Single gene resistance

​Resistance to leaf diseases in corn that limits the number and size of lesions is called horizontal or quantitative resistance and usually involves 3-5 genes.  Ratings for this type of resistance involve a scale developed after considering disease pressure from different environments but does provide a stable type of resistance. Resistance to some leaf pathogens can have another system, usually involving a single gene, that stops some races of a pathogen more completely than horizontal resistance. This latter type of resistance is called vertical or qualitative resistance.
 
The Ht1 gene was discovered in 1964.  It prevented the northern leaf blight pathogen, Exserohilum turcicum, from developing the normal wilted lesion symptom after it reached the vascular tissue.  Instead of the normal lesion formation, a small yellow streak developed and, most importantly, the fungus failed to reproduce with spores capable of spreading the disease within the corn field. This seemed ideal in the USA because its presence was easily identified and damage from the disease was eliminated.  Consequently, the Ht1 gene was utilized in commercial hybrids during the 1970’s.  In 1979, a race of this fungus was found in several locations in the U.S. Corn Belt that overcame the Ht1 gene resistance, resulting in normal lesion and sporulation.  It appears that the fungal gene responsible to overcome the Ht1 gene was present in a low frequency within the widespread population of E. turcicum.  Its frequency increased as it gained competitive advantage over those individuals without this gene.  Similar races of this fungus had already been noted in South Africa and the Philippines.
 
Other single genes ( Ht2, Ht3, Htn) for resistance to E. turcicum have also been identified, as well as races of the fungus that overcome those resistance systems.  This is not a new phenomenon.  Genetic diversity within pathogens have repeatedly shown an increase of individual genes producing products to overcome single gene resistance. It should be noted that the term race for a pathogen refers to only a single gene difference within the pathogen population.  It probably existed as a mutation, allowing a slight structural change in a protein that happened to be attacked by the host plant’s resistance product.  Qualitative, vertical resistance to a disease in corn offers quick answers but stable, long-term benefits are best when quantitative, horizontal involving several genes are employed in corn hybrids.