The Leyland Cypress.
The Leyland cypress is a popular evergreen for the urban landscape, due to its quick growth, hardiness/soil tolerance, and affordability. The Leyland Cypress formally known by its botanical name x Cuprocyparis leylandii, or x Cupressocyparis leylandii is a hybrid from two United States native conifers the Monterey Cypress and the Nootka Cypress. For more history and information on the leyland cypress check out leylandii.com
Their growing popularity throughout the US, however, has resulted in them being over-planted and planted in such environmental extremes as to become susceptible to a number damaging pests both insect and/or disease. Pests/problems that have become common to the Leyland Cypress are: bagworms, canker, root rot, spider mites and scale. In this article we will list each condition and provide some tips on how to identify and treat each issue that might be negatively affecting your Leyland Cypress.
Bagworms
Bagworms are a very common, if not the most common pest associated with the Leyland cypress; and while these caterpillars infest a number of trees and shrubs, but prefer conifers. The bagworms build a cone like bag out of silk and leaves from the Leyland cypress which blend in very well and make it difficult to detect. They will carry this bag with them while feeding on the tree, until the end of the summer at which point the caterpillar transforms to the moth stage. As devastating of a pest as these caterpillars are, the most effective and environmentally friendly method of control is to pick off the bags and burn/destroy them. If infestation is severe, or it is not feasible to pick each bagworm off the tree then there are chemical options that will work well.
Cankers
Seiridium Canker & Botryosphaeria are two common canker diseases that affect Leyland cypresses. While both of these share dieback as a symptom, generally yellowing to brown, they do have a couple unique characteristics. For instance, Seiridium canker produces multiple shallow or elongated cankers (often accompanied by oozing of resin) that do not girdle the branch, but significantly restrict water, while Botrysphaeria produces deep V-shaped cankers that girdle the stems killing the tissue beyond the canker. Both fungal pathogens develop when the environmental conditions add stress to the tree(s), these pathogen infiltrate the tree during the late spring through the summer months when the high heat and drought like conditions cause the stomata (the pores/cells in the leaf epidermis that regulates transpiration) to open up. Some of the conditions which contribute to the development of these diseases are drought, poor planting, poor soil conditions, over watering, poor airflow, and winter damage. There is unfortunately no chemical treatment to prevent or stop these diseases, rather there are a few cultural practices to help prevent development. These practices range from improving soil, adequate watering, ensuring good airflow, and removing dead possibly infected leaves from the ground below and destroying them. If detected early, it is possible to prune out the infected tissue, but be sure to disinfect your pruning tool prior to every cut as the pathogen can spread rather quickly through the pruning wound if spores attach themselves to the pruning blade. A good mixture for a DIY disinfectant is 6 parts water to 1 part bleach.
Needle blight
Cercospora or Cercosporidium needle blight is a fungal pathogen that infects growth at least one year or older beginning low in the canopy and progressing outward and upward over time. Similar to Seiridium Canker and Botryosphaeria, Cercospora pathogenic spores will entire through the stomata cell in the high heat of the summer. Symptoms appear during the summer months and can be more prevalent depending on airflow and irrigation spray. In severe cases all needles except the current years growth will turn brown, eventually leading to the death of the tree. While fungicides such as myclobutanil and thiophanate-methyl are recommended for chemical treatment, they are not always practical or cost effective as these fungicides are applied by spraying the foliage beginning in the late Spring and are performed every 10-14 days through the Summer months ending sometime early to middle Fall. The best solution is prevention, to do this it is best to properly space new plantings to allow for the best airflow, provide adequate watering during drought conditions by way of drip irrigation and avoid overhead/spray irrigation methods.
Mites & Scale
Less common and an occasional pest at best are spider mites and scale. These arachnids and insects are extremely small and difficult to detect with the naked eye. Equipped with piercing mouth parts they affect the tree by sucking on the sap. Spider mites will feed on the leaves which begin to turn yellow, or even brown and die whereas the scale will feed on everything from stems, branches or leaves, turning the leaves yellow then brown as they die; this also weakens branching which can result in branch failure. Spider mites are cool-weather pests that are typically kept in check by summer predators, or can be knocked off with a strong spray of water. If infestation is severe, chemical options such as miticides, insecticidal soap, horticultural oil, and neems oil can be used.
Scale insects are generally also kept in check by beneficial insects/predators and rarely need intervention, however, when severe infestation is detected they can be a little more difficult to treat and if not addressed can kill a tree in just two or three seasons. Chemical treatments by use of insecticides are typically applied through soil injection systemically treating the tree in order to poison the sap and thus poisoning the scale. Caution must be used when using insecticides as these treatments can also target pollinating insects which will adversely affect the surrounding landscape and potential create a larger problem. Other treatment options typically consist of insecticidal soaps and horticultural oils, and will vary depending on level of infestation and time of season.
Natural dieback
Natural dieback or seasonal needle loss is the process in which Leyland Cypress once a year shed their oldest needles, usually the ones closest the trunk. While this can easily be mistaken for death and disease, it is however, natural and does not require treatment. Natural dieback presents itself by the browning or yellowing of the needles from the inside out and can vary in severity season to season, but by the end of the process your tree will once again look healthy. If you notice your Leyland Cypress is beginning to brown but there are no other symptoms of cankers or pests, give it time it may just be natural dieback.
By familiarizing yourself with common issues and employing preventive measures, such as regular inspections, appropriate cultural practices, and early intervention, you can maintain the health and beauty of your Leyland Cypress trees for years to come. For more specific concerns or severe infestations Consult with an ISA Certified Arborist.