The appearance of brown, dead tissue (necrosis) on corn leaves signals that the plant is under significant stress. This browning is the plant’s final response to environmental, chemical, or biological problems that prevent basic functions like photosynthesis and water transport. Identifying the precise pattern of necrosis is the first step toward diagnosis, as the location and shape of the brown areas often point directly to the underlying cause. Understanding these visual cues allows for a targeted solution to restore the plant’s health.
Environmental and Hydration Factors
Water availability is a frequent cause of leaf browning, with symptoms differing based on moisture levels. Under drought stress, corn leaves roll inward to reduce surface area and minimize water loss through transpiration. If drought persists, leaves turn dull, grayish-green before the tips and edges become dry, brown, and often crispy. This browning results from the plant’s inability to move sufficient water to the farthest points of the leaf.
Conversely, overwatering causes browning through a different mechanism, usually starting with overall leaf yellowing. Excess water saturates the soil, displacing oxygen and causing root asphyxiation. Damaged roots cannot absorb water or nutrients, leading to symptoms that mimic drought, though the soil will be wet or soggy. This root rot progresses to generalized necrosis and browning.
Extreme temperatures and wind also inflict physical damage. Intense heat and sunlight combined with water stress can cause sunscald, resulting in dead, brown patches. This injury occurs when the plant cannot transpire water fast enough to cool its tissues, causing them to overheat. High winds exacerbate water stress by removing the layer of moist air surrounding the leaves, increasing evaporative demand and leading to wind burn seen as browning along exposed edges. Cold damage, such as from a late frost, causes water-soaked areas that quickly turn black or brown as the tissue collapses.
Soil Chemistry and Nutrient Issues
If environmental conditions are stable, browning often relates to the soil’s chemical composition, specifically nutrient availability or toxicity. Deficiencies cause necrosis because the plant lacks elements needed to maintain cell structure. The appearance of browning depends on whether the nutrient is mobile or immobile within the plant.
Nitrogen (N) is a mobile nutrient, meaning the plant moves it from older leaves to support new growth. A nitrogen deficiency first appears as pale-green or yellowing on the oldest, lowest leaves. This progresses to browning that starts at the leaf tip and moves down the midrib in a characteristic V-shaped pattern. Potassium (K) is also mobile; its deficiency causes marginal leaf scorch where yellowing starts at the tip and progresses along the outer edges, eventually turning brown while the leaf center remains green.
A nutrient excess can cause browning through fertilizer burn, a form of salinity stress. Applying too much mineral fertilizer creates a high concentration of soluble salts, which draws water out of the roots through osmosis. This chemical dehydration causes leaf tips and margins to turn brown rapidly, often appearing uniform and severe across the entire plant. Soil pH extremes also affect nutrient chemistry; a pH that is too high or too low can lock up essential micronutrients, leading to deficiencies that manifest as chlorosis before progressing to necrosis.
Pests and Pathogens
Browning not attributed to water or nutrient problems often signals infestation by a biological agent (insect pest, fungus, or bacteria). These organisms cause necrosis by feeding directly on plant tissue or by secreting cell-destroying toxins. Fungal diseases often present with distinctive patterns that differentiate them from cultural issues.
Leaf blights, such as Northern corn leaf blight, produce long, elliptical, grayish-green lesions that eventually turn tan and brown, resembling cigars or boat shapes. Southern corn leaf blight causes similar lesions that are smaller and more numerous. Physoderma brown spot is recognized by very small, round yellow or brown spots appearing in bands across the leaf blade, often alongside dark purplish to black spots on the midrib. These patterns indicate a pathogen is active.
Pest damage also results in brown tissue, though initial signs differ from disease. Pests like spider mites feed by piercing cells and sucking out the contents, initially causing fine, yellowish-white stippling. As the infestation intensifies, the stippling merges, and affected areas dry out, taking on a bronzed or brown appearance. Corn borers primarily damage the stalk but can cause upper leaves to die and turn brown (a symptom called “dead heart”). This occurs because their feeding destroys the plant’s vascular system, preventing water and nutrient transport to the upper canopy.