Brown discoloration on an avocado tree’s stem or trunk often signals a problem, ranging from natural aging to a life-threatening disease. The stem is the tree’s vascular highway, and damage here interrupts the flow of water and nutrients, leading to decline. Identifying the type of browning is the first step in protecting the tree, as the cause can be benign, environmental, or pathogenic. Understanding the texture, location, and spread of the discoloration provides clues for accurate diagnosis.
Differentiating Normal Stem Changes from Pathological Browning
Not all brown coloration indicates poor health; browning can be a normal part of an avocado tree’s development. This natural process is called lignification, where soft, green tissues of younger stems harden and turn woody to provide structural support. This benign browning typically occurs at the base of the stem or on older wood and feels dry and hard to the touch.
Pathological browning, in contrast, is characterized by a change in texture and rapid spread. Diseased areas may appear water-soaked, soft, or mushy, sometimes accompanied by liquid oozing from the bark, known as cankers. Browning that begins on new growth, is patchy, or develops quickly on the upper stem is a cause for concern. Observing the specific location and consistency of the brown area is a powerful diagnostic tool.
Diagnosing Waterborne and Fungal Diseases
The most significant threat concerning stem browning often involves waterborne pathogens. The soil-dwelling water mold, Phytophthora cinnamomi, causes Phytophthora root rot, the most serious disease affecting avocado trees worldwide. This pathogen thrives in saturated, poorly draining soil, destroying the tree’s fine feeder roots and interfering with water and nutrient uptake.
While initial damage is underground, the disease manifests on the stem as trunk or collar rot. Symptoms appear as dark, water-soaked cankers near the soil line or graft union, often weeping reddish-brown or dark sap. If this canker fully girdles the trunk, it cuts off the tree’s vascular system, leading to rapid death. Trees suffering from root rot also show secondary canopy symptoms, such as sparse, pale green leaves that wilt and develop brown, necrotic tips.
Other fungal issues, such as Phytophthora cankers caused by P. citricola, produce dark lesions on the lower trunk, sometimes accompanied by a seeping red fluid. Phytophthora diseases result from an imbalance between the root system’s capacity to absorb water and the leaves’ need for it. This condition is exacerbated by poor drainage and excess soil moisture.
Environmental Stressors and Pest Damage
Browning unrelated to fungal diseases often results from environmental stress or insect activity. Sunscald occurs when a tree’s trunk or exposed limbs are suddenly subjected to intense, direct sunlight, often after heavy pruning or defoliation. This damage appears as dry, cracked, or sunken brown areas on the side of the trunk facing the afternoon sun.
Cold damage, or frost injury, also causes browning and discoloration on stems and branches. In younger wood, this may look like water-soaking or simple discoloration. In older trunks, severe cold can cause the bark to split or loosen as the underlying cambium tissue is killed. New growth and small branches are the most susceptible parts, with damage becoming apparent days or weeks after the temperature drop.
Certain insect pests, particularly borers, cause localized stem browning and dieback. The branch and twig borer creates small entrance holes in the wood, which may exude a white, flaky, sugary sap. The invasive shot hole borer is an ambrosia beetle that attacks the wood to cultivate a symbiotic fungus. This leads to Fusarium dieback, resulting in branch dieback and discoloration in the cambium layer.
Treatment Protocols and Ongoing Care
Immediate action is necessary to address pathological stem browning and prevent further decline. For fungal cankers like Phytophthora collar rot, the most important step is correcting the soil environment by improving drainage. This involves reducing irrigation frequency, ensuring water does not pool around the trunk, and promoting soil aeration.
If cankers are weeping or clearly defined, carefully scraping away the infected bark and underlying tissue until only healthy, green wood remains can sometimes halt the spread. For both fungal and cold damage, use a protective trunk paint, such as 50% diluted white, water-soluble, acrylic paint. This paint reflects sunlight and helps prevent sunscald on exposed wood, which is important for trees that have lost their protective canopy due to disease or frost.
For borer damage, removing infested branches by cutting them several inches below the entrance hole and destroying the wood is the best practice, as internal larvae are protected from topical treatments. Long-term prevention centers on cultural practices that maintain a vigorous, healthy tree. These practices include regular, non-excessive irrigation that avoids waterlogging, applying mulch away from the trunk to maintain soil moisture, and avoiding injury to the bark.