Weather and corn leaf diseases

​Variables affecting corn leaf disease damage nearly always involves moisture and temperature within a corn growing region during a critical corn growth period.  Moisture of debris from a previous corn crop is usually critical to spore production by potential pathogens causing many leaf blights.  Very slight air movement within a field is sufficient to move spores of many pathogens the short distance from the soil surface to young plant whorls where moisture is usually available, allowing germination and penetration into the leaves.  Further distribution from the initial infection can be associated with gentle winds associated with rain storms.  Violent storms with hail cause physical damage to leaf tissue, allowing entrance of some pathogens such as the cause of Goss’ Bacterial wilt.  Long distance distribution of pathogens is often associated with direction of storms as spores of some pathogens are easily carried in these winds.  Pathogens dependent on reproduction on living corn plants are moved from those areas to more temperate zones by storms.
 
Air temperatures during the corn growing season affects corn leaf diseases as well.  Warm and dry environment general inhibits fungal spore production. Cool evening temperatures are usually associated with dew forming on corn leaves, providing the moisture for germination of fungal spores and penetration of the pathogen into the corn leaf epidermis.  Warm and humid summer evenings is ideal for some pathogens like Cercospora zeae-maydis, cause of gray leaf blight.  Frequent rain favors the spread and infection of pathogens such as Exserohilum turcicum, cause of northern leaf blight.
 
Vectors of virus diseases are also affected by weather as aphid intensity is associated with drier weather.  Corn flea beetles, vector of the bacteria causing Stewarts Bacterial Wilt, movement from environments where the bacteria are maintained on other grasses to new corn planted as the soils warm.  Distribution of the insects are often affected by direction of wind.
 
Annual fluctuations in weather not only affect a corn variety’s physiology and resulting grain production, but also the significance of resistance to a specific disease.  A variety may be regarded as adequately resistant to a specific pathogen when under usual low intensity of that pathogen but inadequate when the weather factors change.  If we are entering into a period of more erratic weather patterns, we should expect some surprising vulnerability of some varieties to diseases.  Corn, as a species, appears to have adequate resistance within its genetics to any pathogen, but it requires time and effort by many people to identify the cause of a new disease occurrence, to identify the source of resistance and incorporate the resistance into productive corn varieties.