Juniper (Juniperus)
Plant Health Problems
Diseases caused by Fungi:
Tip blight, Phomopsis, Kabatina.
Tip blight is fairly common in landscape plantings and can occur on plants of all ages. Symptoms include browning of needles and tip and branch dieback. Infected tissues develop an overall ash-brown appearance. Infection of the youngest growth is most common although older wood and entire branches can be attacked. In extreme cases the entire plant is killed. Phomopsis typically infects newly developing foliage in the spring whereas Kabatina infects wounded, year-old twigs in midsummer. Plants under stress due to environmental factors (e.g., drought, excess moisture, growing in heavy shade) are more susceptible to infection. In advanced stages, small black fruiting structures of the fungus may be visible with a hand lens.
This disease can be managed by pruning and removing affected tissues when the bark is dry. It is also helpful to avoid plant stress by following good cultural practices and to avoid overhead watering. New plantings should be spaced to provide good air circulation. Resistant varieties are also available. Juniperus chinesis cv. Femina and Pfitseriana and J. communis cv. Depressa and Saxatalis are resistant to Phomopsis. J. chinensis cv. Hetzii and J. communis cv. Hibernica are resistant to Kabatina. Control can also be achieved with the use of fungicide sprays. Among the compounds registered for use in Connecticut are chlorothalonil, thiophanate-methyl, mancozeb, and thiophanate-methyl plus mancozeb. For control of Phomopsis, applications begin when new growth emerges in spring. For Kabatina, applications usually begin in midsummer. More than one application may be necessary. Consult the label for dosage rates and safety precautions. For more information, see the fact sheet on Juniper Tip Blight.
Cedar-apple rust, Gymnosporangium juniperi-virginianae.
This disease requires two types of hosts in order to occur: juniper and apple and crabapple. Junipers are infected during the summer by the rust spores produced on the leaves of apple and crabapple. The symptoms on red cedar and juniper are inconspicuous during the winter and appear as green to greenish-brown kidney shaped galls which vary in size from 1/4-2" in diameter. During cool, rainy periods in the spring, distinctive bright orange gelatinous spore-horns up to four inches long protrude from the surface of these galls. Spores are released from these spore-horns and are carried by wind and driving rain to infect the alternate apple and crabapple hosts and the cycle begins again. This disease can result in twig dieback on junipers but is usually not considered to be very serious.
The most effective way to manage this disease is to avoid planting the two hosts in the same vicinity. It is also helpful to remove the galls on junipers during the winter, before they begin to swell. Resistant varieties of juniper are also available including Juniperus chinesis var. sargentii, J. communis cv. Aureospica and J. virginiana cv. Tripartita. Use of fungicides to protect Juniperus species is not effective and has yielded disappointing results.
For more information, see the fact sheet on Cedar-Apple Rust.
Cedar-hawthorn rust, Gymnosporangium globosum.
This disease is closely related to cedar-apple rust and also requires two types of hosts in order to occur: juniper and several members of the Rose family, including apple, crabapple, quince, and hawthorn. Junipers are infected during the summer by the rust spores produced on the leaves of the Roseacous hosts. The symptoms on red cedar and juniper are inconspicuous during the winter and are similar to cedar-apple rust galls but are round, mahogany red. During cool, rainy periods in the spring, distinctive bright orange gelatinous spore-horns protrude from the surface of these galls but these are shorter and flatter than those of cedar-apple rust galls. Spores are released from these spore-horns and are carried by wind and driving rain to infect the alternate apple and crabapple hosts and the cycle begins again. This disease can result in twig dieback on junipers but is usually not considered to be very serious.
The most effective way to manage this disease is avoid planting the two hosts in the same vicinity. It is also helpful to remove the galls on junipers during the winter, before they begin to swell. Resistant varieties of juniper are also available including Juniperus chinesis var. sargentii, J. communis cv. Depressa and J. sabina cv. Broadmoor. Use of fungicides to protect Juniperus species is not very effective and has yielded disappointing results.
For more information, see the fact sheet on Cedar-Apple Rust.
Diseases caused by Physiological/Environmental Factors:
Winter injury, environmental.
Symptoms include browning of needles and tip and branch dieback and are often confused with those associated with tip blight. Plants of any age can be affected, especially those under stress due to environmental factors such as drought or excess soil moisture. Affected tissues develop an overall ash-brown appearance and in extreme cases, whole plants may die.
Pruning of affected tissues helps to minimize problems due to secondary organisms or opportunistic pests. Good cultural care also helps to minimize the impact of this problem. For more information, see the fact sheet on Winter Injury on Woody Ornamentals.
Insect Problems:
Aphids.
Red cedar trees may be injured in May and June by infestations of aphids. Sprays of malathion or insecticidal soap, which are among the compounds registered for use against this pest in Connecticut, should control them. Imidacloprid may also be used as a systemic to be taken up by the roots. Consult the labels for dosage rates and safety precautions.
Bagworm, Thyridopteryx ephemeraeformis.
Spindle-shaped bags hanging from the trees indicate infestation. The larvae carry the bags around with them as they feed. Serious infestation may result in partial or complete defoliation of a tree. The adult is a moth. Winter is passed in the egg stage. There is one generation a year. Larvae can be controlled by spraying with carbaryl, diazinon, spinosad, or Bacillus thuringiensis var. kurstaki, which are among the compounds registered for use against this pest in Connecticut. Consult the labels for dosage rates and safety precautions.
Juniper webworm, Dichomeris marginelIa.
Low juniper plants in particular are infested by the juniper webworm, which webs together the leaves and twigs. The larva is a light brown caterpillar, about ½" long, which feeds on the leaves in the webs. The moths appear early in June and have a wingspread of 5/8", with dark brown forewings edged in white at the front and rear. Spraying with malathion, which is among the compounds registered for control of this pest in Connecticut, has given good results. Other options are Bacillus thuringiensis var. kurstaki and spinosad. Consult the labels for dosage rates and safety precautions.
Redcedar bark beetle, Phloeosinus dentatus.
This is a light brown or black beetle, 1/16" long, which excavates broad galleries under the bark, especially in stressed trees. If trees are kept in vigorous condition through adequate irrigation there will be little injury by this insect. Severely injured trees should be removed and either chipped or disposed of as municipal waste.
Spruce spider mite, Oligonychus ununguis.
The spruce spider mite injures juniper by feeding on the needles and webs them like twospotted spider mites. The trees take on a faded grayish or rusty brown appearance. Ultrafine horticultural oil, which is among the compounds registered for use against this pest in Connecticut, sprayed during the dormant season will control the overwintering eggs. The same material applied during the growing season at a low rate (1/2 to 1%) effectively suppresses spider mites while not eliminating beneficial predatory mites. Horticultural oil may permanently darken the waxes on needles, thereby changing the color of the plant. Hexythiazox or abamectin (a restricted use product) are other effective options. Consult the labels for dosage rates and safety precautions.