Two related corn pathogens (part 1)

Prior to 1940, the cause of a small leaf spot in the northern USA and of a larger leaf spot in the southern USA were considered one species, Helminthosporium maydis.  Both had similar conidia with dark pigments.  Upon further study, and especially when a variant of the one with shorter and darker spores caused considerable leaf death and ear rot on a few corn inbreds, the two variants were categorized as separate species.  Helminthosporium carbonum became associated with northern leaf spot and the race associated with the extreme damage as Race 1 of this species.  Race 0 was associated with the much smaller lesions on most corn varieties.  The other species remained as Helminthosporium maydis and was more common in the warmer environments of the southern USA caused the disease called southern corn leaf blight.  H. carbonum had less curved and darker conidia, as viewed with a light microscope and was generally considered less pathogenic when race 1 was not effective. 
 
Further study of these two species revealed that both had two mating types resulting in separate sexual recombination within each species.  Rules for fungal names therefore caused Helminthosporium carbonum to be name Cochliobolus carbonum and H. maydis as Cochliobolus heterostrophus.  Asexual stages of these two are currently named Bipolaris zeicola and Bipolaris maydis.
 
Like many leaf pathogens, both enter the leaf by setting us an appresorium on the epidermal surface from which it penetrates into the mesophyll of the leaf.  A combination of toxins and enzymes allows the fungus to kill some tissue, from which it asexually produces conidia to spread the pathogen to new leaf tissue. Race 1 of B. zeicola produces slightly different toxin (HC-toxin) that is destroyed by an enzyme (HC-toxin reductase) in corn plants with a dominant gene called HM.  Consequently, the toxin is only effective in varieties that are homozygous recessive (hmhm) for this gene.  Fortunately, the dominant version of this gene is found in most corn inbreds.  Unfortunately, the fungus B. zeicola is a common pathogen of many grass species including those frequent near corn fields.  The rarity of severe damage to corn from this pathogen allows it to be overlooked in corn breeding nurseries.  I recall finding Race 1 in one entity of a breeding project in Nebraska in 1988 but if that natural infection had not occurred that susceptibility of that new inbred may have been advanced to hybrid seed production.  This apparently did happen a few years ago when seed fields in South Dakota, Central Illinois and Central Ohio showed significant leaf lesions, ear rot and dead plants from Race 1 of B. zeicola even after fungicide application.  The fungus, including race 1, perhaps maintained on other grasses in several areas of the corn belt or perhaps spread unnoticed with seed from a seed increase found an inbred with hmhm gene.  It is unlikely that it ever showed on hybrids made with this inbred because the dominant gene HM would come from the other hybrid parent.
 
Another pathogenic variant of B. zeicola was identified in the 1960s giving slightly larger leaf lesions and some ear rot.  It was mostly limited to northern corn belt and specific genetics especially those related to the inbred W64A.  This was designated as B. zeicola race 2. 
 
Things between these two corn pathogens became even more interesting after Biploaris maydis race T evolved.  This is the subject of the next blog.