The continual battle between the carbon producer, like corn, and the organism seeking carbon nutrition involves biology of both organisms. Each corn pathogen has evolved different biological features for success. Exserohilum turcicum, the fungus causing northern leaf blight of corn produces spores (conidia) with thick wall that do not immediately desiccate, germinate within a few hours of exposure to moisture and set up an appresorium and penetrate the leaf surface within a single day. The relatively heavy spores tend to not travel far within a field from the original infection site.
Cercospora zeae-maydis, cause of gray leaf spot, by contrast has relatively thin-walled conidia that germinates on leaves with only high humidity (80-90% RH). After germination the mycelium remains on the leaf surface until being exposed to 95% humidity for a total of nearly 100 hours. The mycelium can tolerate as low as 60% humidity in a somewhat dormant and then continue to grow when higher humidity returns. After meeting the minimum, an appresorium forms and the fungus enters to leaf surface. The thinner walled spores of Cercospora zeae-maydis is more easily moved in the winds and therefore spreads easier within a field.
Common rust fungus, Puccinia sorghi, is even more easily spread in the wind. Spores (urediniospores) have thick walls but are round and about a third the size of the E. turcicum spores. Thick walls prevent desiccation allowing long distance travel and light weight encourages it. The spores cannot withstand the absence of susceptible hosts between live corn presence during the temperate winters. Consequently, production of a new urediniospores occurs in more tropical areas with continuous corn growth. High altitude winds carry the spores to the temperate zones where new corn plants are growing. These spores quickly germinate in moist corn leaf surface, germinate, then an appresorium above a stomata, penetrating the leaf. This requires about 6 hours of moisture. The leaf whorl, always moist, fits this process. New spores are quickly produced and spread within the field can happen quickly.
These are three examples of corn fungal leaf pathogens with different infection and biology.
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.