to have distinct sexual mating types.
 
Seed companies, including the one that I had just joined, were checking their inbreds and hybrids in the summer of 1972 to make sure there was no remnant susceptibility left among their materials. I was surprised to find a wide range of shapes and sizes of lesions naturally occurring among materials that looked like southern corn leaf blight in our central Illinois nursery.  Examining the spores under microscope showed a range of spore shapes intermediate to H. maydis and H. carbonum.  Other pathologists found the same thing.  It had been shown previously that these two species could cross in lab experiments and now it appeared that the wide-spread distribution of H. maydis into regions where H. carbonum was common allowed multiple opportunities for sexual crosses between the two.  This apparently accounted for the range of conidia morphology seen in the summer of 1972. 
 
The resulting population of these multiple crosses further sorted in virulence on corn.  H. carbonum already had been found, with one race (race 1) to produce a toxin affecting a few inbreds homozygous recessive to the toxin. Another group of this pathogen appeared to mildly pathogenic on corn leaves was defined as Race 2.  After 1972, some inbreds and hybrids were found to be susceptible to Race 3, resulting in distinctive long, narrow lesions. In 1980, inbreds with B73 backgrounds, commonly used as female parents in seed production fields, were infected with a distinct race 4 of H. carbonum.  Apparently, the crosses of the two related fungal species resulted in new genetic combinations.
 
This event occurring 50 years ago not only changed corn breeders wariness of corn’s disease vulnerability but also all involved with corn to be constantly observing for subtle changes in pathogen-host problems.  Biology of one affects biology of others.