There have been numerous reports of corn that is slightly yellow, purple or striped. There are many reasons for these symptoms to occur. The top three that may cause this are 1) temporary or transient deficiency due to environmental conditions, 2) sidewall compaction and 3) nutrient shortage in the soil.
Transient deficiency occurs when the plant is young and tends to be associated periods of weather that remain extra cloudy and/or wet and/or cool. Soil compaction is not a problem and soil fertility is adequate in these situations. An example of transient deficiency is displayed in these two images. The first image was taken on May 31, 2013 and the corn was striped. We normally associate these symptoms with zinc (Zn) deficiency in this region of the country. The fertilizer regime in this field was such that Zn should have been adequate. The roots were not hindered by sidewall compaction. The next image was taken 12 days later in the same area of the field and the corn is a healthy green at this point. No additional fertilizer was applied between the first and second images. The corn simply received some sun and warmer weather. These types of deficiencies rarely result in yield losses, even in corn that yields more than 250 bushels/acre.
Sidewall compaction is common this year since many fields were planted a little too wet. In these cases, the roots need to break through the sidewall of the seed furrow to get into the rest of the soil where the nutrients are residing. While the foliar product might improve the appearance of the leaves, they will not help the roots get through the sidewall.
Soils short on nutrients will result in nutrient deficiency symptoms in corn as well. If a soil test reveals low nutrient levels, then that nutrient needs to be applied to the field, if possible. In the case of Zn, a foliar Zn fertilizer may be adequate for that crop that year. If potassium or phosphorus are short, a foliar fertilizer will not provide enough nutrients. In these cases, a soil-applied fertilizer (i.e. muriate of potash, DAP, triple super phosphate, etc.) are needed.
Depending on the nutrient deficiency and how quickly a plant recovers (whether breaking through the sidewall or a soil amendment is made) there may be minimal yield loss.