Helminthosporium carbonum races

​Among the fungi that are usually barely pathogenic on corn is a relative of the one causing southern corn leaf blight. The old name for this fungus is Helminthosporium carbonum and the most recent name is Bipolaris zeicola.  It has slight spore structure and pigment differences from the cause of southern corn leaf blight (Helminthosporium maydis = Bipolaris maydis) and traditionally was found more common in the northern parts of the US corn belt.  This fungus is a common inhabitant of many grasses and host resistance usually restricts the fungus to feeding on dead or senescing leaf tissue.  H. carbonum and H. maydis have similar sexual reproduction structures, causing fungal taxonomists to place them in the genus Cochliobolus.  H. carbonum, thus is Cochliobolus carbonum and H. maydis is known as Cochliobolus heterostrophus.  Sex within a species provides new genetic combinations, including single gene combinations that can fit with single gene changes in hosts, resulting in a traditionally weak pathogen such as Helminthosporium carbonum to become an aggressive pathogen.
 
In the late 1930’s, a race of H. carbonum was found that produced a toxin on specific corn varieties, resulting in large leaf lesions and significant moldy corn grain.  Now we had race 0 for the traditional form of the species and race 1, defined as producing the HC toxin. 
 
I came into the seed industry as a mycologist in 1972, immediately after the H. maydis Race T epidemic. I and many other corn pathologists in the summer of 1972 found a wide range of lesion types on corn inbreds in breeding nurseries with fungi producing spores that looked intermediate between H. maydis and H. carbonum but not restricted to the T sterile cytoplasm genetics.  Apparently, the large distribution of H. maydis in 1969 and 1970 allowed the sexual crossing of the two species, resulting in multiple new combinations of the two related fungi.  Within the next 20 years these segregating fungal populations became sorted into more distinct races of H. carbonum.  The races are defined by the symptom and are generally host specific.  Race 2 tends to result in oval-shaped leaf lesions on inbreds and hybrids with susceptible genetics, often with inbred W64 background.  Race 3 is defined by giving very narrow lesions, again on specific susceptible host genetics (B73). Race 4 was defined in 1990 by the oval shaped leaf lesions on B73 backgrounds but not on W64 genetics. 
 
It is notable that these ‘races’ were probably only differences in a few fungal genes and that the hosts were specific inbreds.  Corn hybrids usually were not significantly affected because of dominant resistance in one of the parents but they did cause problems for seed producers. Race 1 is the most destructive of these H. carbonum races because of its toxin.  It also still exists, and occasionally shows up in breeding nurseries when a susceptible genotype occurs. It is an advantage to have it show up in a nursery.  Seed fields scattered across multiple central USA states were suddenly affected by Race 1 in the 2014 growing season.  All had the same or closely related inbreds. A summary of Cochliobolus carbonum can be found at https://en.wikipedia.org/wiki/Cochliobolus_carbonum
 
Genetics of corn and of potential pathogens interactions are constant.