The following question was sent to the P&PDL diagnosticians here at Purdue University:
Question: My question is concerning purple corn plants. I have them quite frequently in a particular field (in Nebraska). The corn is to the second or third leaf stage and is quite purple. Not all of them however, I think it is a lack of phosphorous. We deep banded it and I don't think the corn has quite hit it. How serious is this?
Answer: There are a number of reasons for purple corn. Here in the eastern cornbelt there are three common causes: cold conditions slowing growth or cold events such as a light frost, damage to the root system from insects, herbicide carryover or compaction, or genetics. Phosphorous deficiency doesn't make the list because we have a very very high percenatage of our soils with adequate or more P soil tests. In your part of the country, especially in high pH or lower fertility dryland conditions you are more likely to have some P problems.
What effect will it have, hard to say. If it is cold or genetic there should be little if any impact. Problems before the four or five leaf stage are easily overcome and have little lasting effect in many cases.
If it is P, it will eventually tap the deep bands and take off. Again, probably little damage if that occurs in the next few days.
Since you describe it as non-uniform, "quite frequently in one field" I would take a serious look at the root system for signs of injury. Flattened, thickend roots from compaction, insect damage, fertilizer burning from ammonia or your deep banded materials that were not as deep as you thought. Look carefully for patterns.
--Dave Mengal, Agronomist