Leyland Cypress Diseases & Insect Pests

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Leyland cypress (Cupressocyparis Leylandii) has been relatively free of serious disease problems in the Southeast. The tree is very adaptable to soil extremes (acid or calcareous), prefers medium drainage and fertility, and requires full sun.

Seiridium Canker: This canker disease is caused by the fungus Seiridium unicorne. A canker is a definite, localized, usually dry, dead, often discolored, sunken or cracked area surrounded by living tissue. Cankers form on stems, branches and in branch axils causing dieback of shoots. Twig dieback has also been observed with a gray discoloration at the point of infection. Diagnostic features include resin oozing from cracks in the bark, dark brown to purplish patches on the bark, multiple sunken cankers with raised margins, and yellow to brown discoloration of foliage above the canker. Spore-producing structures of the fungus appear on the bark surface as small circular black dots.

Prevention and Treatment: Spores are washed down the tree or splashed from tree to tree by rain or overhead irrigation. New infections develop when spores lodge in bark cracks and wounds. Therefore, diseased plant parts should be destroyed. Branches showing wilt or discoloration should be pruned out and destroyed. Minimize physical damage to plants. Sanitize pruning tools between each cut by dipping in rubbing alcohol or in a solution of 1 part chlorine bleach to 9 parts water. For chemical control, a thiophanate-methyl fungicide may be applied as a full-coverage spray at monthly intervals from April to October. Read and follow all directions on the label.

Phytophthora Root Rot: Leyland cypress is susceptible to the root-rotting fungus Phytophthora. This root rot is mainly a problem where soil drainage is very poor and on young plants. Phytophthora species that cause root rot live in the soil and enter a healthy tree via wounds or the succulent parts of small roots. As the fungus destroys the roots, symptoms of distress appear on branches above. Foliage becomes stunted, sparse, changes color (yellow, purple, tan) and dies. Cankers may be visible at or below the soil line.

Prevention and Treatment: Remove and destroy infected plants, getting the entire root system. There is no practical chemical control for home gardeners. Do not plant Leyland cypress where Phytophthora is known to be present. Improve soil drainage by adding organic material to heavy, clay soils and avoid overwatering.

Botryosphaeria Dieback: This disease is caused by the fungus Botryosphaeria species. Symptoms are dieback of a leader or major side branch. Exam-ination of the dead stem will reveal an elongated, slightly sunken canker which often has a conspic-uous crack along the margin. Beneath the bark there is dark brown discoloration with a definite margin between diseased and healthy tissue.

Prevention and Treatment: This dieback is generally associated with stress, such as heat, lack of water or recent transplanting. Reduce the stress on the plant. Prune back diseased branches or stems below evidence of dead wood. Mulch plants properly and provide adequate water during dry periods. Pruning wounds may be sprayed with a thiophanate-methyl fungicide. Read and follow all label directions.

Bagworm: Bagworm (Thyridopteryx ephemeraeformis) infests many shrubs and trees, but conifers (evergreens) are the preferred hosts. Damage to plants results from feeding by the caterpillar, which causes loss of needles. Mild infestations of this pest slow the growth of Leyland cypress. Heavy infestations can kill a plant.

The adult male bagworm is a dark-colored, hairy moth with a 1-inch wingspan and clear wings. The adult female is yellow and appears almost maggotlike. The larva (immature form or caterpillar) produces a carrot-shaped bag that it carries as it feeds. The bag is formed from silk that the larva produces. As it feeds, the larva adds bits of plant material to the bag. The bag is about 2 inches when complete. In South Carolina, bagworms survive the winter as eggs in a bag. The larvae hatch during May and soon begin to spin their cases. The adult male moth emerges from its bag in late summer. It locates an adult female in her bag. After mating, the female lays 500-1,000 eggs in her bag and dies.

Prevention and Control: Several parasites and predators feed on bagworms, generally keeping their numbers under control. Removal of the egg-containing bags during winter and early spring is a very effective method for preventing problems before the next growing season. Once removed, the bags should be destroyed or placed in a deep container, which allows beneficial parasites that may also be present in the bags to escape while retaining the bagworm larvae.

If the infestation is severe or the bags are out of reach, spray with the bacterial insecticide, Dipel. This insecticide contains spores of the bacterium, Bacillus thuringiensis, which when eaten kill the caterpillar. Young larvae are much more susceptible to the treatment than are older larvae. As such, apply this pesticide in the spring as soon as bagworms are seen. Control is most effective when spraying is done in late afternoon or early evening. This insecticide is very safe to use. As with any pesticide, read and follow all label directions and precautions before using.

Spruce Spider Mite: Mites are not insects but are more closely related to spiders. Spruce spider mites (Oligonychus ununguis) are occasional pests of Leyland cypress. They are very small and not seen easily with the naked eye. They have piercing mouthparts that they use to suck plant sap. Their feeding results in speckling (formation of tiny yellow spots) on needles. Some needles may turn brown and drop off. With heavy infestations, fine webbing may be seen on the plant. Several seasons of heavy mite feeding may kill a Leyland cypress. Although most spider mites increase in numbers during hot, dry weather, spruce spider mites are cool-weather mites. Their populations peak during spring and fall, but drop dramatically during the heat of summer when predators feed on them.

Prevention and Control: Naturally occurring enemies of mites include various predator mites, ladybird beetles (ladybugs) and other insects. These predators will usually suppress mite populations. Since insecticide use kills beneficial predators as well as mites, insecticides should be avoided unless absolutely necessary. Misuse of insecticides can result in increased problems with mites by causing the death of natural predators of the mite. Miticides, labeled specifically for mite control, are less harmful to beneficial insects. Mites can be removed with a strong spray of water, if applied on a regular basis.

To determine whether insecticide use is needed, it helps to know how many mites are present. Hold a white sheet of paper under a branch and strike the branch. The mites that are knocked off will be seen crawling around on the paper. If dozens of mites are seen per whack, serious damage can result. Continue to check population numbers at 7- to 10- day intervals. Pesticides labeled for homeowner use against spruce spider mite include insecticidal soaps and acephate + fenbutin oxide (Ortho Systemic Insect Killer or Ortho Orthenex Garden Insect & Disease Control Concentrate). As with any pesticide, read and follow all label directions and precautions before using.

Juniper Scale: Symptoms of juniper scale (Carulaspis juniperi) infestation are very similar to symptoms of spruce spider mite infestation. Initially, the Leyland cypress appears off-color and infested branches show little growth. The needles eventually turn yellow or brown. Branches may die back. If ignored, juniper scale infestation may kill the plant in two to three growing seasons. Signs of the pest include clusters of tiny bumps or scales about ¹/8 inch in diameter, especially on the undersides of needles. Adult females are white at first but turn gray or black later. Adult females are mostly flat with a slight volcano appearance, if viewed through a magnifying lens. In addition, a shiny, sticky material (honeydew) is often seen on needles of Leyland cypress infested with juniper scale.

Adult females survive the winter on the plant. In early spring, they lay eggs under their shell. The immature forms, called crawlers, hatch and crawl around before settling on the needles to feed. They feed by sucking plant sap. As they mature, they form a crusty shell over their bodies. Their legs become useless and they remain in one location. As they feed, excess sap is excreted as a sugary material, called honeydew. The sooty mold fungus can grow on the honeydew, forming dark splotches on needles.

Prevention and Control: The presence of adults or crawlers determines which treatment will be most effective. Use a horticultural oil as a dormant spray in late winter or very early spring before new growth occurs to control adult females by suffocation. Insecticides recommended for use against crawlers include acephate (Ortho Bug-B-Gon Japanese Beetle Killer, Ortho Systemic Insect Killer or Ortho Orthenex Garden Insect & Disease Control Concentrate), carbaryl (Sevin 50WP) and malathion 5EC. Read and follow all label directions and precautions before using.

NOTE: Chemical control of diseases and insects on large trees is usually not feasible since adequate coverage of the foliage with a pesticide cannot be achieved.