For the past two weeks, purple corn has been observed in several fields throughout southern and western Minnesota. Although corn that is deficient in phosphorus has a purple color, it is doubtful that phosphorus deficiency is responsible for the observations this spring and early summer.

The purple coloring this year, as was the case in 1997, is probably caused by a situation called the “fallow syndrome.” This situation usually occurs in areas where a crop was not grown last year or following sugar beets.

An explanation of the “fallow syndrome” involves an understanding of a symbiotic relationship between the corn plant and a group of fungi called mycorrhizae. It’s a “ You scratch my back, and I’ll scratch yours” relationship. The mycorrhizal fungi grow and develop around corn roots and assist the root in taking up nutrients. The mycorrhizal fungi are usually associated with uptake of phosphorus and zinc. The fungal growth is stimulated by excretion of sugars and other organic compounds from the root system. So, the mycorrhizal fungi and the corn plant help each other – a symbiotic relationship.

The population of the mycorrhizal fungi reaches a minimum following a non-host crop or where no crop has been grown. The sugar beet crop is not a host for these fungi. Therefore, the “fallow syndrome” is frequently observed when corn follows a sugar beet crop. Likewise, repeated tillage of soil where a crop was not grown last year can produce a “fallow syndrome.” For example, the “fallow syndrome” may appear where heavy rains flooded soybean fields and the flooded area was tilled repeatedly to control weeds.

There are no management practices that are appropriate for correcting “fallow syndrome” this year. As temperatures warm and soils dry, the purple coloring usually disappears.

A banded application of phosphate fertilizer near the seed at planting can prevent the problem. Broadcast applications of phosphate are not effective. A banded application of 20 lb. P2O5 per acre at planting is suggested.

The “fallow syndrome” does not appear every year. A banded application of phosphorus is good insurance to prevent the problem. This is especially true when corn follows a sugar beet crop.