Weather and Gray Leaf Spot

Cercospora zeae-maydis infection is strongly related to relative humidity.  Weather in Northern Illinois in 2018 has provided a prime example of the difference between the dependence on moisture from rain for infection by Exserohilum turcicum, cause of northern corn leaf blight, and that of the fungus causing gray leaf spot.  Both fungi sporulate on infected corn debris.  C. zeae-maydis spores are considerably lighter allowing further distribution within and outside the corn field.  E. turcicum requires some free moisture on the leaf to germinate and penetrate the leaf epidermis within a few hours after germination.  C. zeae-maydis depends more on humid air than free water to germinate and grow on the surface of the leaf. Growth chamber experiments showed that penetration into the leaf only occurred after 100 hours of 90-100% relative humidity. It is remarkable that the hours do not need to be consecutive but can occur after a series of humid nights, for example.  The fungus appears to have the ability to halt hyphal growth until the next high humidity period.
 
Most of the midseason weather here this summer has featured little rain but warm humid days and nights.  Northern leaf blight is not frequent in corn but scattered gray leaf spot lesions are easily found.  This example is typical of occurrence of this disease.  Although the disease was known in USA more than 50 years ago, it gained notice in the humid environments of Virginia in early 1980s.  Wide use of susceptible genetics allowed spread first to areas around river valleys further west until it had spread to the irrigated areas in western corn belt. The pathogen is now present throughout most areas of USA corn belt, becoming especially noted during warm humid seasons.
 
Weather during corn growing season affects these two corn pathogens in distinctly different ways.  Heavy spores of the Northern Leaf Blight fungus tend to not be carried far by wind whereas the lighter spores of the Gray Leaf Spot fungus are easily moved field to field.  Northern Leaf Blight fungus is favored by frequent rain showers, but the Gray Leaf Spot fungus is favored by less rain but warm humid weather.