Weather and Northern Leaf Blight

​This corn disease caused by the fungus Exserohilum turcicum should have a different name because it occurs tropical, subtropical and temperate zones in all continents.  Its occurrence and severity, however, is greatly affected by weather.  Spores (conidia) are produced on moist, infected dead or living leaf tissue in humid conditions.  These spores are bigger and heavier than those of some corn pathogens such as the cause of gray leaf spot (Cercospora zeae-maydis) and thus tends to move a short distance within a field. The small percentage that are picked up during strong storms do spread the disease longer distances however. There are instances where the disease will be concentrated in a small area of a field apparently initiated with a few early infections and spread mostly to adjacent plants.
 
Spore production is encouraged on a moist substrate in a humid environment, but new infection requires the spore to be in moisture for a few hours. Condensation on leaves with cool evening temperatures can be adequate, moisture in the leaf whorl while plant is young, and frequent rain showers are usually adequate for the spores to germinate. These elongate, multicellular spores send out hyphae from cells on each end. The hyphae set up clusters that enzymatically drill into the host epidermal cells to absorb nutrition.  At his point the fungus is no longer dependent on weather.  
 
The fungus grows within the leaf towards the vascular bundle.  As it feeds and grows in the local vascular cells, a small wilt symptom develops about 2 weeks after initial spore germination. High relative humidity that can be associated stimulates the creation of more spores from the lesion and with rain showers the cycle is repeated.
 
Occurrence of severe outbreaks of this disease in seasons and locations with frequent rain showers and higher humidity has resulted in natural and intentional selection of higher levels of resistance in some corn-growing regions than others.  Open pollinated varieties of the early 1900’s with highest resistance were selected in the Eastern United States.  Many tropical varieties tend to have higher levels of resistance than those selected in drier environments. 
 
This disease frequently is more severe in a small region that happened to have frequent rain showers during the corn midseason development. Resistance differences will be more evidence in these conditions than when corn is grown under drier and less humid environments, which may occur in the same field the following season.