Yellowing leaves, known as chlorosis, are a frequent symptom observed by corn growers. This discoloration indicates reduced chlorophyll production, which impairs the plant’s ability to photosynthesize and grow. Chlorosis is not a specific disease but a general sign that something is interfering with the plant’s normal function. Factors ranging from nutrient scarcity to environmental stressors or biological threats can trigger this response. Diagnosing the pattern of yellowing provides immediate clues about the underlying cause and directs the appropriate remedy.
Reading the Yellowing Pattern: A Diagnostic Guide
The location where yellowing first appears offers the primary clue in determining the cause of chlorosis. Nutrients are classified as either mobile or immobile, dictating whether symptoms appear on older or newer leaves. If yellowing begins on the lower, older leaves, it suggests a mobile nutrient deficiency, as the plant relocates these nutrients to support new growth. Conversely, chlorosis manifesting on the upper, newer leaves points toward an immobile nutrient deficiency.
Observing the specific pattern of discoloration on the leaf further refines the diagnosis. Uniform yellowing, where the entire leaf turns pale, is distinct from interveinal chlorosis. In interveinal chlorosis, the tissue between the veins yellows while the veins themselves remain green. A third pattern is the V-shaped yellowing that begins at the leaf tip and progresses down the midrib, indicating a specific nutrient issue.
Other symptoms can help narrow the possibilities beyond color. Stunted growth often accompanies severe chlorosis, indicating a long-term resource deficit. The appearance of purple or reddish tints suggests metabolic stress. Noting whether symptoms appear on old or new growth, the pattern on the leaf, and associated visual changes helps create a diagnostic profile.
Key Nutrient Deficiencies and Treatment
The most common cause of chlorosis is Nitrogen (N) deficiency, a highly mobile nutrient frequently depleted from the soil. Nitrogen deficiency presents as V-shaped yellowing that starts at the tip of the oldest leaves and moves down the midrib toward the stalk. Since the plant moves Nitrogen to support new growth, the lower leaves exhibit this chlorosis first. Addressing this requires immediate application of a Nitrogen-containing fertilizer, often applied as a side-dressing for rapid uptake.
Another mobile nutrient causing yellowing is Magnesium (Mg), which forms the central core of the chlorophyll molecule. Magnesium deficiency appears as interveinal striping on older leaves, often starting along the margins. The tissue between the veins turns yellow or white, while the veins retain their green color. Treatment involves applying fertilizers containing Magnesium sulfate (Epsom salts) or other specific Magnesium supplements.
Iron (Fe) and Zinc (Zn) are immobile nutrients, meaning deficiencies manifest first in the newest growth. Iron deficiency causes severe interveinal chlorosis on the youngest leaves, leaving the major veins distinct dark green against a pale yellow background. Zinc deficiency presents similarly, often causing white or pale yellow bands on either side of the midrib in the upper leaves. Treating these deficiencies requires a foliar spray containing the micronutrient, allowing the plant to bypass potential soil uptake issues.
Environmental and Soil Stress
Environmental factors and poor soil conditions frequently interfere with a corn plant’s ability to absorb necessary nutrients. Water stress, whether too little or too much, can induce chlorosis. Overwatering saturates soils, depriving roots of oxygen and causing damage that prevents nutrient absorption, resulting in general yellowing. Conversely, prolonged drought also restricts nutrient movement to the roots, leading to uniform chlorosis across the canopy. Consistent irrigation that balances soil moisture without saturation is the best preventative measure.
The physical structure of the soil plays a role in nutrient availability. Soil compaction limits root growth and restricts the exchange of oxygen and carbon dioxide, which hampers nutrient uptake. Poor drainage exacerbates this issue, promoting conditions that favor root diseases and nutrient lockout. Improving soil structure through deep tillage, aeration, or incorporating organic matter helps ensure roots access both oxygen and available nutrients.
Extreme Soil pH levels can chemically lock up nutrients, rendering them unavailable to the plant roots. Iron and Zinc, for example, become less soluble and unavailable in soils with a high (alkaline) pH, even if present. Testing the soil pH is crucial; if the pH is too high, adjustment may be needed through soil acidifiers like elemental sulfur. Additionally, young corn plants can exhibit temporary yellowing or purpling when exposed to sudden cold temperatures, a stress response that typically resolves once warmer weather returns.
Addressing Pests and Diseases
If yellowing does not conform to nutrient deficiency or environmental stress patterns, pests or diseases may be the cause. Certain sap-feeding pests, such as spider mites and aphids, inject toxins or drain fluids, causing localized, spotty, or mottled yellowing on the leaves. Visual inspection is necessary, looking for fine webbing on the undersides of leaves or clusters of small insects on the stalks. Managing these infestations involves targeted insecticides or introducing natural predators.
Infectious diseases caused by fungi, bacteria, or viruses can result in chlorosis, but the pattern is irregular and distinct from systemic deficiencies. Fungal diseases, such as Northern Corn Leaf Blight, often cause long, elliptical, yellow-to-tan lesions or streaks rather than uniform yellowing. Viral infections are characterized by mosaic patterns, ring spots, or general mottling across the leaf surface. Since chemical control for viral diseases is not feasible, removing and destroying affected plants prevents the pathogen’s spread and ensures proper field sanitation.