Well, a month ago, we were received reports of white in corn fields. Some Southern Illinois growers were "spotting" white on leaves (Holcus spot), while others were trying to solve the mystery of white corn plants in Central Illinois. For more on this you can refer to my May 19th blog: Everyone is Talking about White Corn
Now, Northern Illinois (Ottawa and Southeast of Mendota) must need " have a turn" to have a corn, color issue, but this time, their color is purple. Today, I was asked if there had been any reports of areas or entire corn fields (V5-V7) that were purple. I had no additional information, nor pictures, but I asked some of the U of I Crop Science Extension Specialists if they had heard of any reports of "purple corn fields" and I asked what, in their opinion, could cause a large area of corn, growing in moderate to high temperatures, beyond a growth stage of V5, to become purple?
Dr. Mike Gray, U of I Crop Sciences Extension Entomologist:
"I have not heard of any reports. Because of the clumped distribution of insects within a field -- it is unlikely that an entire field showing purple discoloration is insect related. However, some insect feeding (e.g. grape colaspis and/or white grubs) that prune root hairs on seedling corn plants can lead to purple discoloration on plants. The mechanism -- poorer absorption of nutrients (especially phosphorus) by the root systems."
Dr. Fabian Fernandez, U of I Crop Sciences Soil Fertility and Plant Nutrition Extension Specialist
"I have not had any reports. Would it be possible to get a picture of the field and a close up on the plant? Purpling of corn leaves is typical for phosphorus deficiency, but often the problem is observed with younger plants when it’s very wet or cool. I suspect if they are seeing this at V5-7, and is due to phosphorus, the field is either very deficient in phosphorus, or something terribly wrong is going on with the root system that is not allowing plants to take up phosphorus. Both situations seem very unlikely to me because 1)it is not very common to see entire fields completely phosphorus-deficient and 2) if the problem is related to the roots there are other nutrients that would show deficiencies too, making for very sorry-looking plants. Again, if you can get a couple of pictures, that would be beneficial."
Dr. Aaron Hager, U of I Crop Sciences Weed Science Extension Specialist
I was lucky enough to have Dr. Hager looking at 7 crop samples, with potential herbicide injury, today at the U of I Plant Clinic. In passing, in the hallway, I quickly asked him his thoughts on purple corn. He said that some herbicides can cause corn to turn purple. However, it is very, unlikely for an entire field to show symptoms for very, long. For example, glyphosate may cause purple leaves in corn, but it will not stay purple, in time, it could become "smoked".
As noted by Dr. Nafziger, in his comments below, other herbicides can cause injury by limiting root growth, but they just are not used that much anymore.
Dr. Emerson Nafziger, U of I Crop Science Agronomic Extension
"The “proximate cause” of purple corn is sugar accumulation. This can arise from P deficiency, since P is needed for sugar transport. In young plants, though, it usually results from inability of the root system to grow out into the soil around the base of the plant. Sugars accumulate when roots can’t grow, and the anthocyanin pigment then forms.
We’ve heard of some of this in areas where the surface soil is dry this year (I wrote about it in this week’s Bulletin) -The Bulletin: What Crops Need Now If it’s only in parts of the field, it will often be in places where the surface soil dried out before nodal roots could penetrate. Sometimes it can be in heavier soils as well, where the sidewall that forms during planting dries to form a barrier.
So I’d suggest looking first at root growth. In some cases the seminal roots will have grown more than normal, but at some point they just can’t support the plant anymore. If the field recently had rain, the problem will likely go away very quickly. If the crop is still purple several days after rainfall, then other possible causes need to be looked at. But since we don’t use many herbicides that limit root growth anymore, there aren’t that many causes to consider."
The Agronomist that is dealing with the issue of corn turning purple made the following comment after receiving the above information and consulting further with Dr. Fabian Fernandez:
"We have been extremely wet, and I wonder if this problem is related to saturated soils. I am beginning to think so. Dr Fernandez also mentions extremely low P levels. In one area that I see the purpling, this is a possibility, in another, it is a field that is extremely HIGH in P."