Like the swallows that return to Capistrano, it seems that purpling in young corn returns every year somewhere, says Bob Nielsen, Purdue University corn agronomist.

Nielsen says purplish plants are typically more prevalent in wetter areas of fields and almost nonexistent in the drier areas.

"While mildly attractive from an ornamental standpoint, landlords and tenants alike often become concerned when they see their fields take on a purplish hue that is clearly evident from the window of the pickup at 60 mph," he says.

Nielsen says purpling of corn plant tissue results from the formation of a reddish-purple anthocyanin pigments that occur in the form of water-soluble cyanidin glucosides or pelargonidin glucosides.

He adds that a hybrid’s genetic makeup greatly determines whether corn plants are able to produce anthocyanin. A hybrid may have none, one or many genes that can trigger production of anthocyanin.  Purpling can also appear in the silks, anthers and even coleoptile tips of a corn plant.

So what triggers the color change? (See images from Purdue University.)

"The answer is not clearly understood, but most agree that these pigments develop in young plants in direct response to a number of stresses that limit the plant's ability to fully utilize the photosynthates produced during the day," Nielsen says. "These stresses include cool night temperatures, root restrictions and water stress — both waterlogged and droughty conditions.

While he says many Eastern Corn Belt corn fields have suffered through wet soil conditions during the past several weeks, Soil compaction can also restrict the development of the initial root systems. The additional stresses imposed by quite a few relatively cool nights (upper 30’s to low 40’s) and the occasional bright sunny days (high levels of visible and UV radiation) may be the final “triggers” that result in fields of purple plants.

"I have rarely diagnosed phosphorus deficiency as the primary cause of purple plants early in the season," Nielsen says. "Nonetheless, cold or wet soils inhibit root development and can aggravate a true phosphorus deficiency situation, frequently causing even more intense leaf purpling."

When it comes to yield losses, Nielsen says the cause of leaf purpling, not the purpling itself, will determine whether yield loss will occur by harvest time.

"If the main cause is the combination of bright, sunny days and cool nights, then the purpling will disappear as the plants develop further with no effects on yield," he says. "If the stress of restricted root systems is a major contributor to the purpling, then the potential effects on yield depend on whether the root restriction is temporary or more protracted.

"Plants can recover from temporary root restrictions with little to no effect on yield. If the root stress lingers longer, the purpling may continue for some time and some yield loss may result if the plants become stunted."