Biology of Northern Leaf Blight

​The disease is caused by the fungus Exserohilum turcicum (Setosphaeria turcica).  The fungus lives in infected, dead or live leaves.  It asexually produces spores (conidia) when the diseased tissue is moist and temperatures are proper for corn growth.  The conidia have 4-6 cells arranged in a row and are light enough to be distributed by air currents within a corn field. After landing on a corn leaf, with a little moisture, in 3-6 hours the cells on both end of the conidia, begin dividing, emerging as germination tubes. These new hyphae quickly form a base on the surface called an appresorium, from which the fungus grows into the leaf epidermis. Within 12-18 hours after the conidia have landed on the leaf, it has successfully penetrated the leaf.  There appears to be no difference in time to leaf penetration between the susceptible and resistant corn hybrids.
 
Chloroplasts near the infection point soon lose pigments as nearly 100 cells die, perhaps because of enzymatic activity of the fungus.  This can be observed when small (0.5-1cm) circular, yellow spots show in the leaves in 24-48 hours after infection.  From this initial location, the hyphae grow between cells towards the vascular bundles.  Penetration of the vascular bundles is followed by plugging the xylem causing further death of surrounding cells dependent upon the water supplied through these tubes.
 
Resistance systems appear to begin after initial infection and perhaps mostly once the fungus has reached the vascular system.  There does seem to be differences in that initial yellow spot in a couple days after infection and I wonder if the brighter color is not associated with greater quantitative resistance.  Could part of quantitative resistance be self-destruction of cells near the infection point, depriving the fungus of nutrition?  It does seem that initial spot is less obvious and possibly smaller in the more susceptible corn genotypes.
 
The wilted areas surrounding the plugged xylems eventually are depleted of living host cells. The fungus responds by producing new conidia within 14 days of the initial infection, ready to spread to more live leaves.  Exserohilum turcicum remains viable in dry leaves for a number of years in dry environment, ready to produce conidia within 24 hours after moistened.  Tillage, crop rotation and hybrid resistance become major factors in crop damage from this disease.
 
Much is written about this disease. An interesting summary of the morphology aspects in included in this report: Journal of Applied Biosciences (2008), Vol. 10(2): 532 - 537.
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