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Periodically, corn fields experience conditions that cause ears to exhibit some colored kernels. Several internal and external influences cause kernel coloration. Weather and temperature during grain fill could affect pigmentation. Sometimes darkened kernels appear as the result of diseases, such as fusarium, which can cause a starburst pattern of whitish-pink, moldy discoloration on a kernel. However, if kernels lack the color and pattern of fusarium or other fungal infections, the diagnosis points toward other non-disease influences.
A dissected view of a corn kernel reveals four main parts: embryo (germ), endosperm, aleurone, and pericarp (Figure 1). Depending on the genetics of the corn product, the aleurone and pericarp layers can individually express purple, red, bronze, or white/clear coloration under certain conditions.
Wheat curl mite feeding may trigger red streaking within the pericarp of otherwise yellow kernels. The red streaks do not necessarily signify the presence of ear molds, toxins, or grain quality issues. Purple kernel streak has been documented as triggered by cold temperatures late in the grain filling period of white corn products.1
Late-season stink bug damage can darken areas of kernels. Stink bugs seek developing tissues and can pierce husk leaves to inject kernels with enzymes. Enzymes help stink bugs dissolve tissue and suck up pre-digested plant fluids with their stylet mouth parts. Damage from stink bugs is more likely near field edges and in no-till fields.2 Stink bug damage may be partly concentrated on parts of the outer half of the ear. Once kernels become too hard to pierce, stink bugs are expected to move on into soybean fields that have developing seeds. This fall, live stink bugs may not be found in the affected corn fields as they have likely moved to nearby soybean fields and then to overwinter into wooded and grassy cover. Late-season kernel feeding is not expected to reach economic levels, and thresholds for control are not developed for late-season corn fields.
Differences in kernel appearance can be the result of gene expression similar to the non-disease lesion mimic. Corn has unique fertilization with pigmentation of the aleurone layer and pericarp determined by the father and mother, respectively. Positions of particular genes can "jump", and these “jumped genes” are evident when associated with pigmentation.3
Normal yields are expected in fields with colored kernels. Midwest elevators have reported the levels of colored kernels as two percent or less this year. A kernel with a damaged or colored embryo could be considered damaged. This season, colored kernels have been observed to have healthy embryos, with colored crowns (Figure 2). Early-season stink bug damage can affect corn stands; however, sporadic, late-season damage is unlikely to affect grain yield or quality. Cool weather can affect pigmentation and lengthen the grain fill time. A longer grain fill period could benefit yield potential.4
There are three classes of corn: yellow corn, white corn, and mixed corn. Corn classified as ‘Yellow’ is yellow-kerneled and contains no more than five percent of corn of other colors. A slight tinge of red is still considered yellow corn. White corn is white-kerneled with no more than two percent of corn of other colors. A slight tinge of light straw or pink color is considered white corn. Mixed corn is corn that does not meet the above standards or white-capped yellow corn.5 These classifications may be important for food-grade corn; however, a kernel needs to be greater than 50% colored in order to be considered 'corn of other color'.In the ethanol industry, purple pericarp corn was introduced to mark loads of corn intended with an ethanol end use. These purple plants are intentionally grown within fields for visual tracking. Purple-colored pericarps are removed in ethanol processing.
Elevators have accepted grain with this type of kernel coloration because grain remains of good quality, and disease-free grain has not been discounted as damaged. An official grade by a grain inspection agency can be requested if the ‘house grade’ by an elevator counts discolored kernels as damaged. While each elevator operates individually, an undamaged germ (embryo) is what matters when grading grain.
Field scouting for insects and disease helps manage fields and schedule harvest order. There are a number of kernel color possibilities that are considered safe and do not affect quality or end use. Disease-free kernels with crowns that are slightly colored are not expected to be discounted at elevators.