Avocado trees are popular for their fruit and as ornamental additions. When the stem or trunk of an avocado tree turns brown, it often indicates a deeper health problem. This visible discoloration is not a disease itself, but a sign that the tree is reacting to a biological infection or an environmental stressor. Understanding the precise appearance of the browning helps in accurately diagnosing the underlying issue.
Browning Caused by Disease and Fungi
Stem browning caused by pathogens often presents as localized, dark, and wet lesions on the bark. Common culprits are water molds, such as Phytophthora cinnamomi or P. citricola, which cause trunk cankers and collar rot. These infections typically appear as a dark, water-soaked area at or just above the soil line, sometimes accompanied by a reddish or white sugary exudate oozing from bark cracks.
If the outer bark is scraped away from a Phytophthora canker, the internal wood tissue shows a distinct orange-tan to brown discoloration, contrasting sharply with the healthy, creamy-white wood. This discoloration indicates the pathogen is actively killing the cambium layer, which can ultimately girdle the tree. Another fungal issue, Dothiorella canker, begins with a small discoloration on the bark and may cause the bark to split and peel away. Unlike Phytophthora, Dothiorella cankers are less severe and may show a white powdery appearance on the bark surface.
Browning can also begin as dieback on younger stems and move inward toward larger branches. This progression is often associated with fungi entering through wounds or natural openings. When a stem is infected, the vascular tissue beneath the bark may exhibit dark streaks, indicating disrupted water and nutrient flow. This internal discoloration confirms a systemic pathogen, distinguishing it from superficial bark damage.
Browning Caused by Environmental Stress
Browning not caused by a biological pathogen is typically the result of abiotic factors that physically damage the stem tissue. Sun scald, a form of sunburn, occurs when the stem is suddenly exposed to intense sunlight, often after heavy pruning removes the protective leaf canopy. This damage appears as a diffuse, scorched brown patch on the side of the trunk or stem facing the afternoon sun. The browning is usually limited to the outer bark layer, and scraping the tissue reveals healthy wood underneath, unlike deep discoloration from fungal cankers.
Cold damage also causes stem browning, usually manifesting as a generalized, uniform discoloration or splitting of the bark. Freezing temperatures can rupture plant cells beneath the bark, resulting in a dark, bruised appearance that is relatively superficial. This browning is typically diffuse and lacks the distinct, sunken margins or the resinous ooze characteristic of a canker infection.
Physical injury from lawnmowers, string trimmers, or animal chewing can create wounds that expose the inner wood, causing the edges to dry out and turn brown. This browning is always confined to the area of the abrasion or impact. If the wound is clean, the tree attempts to compartmentalize the damage, leading to a localized patch of brown, dried bark linked to an external impact rather than a spreading infection.
Corrective Action and Prevention Strategies
Once the cause of stem browning is identified, immediate action is necessary to halt the damage and support tree recovery. If the browning is confined to a branch tip or small stem, pruning the affected wood back to healthy tissue is the first step. When pruning, all tools must be sterilized with a bleach or alcohol solution between cuts to prevent spreading pathogens. The removed, diseased wood should be disposed of away from the orchard or garden area.
For browning caused by sun scald or cold injury, protective measures can be applied directly to the trunk and main stems. A diluted white latex paint, often called whitewash, can be applied to the exposed bark to reflect sunlight and prevent future sun scald damage. For cold protection, a physical tree wrap can insulate the trunk during freezing temperatures. These protective coatings help the tree recover by shielding the vulnerable cambium layer from further stress.
Long-term prevention relies on proper cultural practices, especially managing soil and water. Since Phytophthora thrives in saturated conditions, improving soil drainage is paramount, often involving amending heavy soils or adjusting irrigation to avoid waterlogging. Additionally, mulch should be maintained at a shallow depth, kept several inches away from the trunk to prevent moisture buildup that encourages crown rot. Appropriate irrigation prevents both drought stress and over-watering, significantly reducing the tree’s susceptibility to disease and environmental stress.