Fast-growing corn in the last week has resulted in a few scattered plants in some fields - at least in central and west-central Iowa - displaying bright yellow leaves waving above the canopy (see photo). Corn with these symptoms is around the V5 to V8 growth stage. The yellow upper corn leaves are sun-starved, entrapped earlier in the whorl by more mature leaves. They will wave for a day or more above rapidly growing crop canopies before turning green.
The symptom is not unusual; we reported it in the ICM News in both 2008 and 2009 occurring at the same growth stages, as well as late as the tenth to twelfth leaf stage. An article posted on the ISU Extension and Outreach corn website also addressed the subject.
Most of the leaves should unfurl within a few days. The pale-yellow leaves will green as they accumulate chlorophyll. If the deformity causes a delay in either growth or development, it may reduce yield on a plant basis, but the effect - if not widespread - likely will not influence overall yield on a field basis.