Avocado leaf browning, or necrosis, is the most common visual complaint for owners of avocado trees. While the appearance of scorched, brown foliage can be alarming, it is rarely fatal, but rather a clear sign that a cultural or environmental issue requires attention. Necrosis represents dead leaf tissue caused by an inability to properly manage water, nutrients, or external conditions. Solving the problem depends on correctly identifying the specific cause, which often requires examining the pattern of the browning and the tree’s overall condition.
Irrigation Practices and Salt Buildup
The most frequent cause of brown leaf tips, especially in container-grown avocado trees, is the accumulation of mineral salts within the soil and leaf tissue. Avocado trees are highly sensitive to these dissolved solids, particularly chloride and sodium ions, which are present in most municipal water sources. As the tree transpires, water evaporates from the leaves, but the salt ions are left behind, gradually building up to toxic concentrations. This toxicity manifests as “tip burn,” where the browning begins precisely at the leaf tip and progresses inward along the margins.
Chloride is especially problematic because the roots absorb it and transport it to the leaf tips where the veins terminate. Once the chloride concentration reaches a threshold, often around 0.75% to 1% of the leaf’s dry weight, the tissue dies. Varieties like Hass are known to be particularly susceptible to this chemical burn. The problem is compounded when the tree receives frequent, light irrigation that fails to push the salts below the shallow root zone, leading to concentrated buildup in the topsoil.
The necessary practice to manage this is called “leaching” or “flushing” the soil. This involves applying a much larger volume of water than is typically used during a normal watering session. This heavy application washes the accumulated salt downward and out of the root zone, effectively resetting the soil salinity. For potted trees, this means watering deeply until a significant amount of water drains out the bottom, and then discarding the runoff so the salts are not reabsorbed. Deep watering must be balanced, however, as excessive watering causing soil saturation can lead to other serious root problems.
Under-watering also exacerbates salt burn because it reduces the overall volume of water available to dilute the salts, increasing their concentration in the remaining soil moisture. High concentrations of fertilizer salts or other chemical additives in the soil also contribute to the overall salinity load and should be applied cautiously. Consistent use of water with a chloride content over 75 parts per million (ppm) without proper leaching will result in leaf tip burn over time.
Extreme Temperatures and Light Exposure
Beyond soil chemistry, environmental stressors can also cause leaf browning. Cold damage often results in large, sudden patches of browning or blackening across the leaf surface, distinct from the margin-only pattern of salt burn. If the air temperature dips into the low 20s Fahrenheit, the leaves may completely brown and dry up within days, often remaining attached to the tree. Young leaves are particularly vulnerable.
Sunburn, or sunscald, is another common cause of leaf necrosis, presenting as chlorotic blotches that turn necrotic, typically on the upper surfaces of leaves facing the most intense light. This damage is often seen on the south or southwest sides of the tree. It is especially likely to occur on newly planted trees or those that have recently lost significant foliage. When a tree defoliates, previously shaded leaves and bark are suddenly exposed to direct, intense sunlight, leading to tissue damage.
Low atmospheric humidity can intensify the effects of salt accumulation and desiccation. Dry air increases the rate of transpiration, causing the tree to lose water more quickly, which accelerates the buildup of chloride in the leaf tissue. Maintaining good soil moisture helps the tree cope, but the physical environment plays a large role in the tree’s susceptibility to leaf scorch.
Fungal Infections and Root Health
Browning and overall decline can also signal biological issues related to root health and pathogens. The most serious root-related cause is Phytophthora root rot, a devastating disease caused by the water mold Phytophthora cinnamomi. This pathogen thrives in saturated soils with poor drainage, making overwatering a major contributing factor. The organism attacks the fine feeder roots, causing them to blacken and become brittle, severely hindering the tree’s ability to absorb water and nutrients.
The above-ground symptoms of root rot often mimic severe water stress or drought. This leads to a general thinning of the canopy, pale or yellowish leaves, and browning or necrotic tips. A tree with root rot will wilt and show leaf browning even when the soil is wet, confusing many growers who mistake the symptoms for underwatering. This widespread browning and decline contrasts with the localized tip burn associated with salt toxicity.
Fungal leaf spots present a different pattern of browning, appearing as distinct, often circular or irregular necrotic spots on the leaf surface rather than a uniform burn along the edges. Pathogens like Colletotrichum gloeosporioides, which causes Anthracnose, create dark, sunken lesions, especially in warm, humid conditions. Certain sap-sucking pests, such as mites, also damage the leaf tissue, causing stippling and eventual bronzing or browning of the leaves as the cells die.