Gray Leaf Spot emergence

​The pathogen, Cercospora zeae-maydis, had been known for 50 years before it became a major corn disease pest.  Two major factors in its emergence as important were increased susceptibility and tillage practices that allowed more diseased leaves from the previous season to remain above ground.  Both factors emphasize important dynamics in emergence of new diseases.
 
An important hybrid parent inbred that became widely used either directly or as a breeding parent in the 1970s was B73.  It is susceptible to this pathogen but at the time of its early use this disease was scarce in the U.S.A. corn belt.  Concurrent with the introduction of this germplasm, minimum tillage, continuous corn practices were increasing.  Both factors were, and still are, important to corn grain production. 
 
This fungus cannot compete with soil microorganisms if buried but can overwinter on exposed diseased leaves. When moist, the lesions from the previous season will produces spores (conidia) that are easily picked up by the wind. After landing on a moist corn leaf the spore germinates with hyphae attaching to the epidermal wax surface.  It does require a unique accumulation of nearly 100 hours of 90-100% humidity before it will form an appresorium to enzymatically drill into the leaf tissue.  This may take only a few days or many days.  Initially the disease was prominent in the valleys in humid Eastern U.S.A. but eventually was seen in Ohio River valley.  Later it became a major problem in much of the Midwestern USA.  Reaching the high humidity is most commonly associated with dew formation, that is not uncommon with evening cooling in the Midwest and especially in corn fields.
 
The resistance factor was also an important variable.  This disease increased with susceptible hybrids, a few of which were very popular in the late 70’s.  Even with low intensity of the spores in tilled fields these few hybrids would show the disease.  Most current hybrids will have some lesions in many fields but only occasionally enough to cause significant yield damage.
 
Most ‘new’ corn diseases have a similar history.  The pathogen is mostly obscure and regarded as insignificant until susceptible genetics becomes widespread and environment changes in some way that favors the pest. With realization that the disease is significant, corn breeders tap the vast genetic diversity of maize to identify and incorporate genes for moderation of the pathogens effect on corn grain production. Often those genetics are available within their breeding program.
 
Genetic diversity among pathogens changes in corn growing environments will forever allow new disease pressures on corn.  Thanks to its unique reproductive structure encouraging cross-pollination and its unique history of interactions with humans, resistance to ‘new’ diseases will always be found.