[Text originally appeared in the Integrated Crop Management extension newsletter on pages 176-177 of the IC-498 (14) - June 11, 2007 issue.]
This is the year of multi-colored corn due to the wet spring. There are noticeable differences dependent on management practices, such as the rotation used: corn following corn or corn following soybean. What impact will this have on growth and final yields? As of June 11, corn across Iowa ranged from V2 to V9 (two to nine leaves), with a rough average of V6.
Most of Iowa was saturated with precipitation amounts that are from normal to two times the normal amount since mid-April. Although a good share of Iowa's corn appears green and healthy, reports of yellow and purple corn are coming from several parts of the state. Differences in overall vigor and growth between corn following corn and corn following soybean are even more obvious than in prior years. In the long run, these could contribute to the yield penalty for corn following corn. Long-term data has shown that in good years, corn following corn can yield close to or the same as corn following soybean. In stressful years though, corn following corn will not be able to compete with corn following soybean.
Cool, wet weather is a major cause of the yellow corn seen this growing season as it is a symptom of crop stress. Yellow corn does not necessarily point to a nitrogen deficiency. Corn grown in waterlogged soil turns yellow and may die if conditions are severe. Nitrogen and potassium deficiencies, herbicide applications, and soil compaction may all contribute to leaf yellowing. Leaf yellowing in most cases though is likely associated with a poorly developed root system. See yellowed plants from a water saturated field: field photograph and close-up photograph.
Once secondary roots develop and temperatures warm, plants are expected to recover. Some lasting results of yellow or stunted corn are uneven plant heights and/or stand reduction; these will both reduce yield potential if they occur. Once the soil dries, if the plants remain yellow, then nitrogen may be limited, which will also reduce yield.
Another symptom of cool air and soil temperatures, combined with wet conditions, is a purplish tint to corn seedlings. See image. Purple leaf coloring is more pronounced in some hybrids' genetics than others. Most often though, the leaf purpling is related to stress experienced by the young seedling and/or restricted root development. Phosphorus unavailability is often mentioned as a culprit for the purple leaves. Phosphorus deficiency will result in reddish purple leaves, yet it is not likely the primary cause. A reddish purple tint on leaves can be due to anything that disrupts sugars within the plant. Cool and/or compacted soils, as well as shallow planting, can each create the opportunity for purpling to be expressed in corn leaves. If root development is restricted (due to temperature or seedbed problems), then the observed symptoms are simply an expression of this since the plant is not developing normally.
Whenever we see purple corn seedlings we can assume one of two things: either the plant is not translocating sugars well or there is a phosphorus deficiency. Although the plants are purple, phosphorus is not likely deficient in the soil. The plants are simply responding to wet conditions in which root growth was stalled for a period of time. The plants should grow out of this condition with no significant long-term impact (except as noted above with yellow corn). For additional information on purple corn and cold season stress, read "Early season cold stress".
Root restrictions that cause purpling or yellowing are expected to be temporary and seedlings should regain a healthy green color as weather conditions improve. Overall, yield should not be affected. It is good practice to check a few plants after they 'green up' to ensure that root development resumes.