Infection of young corn plants by pathogens affects the disease development in older plants. Exserohilum turcicum, cause of northern corn leaf blight, spreads spores from infected corn leaf debris to the whorl of young plants. About two weeks later, lesions develop producing more spores spread by the wind to other plants. If the spores land in the whorls, infection is likely because of the moisture. Spores of this fungus are most often spread within a field or to nearby fields because of the size of the spores. The gray leaf spot fungus, Cercospora zeae, on the other hand has lighter spores that are more likely to be spread over greater distance.
Puccinia sorghiand Puccinia polysora, causes of common and southern rust respectively, have small urediniospores that are carried great distances by wind. These fungi requiring a living host plant, are maintained. Infection of corn in USA Midwest occurs after spores are carried from winds from the Mexico and Caribbean Islands to be deposited in corn whorls after rain storms. These fungi quickly produce more urediniospores that spread and infect more plants.
Goss Wilt bacteria, Clavibacter michiganensissubsp. nebraskensis, mostly need plant tissue injury to enter the corn plant. This pathogen is spread from diseased leaves of the previous season to young plants during storms, especially with hail, that causes injury in the leaf tissue. The bacteria can spread within the plant, eventually killing young plants.
Young corn plants are also vulnerable to damage from virus infections. These diseases are usually associated with transmission of the virus by an insect vector. Maize dwarf mosaic virus, vectored by aphids, causes significant damage if infected by V2-V6 stage. Aphids, thrips, beetles and mites are often the vector associated a specific corn virus. Severity of the disease often depends upon the timing of the vector presence and the maturity of the corn plant. Severity of the disease often does not become apparent until the plant reaches the flowering stage.
Environments of the early season that favor good growth can make a corn field look beautiful. That same environment can also favor pathogens. Resistance to the pathogens not obvious until the plants approach maturity. Dynamics of diseased debris from the previous season, intensity of the disease in corn far away, origin of storms, and presence of pathogen vectors all affect the eventual disease development in the corn field.
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.