Single gene resistance and fungal races

​Northern leaf blight damage is controlled by two distinct systems.  Neither system appears to limit the ability of the fungus (Exserohilum turcicum (=Setosphaeria turcica) to penetrate the leaf surface. Once inside, the fungal hyphae are detected by the nearby cells resulting in production of chemicals to slow the growth toward the vascular bundles.  This resistance system involves at least 3-4 genes and is regarded as stable.  Fungal variability and intensity causes some range in lesion development but varieties with best multiple gene resistance will show under nearly all levels of fungal pressure by having fewer lesions than more susceptible ones.
 
The complexity of evaluating and selecting genotypes with the multiple resistance genes, especially when appropriate disease pressure is not present has made it attractive to select genotypes with single gene resistance systems.  Such a system was found in a popcorn variety in the 1960s and soon incorporated in many USA corn breeding programs.  This HT1 gene did not limit the fungus from reaching the vascular system but took effect after it reached. The gene products limited the fungus, inhibiting from wilting the host tissue and, more importantly, inhibited the fungus from producing spores and thus spreading within the field. Although isolates of the fungus in Philippines and Hawaii were overcoming this resistance earlier, it took 10 years of widespread commercial corn use in mainland US corn belt before this single gene was found to be overcame with a variant of the fungus. Such variants are commonly referred to a new race.  Because it may only involve a single gene difference in the pathogen, I prefer the term pathotype, but certainly the race term is commonly used to describe the newly describe mutant.  In this case, the E. turcicum variants detected and controlled by the Ht1 gene is called race 0.  Race 1 is given to the fungus not controlled by the Ht1 gene.
 
Three other single genes for severe limitation of this fungus have been identified in corn. These genes are designated as Ht2, Ht3 and HtN, but consequently fungal variants have been found that overcome each of these single gene systems. The races of this fungus are noted as combinations of race 0, race 1, race 2, race 3, race N or combinations such as race 1,2,3 or 2,3,N. 
 
Diversity of genetics within a fungus is not surprising.  Selection pressure for mutants that allow the fungus to reproduce is great.  Best to select for the more stable multiple gene resistance in corn.