The Leyland Cypress (x Cupressocyparis leylandii) is a widely planted evergreen, valued for its rapid growth and ability to form dense privacy screens. Browning foliage is a common symptom indicating the tree is under stress and requires attention. Determining the exact cause is paramount, as the correct intervention depends entirely on an accurate diagnosis.
Water and Soil Related Stress
Improper water management is one of the most frequent causes of Leyland Cypress browning, presenting in two opposite extremes. Drought stress causes the tree to conserve moisture by sacrificing the foliage farthest from the trunk, resulting in browning that often begins at the tips of the branches or the top of the canopy. This lack of water weakens the tree, making it susceptible to secondary infections and pests.
Conversely, overwatering or planting in poorly draining soil creates an environment conducive to root rot, most commonly caused by the water mold Phytophthora. This pathogen thrives in saturated conditions, attacking the fine feeder roots and destroying the tree’s ability to absorb water and nutrients. Paradoxically, a tree suffering from Phytophthora root rot displays symptoms almost identical to drought stress, including yellowing and browning foliage. To correctly diagnose the problem, homeowners must check the soil moisture six inches deep; if the soil is saturated, the browning is likely due to root rot, not dehydration.
Identifying Specific Diseases and Pests
Biological threats are a cause of browning, with fungal cankers being destructive to the Leyland Cypress.
Seiridium Canker
Seiridium Canker, caused by the fungus Seiridium unicorne, is a common disease that primarily attacks stressed trees. The fungus invades through wounds or bark cracks, forming distinct lesions on the branches or trunk that can be sunken, dark brown, or purplish. These cankers disrupt the flow of water and nutrients, causing the entire branch beyond the point of infection to turn reddish-brown and die.
The presence of thick resinous sap oozing from the canker site is a common symptom. Since fungicides are generally ineffective in treating established cankers, the only management strategy involves pruning the infected branch. The branch must be removed at least three to four inches below the visible canker or discolored wood to ensure the complete removal of the fungus. Pruning tools must be sanitized between cuts to prevent the spores from spreading to healthy tissue on other parts of the tree.
Spruce Spider Mite
The Spruce Spider Mite (Oligonychus ununguis) is a tiny arachnid that feeds on the cypress foliage. These mites use piercing mouthparts to suck the fluids from the needles, leading to a fine yellow stippling or bronzing of the foliage. Infestations are often worse during hot, dry weather and can cause needles to turn brown and drop prematurely.
A simple diagnostic technique is the “tap test,” where a branch is sharply tapped over a white sheet of paper. If small, moving specks appear on the paper, and they smear red or green when crushed, spider mites are confirmed.
Crowding and Natural Shedding
Browning is not always a sign of disease or pest infestation; sometimes, it is the result of the tree’s natural growth habit. Leyland Cypresses are dense evergreens, and as they mature, the interior foliage becomes shaded by the outer growth. This lack of sunlight causes the older, inner needles to yellow, turn brown, and eventually shed, a process known as natural interior shedding. This browning is normal and begins inside the tree while the branch tips remain healthy and green.
Crowding, however, can turn this natural process into a health issue. When trees are planted too closely together, they compete intensely for limited resources like water and nutrients. This competition creates stress, which predisposes the trees to opportunistic diseases like cankers. Planting density also reduces air circulation, creating humid conditions that exacerbate the spread of fungal pathogens. Adequate spacing, ideally 10 to 15 feet apart, ensures sufficient air flow and light, maintaining the tree’s vigor.