Pathotypes on corn

​Exserohilum turcicum (Helminthosporium turcicum) has been known as a pathogen of corn probably as long as corn has been cultivated in the humid fields.  This fungus overwinters in diseased leaves, produces spores that spread to new crop leaves that germinate and penetrate the leaf tissue.  The mycelium grows in the leaf tissue, plugging the veins until the plant resistance system restricts the growth, causing the fungus to produce a lesion upon which the fungus produces spores and thus spreads further on the same plant and nearby plants.  Time from infection to lesion formation is about 2 weeks, although the plant’s resistance response may affect this time frame.  
 
Corn genetics is a major factor in preventing this fungus from reaching the vascular system within the leaf.  Selection of genetics for reduced number of lesions is practiced by most corn breeding programs.  This system allows the fungus to reproduce but at a slower and less successful rate than more susceptible genetics.  Three or four genes are involved in this type of resistance.  This type of resistance is called horizontal or multigenetic resistance.
 
Environments favorable to the fungus, such as diseased leaf tissue on surface of non-tilled field and frequent rain may increase initial spore production and thus more infection, resulting in increased lesions on even the more horizontally resistant hybrids.
 
A type of resistance that allowed the fungus to reach the leaf vein but prohibited normal lesion production with fungal sporulation was identified in about 1960 in a variety of popcorn.  It was inherited by a single gene referred as Ht1.  Plants with this gene may have poor horizontal resistance allowing successful invasion but prohibiting the spread from the infected leaf tissue by spores became a major factor in controlling the disease.  Most USA corn breeding programs quickly adapted the use of this gene as crossing in the single gene was a much simpler process than selection for horizontal resistance. 
 
A seed production field in 1979 in Indiana, planted with a Ht1 inbred was found to have multiple susceptible lesions caused by this fungus.  After being identified in that field, similar reactions were found in several locations that summer in the Midwest.  The fungus had a gene that overcame the single gene resistance in corn and produced normal susceptible lesions, sporulating and spreading to others. This became known as Race 1 of Exserohilum turcicum.  Other single genes for resistance have been identified (Ht2, Ht3 and HtN) and likewise so has races of the fungus been found to overcome these individual genes.
 
This is not a new lesson.  Single gene resistance, especially one that restricts pathogen reproduction puts considerable selection pressure on the pathogen to favor the variants that can overcome the resistance.