Brown foliage on a tree, whether sudden or gradual, is a symptom of severe stress, not a disease itself. This browning, known scientifically as necrosis or extensive chlorosis, signals that plant tissue is dying. Correctly diagnosing the underlying cause requires careful observation of the browning pattern, the type of tree affected, and the surrounding environment. The issues can stem from non-living factors like weather and soil conditions, or from living threats such as insects and pathogens.
Abiotic Causes Related to Soil and Water Management
Water management issues in the root zone are the most frequent causes of a tree turning brown, as the foliage is highly sensitive to hydration levels. Drought stress, or underwatering, causes leaves to turn brown, often starting at the tips and margins in a pattern called leaf scorch. This symptom is the tree’s response to severe water deficiency, causing it to shut down peripheral tissue to conserve its limited moisture supply.
The opposite problem, overwatering, can create the same brown-leaf symptoms because saturated soil suffocates the roots by displacing the necessary oxygen. When roots cannot respire, they die and begin to decay, a process known as root rot. Root rot prevents the remaining healthy roots from absorbing water, effectively mimicking drought conditions. A tree with root rot will show browning and dieback despite the soil being wet, so checking for persistently soggy soil is an important diagnostic step.
Physical constraints in the soil can also restrict a tree’s ability to take up water and nutrients. Soil compaction, often caused by heavy equipment or consistent foot traffic, drastically reduces the pore space needed for water infiltration and oxygen exchange. This limited air and water availability inhibits root growth and function, leading to chronic stress and canopy decline.
Another structural problem is the development of girdling roots, where roots circle the trunk or other roots instead of growing outward. As the tree grows, these roots tighten, effectively choking the vascular system. This restricts the flow of water and nutrients from the roots to the crown, leading to a slow, progressive decline and browning of the foliage.
Biotic Causes: Insect Infestations and Mites
Browning results from the feeding activity of various mobile pests that physically damage the tree’s tissues. Sucking insects, such as aphids, scale, and mites, extract sap, which contains the tree’s fluids and nutrients. When sap is removed, the chlorophyll-containing cells die, leading to fine yellow or white speckling (stippling) that eventually turns the entire leaf or needle section brown.
Spider mites, which are technically arachnids, are notorious for causing bronzing or browning, particularly on conifers and broadleaf trees under hot, dry conditions. Heavy infestations of armored scales can also cause localized dieback as they remove cell contents, leading to the browning and eventual death of branches.
Boring pests cause browning by interrupting the tree’s internal vascular system. Insects like the Emerald Ash Borer or various bark beetles tunnel into the phloem and xylem layers just beneath the bark. This tunneling physically severs the vessels responsible for transporting water and nutrients. This quickly leads to the rapid browning and dieback of the entire section of the canopy above the infestation. Physical signs of these pests include small, distinct exit holes in the bark or the presence of ‘frass,’ a fine, sawdust-like material.
Biotic Causes: Fungal and Bacterial Pathogens
Microscopic pathogens cause browning by invading the tree’s internal systems, resulting in systemic failure or tissue death.
Vascular Wilts
Vascular wilts, such as Dutch Elm Disease or Verticillium wilt, are caused by fungi that enter the tree’s conductive tissue (xylem) and proliferate. The tree responds to the invasion by producing gums and tyloses to wall off the pathogen, which inadvertently clogs the water-transporting vessels. This blockage prevents water from reaching the foliage, leading to sudden wilting and browning, often affecting only one sector or branch initially.
Canker Diseases
Canker diseases are localized infections typically caused by fungi that invade the bark through wounds or natural openings. The pathogen kills the underlying cambium and phloem, creating sunken, dead areas on branches or the trunk. If a canker completely encircles a branch, it effectively girdles the limb. This blocks the flow of nutrients and water, causing the foliage distal to the canker site to brown and die.
Needle Cast
Fungal infections specific to conifers, known as needle cast, cause older needles to turn brown and shed prematurely. These fungi colonize the needles, and while new growth may appear healthy, the one- or two-year-old needles will develop reddish-brown spots that expand until the entire needle turns brown and falls off.
Environmental and Chemical Stressors
External, non-biological factors related to weather and chemical exposure can also induce severe browning symptoms.
Temperature Stress
Extreme temperatures, particularly in winter, can lead to desiccation known as winter burn or sunscald, especially on evergreens. Dry winter winds and frozen ground prevent the tree from replacing moisture lost through the needles, causing them to turn brown from excessive dehydration. Sunscald occurs when intense winter sun rapidly warms the bark on the southwest side of the trunk, followed by a sudden drop in temperature at night, which damages the tissue.
Chemical Damage
The improper application of chemicals can cause rapid and distinct browning patterns. Herbicide drift, where weed-killing chemicals are carried by wind to non-target trees, causes rapid necrosis and often unusual, distorted growth on the affected foliage. Similarly, over-application of fertilizer can result in chemical burn, where the high concentration of salts in the soil solution draws water out of the tree’s roots, causing the foliage to brown and wilt.
Salt Stress
De-icing salts used on roads and sidewalks during winter also contribute to browning by inhibiting water uptake. When high concentrations of salt accumulate in the soil, they reverse the osmotic process, effectively pulling water out of the roots. This condition, called physiologic drought, results in browning that appears similar to drought damage, often concentrated on the side of the tree closest to the treated pavement.