Juniper Diseases and Their Control in the Landscape
Ornamental Disease Note No. 15
R. K. Jones, Plant Pathologist (retired)
D. M. Benson, Plant Pathologist
Junipers have been widely grown in North Carolina for many years. Recently, numerous new juniper cultivars have been introduced, particularly the low growing types. Junipers are well adapted to most areas of North Carolina, but several diseases can be damaging.
Phytophthora root rot, caused by Phytophthora cinnamomi, is a serious, widespread and difficult to control disease affecting a wide range of plants in North Carolina. Plants susceptible to the disease include acuba, azalea, rhododendron, dogwood, Camellia japonica, Pieris, Taxus (yew), deodar cedar, mountain laurel, heather, juniper, high-bush blueberries, Fraser fir, white pine, shortleaf pine, leucothoe, and others. Boxwood is attacked by P. parasitica, a similar fungus.
Favoring Disease Occurrence
For individual juniper plants in the landscape, approximately 10 square feet of soil around the plant should be drenched. For large junipers and other large shrubs, 20-30 square feet of soil should be drenched. Mefenoxam can be applied with a Hozon applicator (1:15 proportionator) at the rate of 9 cc per gallon of water for 60 square feet.
In summary, for long-term control of root rot, the following control measures should be utilized in the order given below:
poorly drained areas.
Nematodes are one of the most destructive groups of pests causing decline of established ornamental plants in North Carolina. These microscopic roundworms feed on plant roots, causing various types of damage. For example, root-knot nematodes cause swellings or galls on roots of susceptible plants. Other types of nematodes cause plant roots to be stubby and branch abnormally. Roots damaged by nematodes often are invaded by pathogenic fungi and bacteria, resulting in further damage to the plants.
Several plant-parasitic nematodes, including root-knot, stunt, ring, sting, lance, lesion, stubby root, dagger and spiral, have been associated with decline of ornamentals in North Carolina. Examples of plants that are severely affected are presented in Table 1.
Several juniper cultivars are highly susceptible to the lesion nematode (Praytlenchus vulnus). Damage to plants from this nematode is progressive and often results in poor growth, low vigor, yellowing or bronzing of the foliage, loss of foliage, stem dieback, failure to respond to fertilizer because of root damage and eventually death of the plant. Symptoms of nematode damage usually are most apparent during late summer and fall or during extended dry periods. Affected plants usually decline and die over a period of several years, but occasionally plants may die suddenly.
Other problems such as root rot or too much fertilizer can cause symptoms similar to those produced by nematodes. To determine the cause of plant decline, a laboratory examination may be necessary. For laboratory examination, a composite sample consisting of at least one quart of soil and a generous double handful of small fibrous roots should be collected from several spots beneath affected plants and placed in a plastic bag. Be sure to collect this sample from declining, but still living plants. The soil and roots and some of the affected stems and foliage should be taken to your county extension office.
Control in the Landscape
If damaging nematodes are known to occur in the planting site and highly susceptible plants must be used, the entire area can be treated before planting with SMDC (Vapam). This material cannot be used on existing plants in a bed. Chemical treatment is usually performed by trained professional applicators.
For existing landscape plants with nematode problems, it may help to mulch the plants, to apply adequate water during dry periods, to fertilize and lime properly, and to prune out any dead branches. Plants already showing advanced stages of decline due to nematodes (50% or more of above ground portions of the plant) should be replaced with less susceptible shrubs or with turf.
Twig blight and dieback caused by the fungi Phomopsis juniperovora and Kabatina juniperi occur on redcedar (Juniperus virginiana) and some ornamental junipers. These diseases are more common in the mid-west and northern United States than in North Carolina. Most damage occurs on young plants and nursery stock, while older plants seem to be more resistant. The diseases appear as a blight on the young twigs. Infection takes place during wet periods in the spring and fall. Except on highly susceptible cultivars, the diseases usually can be controlled by prompt pruning. Thiophanate methyl (Cleary's 3336 or Fungo Flo) sprays at 1 ½ pound per 100 gallons of water (3 teaspoons per 1 gallon of water) will provide protection against Phomopsis blight if sprays are applied before infection occurs.
Browning of the older needles of shore juniper (Juniperus conferta) has caused concern in North Carolina. The problem is common throughout the state and is apparently more severe in landscape plantings than in nurseries. Mites, fungi and other agents have been suggested as the cause. Typical symptoms are yellowing and browning of older needles beginning near the soil line and progressing up the plant stem. Often the lower third of the plant appears brown. Sometimes needles have flecks of yellow or brown. Similar above ground symptoms are caused by a variety of quite different types of agents.
As discussed earlier, the soil-inhabiting fungus Phytophthora cinnamomi causes root rot resulting in the yellowing and browning of the needles of shore juniper plants. Shore juniper is very intolerant to extremes of soil moisture. A period of either very wet soil or excessive drought will cause death of older needles. In addition, this cultivar is sensitive to air pollutants such as ozone, nitrogen dioxide and sulfur dioxide that cause brown flecks on the needles.
Since symptoms caused by different agents are so similar, it often will be difficult to determine the exact cause of shore juniper decline from the symptoms alone. During drought, plants should be watered deeply, but not so often that the soil will remain soggy. Control of Phytophthora root rot of shore juniper is the same as that for other plants susceptible to the disease. Shore juniper should be planted on well-drained soils. Because of the widespread occurrence of P. cinnamomi, the poorly-drained, heavy clay soils and the frequency of soil moisture extremes in much of North Carolina, it may be best to avoid extensive use of shore juniper in the landscape.
to Phytophthora root rot, two other root diseases may occasionally affect
juniper. Annosus root rot, caused by Heterobasidion annosum, and
Armillaria root rot, caused by Armillaria spp., infect juniper
through contact with roots from nearby infected trees, usually conifers
for annosus root rot and hardwoods for Armillaria root rot. In addition,
Armillaria spp. produces root-like structures called rhizomorphs
on infected trees that can grow through the soil and infect nearby plants.
Symptoms on infected juniper may range from chlorosis and slow decline
to rapid browning and death of the entire plant. Juniper infected by Armillaria
root rot can usually be diagnosed by the presence of a characteristic
white, fan-shaped mycelial mat beneath the bark at the root collar. When
present, annosus root rot can be confirmed by the presence of small corky
fruiting bodies, brown on top and white beneath, that are produced near
the ground line, often beneath the litter. In the absence of the fruiting
bodies, confirmation can only be made in the laboratory. Trees in the
vicinity of juniper beds suspected of being infected by Armillaria
or H. annosum should be cut and as much of the stump and larger
root system as possible should be removed. When removing living pine trees
or other conifers, the stump surface should be treated with powdered borax
immediately after the tree is cut to prevent infection by H. annosum.
1. Response of selected woody ornamentals
HS - Plant highly susceptible (severe stunting, branch dieback and death); S - Plants susceptible (some stunting); T - Tolerant plants, will grow satisfactorily; O - Have not been tested
For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service
for the use of chemicals are included in this publication as a convenience
to the reader. The use of brand names and any mention or listing of commercial
products or services in this publication does not imply endorsement by
the North Carolina Cooperative Extension Service nor discrimination against
similar products or services not mentioned. Individuals who use chemicals
are responsible for ensuring that the intended use complies with current
regulations and conforms to the product label. Be sure to obtain current
information about usage and examine a current product label before applying
any chemical. For assistance, contact your county North Carolina Cooperative
Extension Service agent. 10/90/1000
Published by North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Reformatted Feb. 2001 by Plant Disease and Insect Clinic