Purpling in corn results from the accumulation of a pigment called anthocyanin, a plant compound similar in structure to chlorophyll. Purpling can be a result of genetic response to weather conditions. Some hybrids will produce anthocyanin (resulting in purple plants) as a genetic response to cool nights following bright, sunny days. The second factor is restricted root development in conjunction with a buildup of sugars produced by photosynthesis. These sugars build-up in the leaves and stem of the plant because the roots are not growing fast enough to utilize the sugars. This in turn triggers anthocyanin production resulting in purple plants. The cause of root restriction will generally indicate whether or not yield losses are experienced. If restricted root development is due to cool temperatures then the purpling should disappear as the plant develops and yield losses should be minimal. The following figure illustrates the accumulated GDD at the Tidewater AREC for 2005 versus the long-term average from April 15-May 21. Obviously, the temperatures in this area (and most of the state) are well below normal. This has likely limited corn development and goes a long way toward explaining why we are seeing some purpling in fields.
However, be sure to diagnose the problem. If the cause of root restriction (compaction, herbicides, insect feeding) continues to limit root growth then a yield loss may result. Digging up plant roots and looking for abnormalities such as clubbed (indication of compaction or herbicide damage) or pruned roots (indication of insect feeding) can give some indication of what may be causing restricted root growth. Phosphorous deficiency can also cause purple color in leaves.
Yellow corn can also be a result of cool, wet conditions. When soils are cool and wet, the plant cannot efficiently collect nitrogen from the soil, another symptom of poor root growth. Similar to cold weather purpling, plants will grow out of this condition as temperatures warm.
Twisted growth of corn whorls occurs when the weather goes from cool to hot. Twisted growth of corn is caused by the inability of cells of older leaves to relax quickly enough to allow younger leaves to push up through the center of the whorl. When the whorls eventually unfurl, yellow, crinkly leaves will appear. These are the young leaves that were unable to photosynthesize in the twisted whorl. These plants will green up after a few days of sun. Generally very little yield loss is associated with twisted growth.
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