Lemon trees are popular for their fragrant blossoms and bright fruit, but leaf discoloration signals a deeper problem with the plant’s environment or health. This browning, termed “leaf necrosis,” indicates that the plant tissue is dying. It can be caused by factors ranging from incorrect watering practices to biological invaders. Understanding the distinct patterns of necrosis is the first step in accurately diagnosing the underlying issue and implementing an effective remedy.
Browning Caused by Water and Soil Issues
The most frequent source of leaf browning stems from imbalances in water availability and the soil environment. Overwatering is a common mistake, leading to saturated soil conditions that starve the roots of oxygen. This quickly progresses to root rot. When the root system is compromised, it cannot transport water and nutrients efficiently, causing the entire leaf to yellow, turn brown, and eventually drop.
Conversely, underwatering, or drought stress, presents a different pattern of damage. When the tree cannot draw sufficient moisture, the lack of hydration first appears as crisp, brown tips and edges. The tree may also curl its leaves inward to reduce the surface area exposed to the sun and minimize water loss.
Another significant soil-related cause of necrosis is the buildup of soluble salts, common in potted citrus trees. Irrigation water contains dissolved minerals like sodium and chloride, which accumulate in the soil as water evaporates. These accumulated ions become toxic, inhibiting the roots’ ability to absorb water and causing “salt burn.” This toxicity manifests as browning along the leaf margins, where the excess ions are concentrated.
To remedy salt buildup, “leaching” or “flushing” is necessary. This involves deep watering the soil until a large volume of water drains freely from the pot’s bottom. For general water management, allow the soil to dry out slightly between waterings. Always check the moisture level several inches down before irrigating again. Proper deep watering encourages a strong root system, making the tree more resilient to stress.
Browning Caused by Environmental Stressors
Acute environmental shifts can cause rapid leaf damage and browning. Cold damage, often resulting from sudden frost or a temperature drop below 30.5 degrees Fahrenheit, causes ice crystals to form within the leaf tissues. This cellular damage first appears as dark, water-soaked areas that quickly turn into large patches of tan or brown, dead tissue.
Sunburn, or sunscald, is another physical stressor resulting in leaf browning. This occurs especially when a tree is moved abruptly from shade to intense, direct sunlight. The sudden exposure overwhelms the leaves’ ability to regulate light absorption, leading to bleached or brown spots on the surface. This damage impairs photosynthesis, reducing the tree’s energy production.
Strong, persistent winds can also cause browning through desiccation, or “wind burn.” The wind rapidly strips moisture from the leaf surface, forcing the tree to transpire at an unsustainable rate. This rapid water loss leads to the drying and browning of the leaf edges and tips. This damage is similar to underwatering, but the cause is atmospheric.
Browning Caused by Pests and Fungal Infections
Biological agents, including fungi and pests, can directly cause leaf necrosis or lead to browning. Fungal root rot, caused by pathogens like Phytophthora, results from prolonged overwatering where fungi thrive in saturated, low-oxygen soil. The fungus destroys the roots, leading to symptoms of water-delivery failure, such as widespread leaf yellowing and browning.
Specific fungal diseases, such as Anthracnose or Citrus Scab, target the leaf tissue, resulting in distinct brown lesions or spots. Anthracnose often causes dark, sunken spots that lead to twig dieback. Scab creates raised, corky, brown lesions on the leaves, twigs, and fruit. These diseases are more prevalent in environments with high humidity and poor air circulation.
Pests damage leaves through their feeding habits, resulting in browning or bronzing. Scale insects attach to the stems and leaves and suck out the plant’s sap, causing localized yellowing that turns brown and necrotic. Spider mites feed by piercing leaf cells, resulting in a fine, speckled pattern called stippling. In severe infestations, stippling merges to give the leaf a dull, bronzed appearance. Fungal issues are managed with copper-based fungicide, while pests are often managed with horticultural oils.
Maintaining Long-Term Nutrient Health
While water issues cause immediate browning, chronic nutrient imbalances contribute to long-term stress that makes the tree susceptible to necrosis. Potassium deficiency is a notable example in citrus. Since this nutrient is highly mobile, the tree relocates it from older leaves to support new growth. This causes the older foliage to display symptoms first, beginning as yellowing tips and margins before progressing into necrotic, brown spotting.
Browning caused by deficiencies is often a late-stage symptom, indicating the tree has been under strain. To prevent this chronic stress, a consistent and balanced fertilization strategy is necessary. Use a formulation specifically designed for citrus trees. These fertilizers contain the appropriate ratio of macro- and micronutrients required for healthy growth and fruit production.
Following a seasonal feeding schedule prevents the boom-and-bust cycle of nutrient availability that stresses the tree. Ensuring the tree receives sufficient potassium, nitrogen, and other trace elements builds a robust cellular structure. This improves its natural defenses against environmental stressors and pathogens. Regular, appropriate feeding sustains the tree’s health, ensuring the foliage remains green and vigorous.