Corn leaf whorl

​As the corn plant approaches the 10-leaf stage, before extensive stem cell elongation, cells in the new leaves rapidly elongate.  Newest leaves are wrapped around the older ones forming a whorl of leaf blades until the leaf blade elongate enough to separate the leaves.  If the growth is too rapid the whorl tends to get very tight or twisted.  This is usually associated with extra warm temperatures, especially when following days of cooler than normal temperatures.  Growth hormone herbicides can have the same effect. If twisted too tightly for more than a few days, sections of the leaves do not get sufficient light for chloroplasts to produce pigments, resulting in a temporary yellow leaf band when those sections finally emerge. Twisted whorls has been associated with drought conditions apparently because of the effect on cell elongation.
 
The corn leaf whorl also provides a nice warm, moist environment for pests and pathogens to infect the plant. European corn borer (Ostrinia nubilalis) larvae and corn leaf aphids (Rhopalosiphum maidis) are favored in this cozy environment. Uredospores of the rust fungi Puccinia sorghi and Puccinia polysora often being their initial infection in the humid, moist environment of the leaf whorl.  Not only does this allow the spores the perfect germination and infection environment but the whorls provide a pocket to catch the spores arriving with rain.  These fungi do not overwinter in the central USA corn belt but the spores are carried north by winds from Mexico and Caribbean corn plants.  Spores deposited by the rain into the Midwestern leaf whorls find a good home to become established in the field, and to spread to the rest of the plants if the season weather favors them. 
 
Spores from many corn fungal pathogens overwintering on the previous season’s corn debris often make the initial infection in the whorls, again because of this favorable environment.  This is usually revealed with a band of a disease symptoms in the leaf a few weeks later, as the leaves emerge.  The smut fungus Ustilago maydis, is believed to enter corn plants only through injured tissue and the leaf smut phase is associated with injury to the whorl when it is twisted.
 
As with most parts of the corn plant’s biology, a lot is going on that is not obvious even during the first few months after planting.