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

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.

Symptoms

The most common symptom is a slow general decline of the plant. New growth may be wilted, light green and stunted. The plant foliage becomes sparse or thin and eventually dies. Some plants die one branch at a time until the entire plant dies. The centers of the roots change from white to a reddish-brown color, and the outer layer of the roots will separate easily from the core.

Factors Favoring Disease Occurrence

Phytophthora root rot is favored by high soil moisture and warm soil temperatures. The disease does not occur as frequently and may not be as severe on well-drained sandy soils as on clay or poorly drained soils. The disease is common and severe in areas where run-off water, e.g., rainwater from roofs, collects around plant roots. Shallow soils with underlying rock or hard pans, setting woody plants deeper than the soil level in the nursery or container, over-watering plants, flooding, or long periods of heavy rain also favor disease development.

Prevention

Phytophthora root rot must be prevented by cultural means, as chemicals are often ineffective in controlling this disease after aboveground symptoms are obvious. The following suggestions may aid in the prevention of root rot:

1. Purchase disease-free plants from a reputable nursery. Avoid plants that lack normal green color, appear wilted in the morning, have excessive winter defoliation or dark, discolored roots.
2. Plant root-rot susceptible plants only in well-drained areas. Avoid areas where excess water from any source collects on the planting site. If soil is heavy clay or does not have good internal drainage, set plants in raised beds and thoroughly mix in porous material such as bark (not sawdust or peat). The material should be incorporated to a depth of 8-12 inches. In some areas drain tile plus gravel placed 6-12 inches below the surface may also help reduce excess soil moisture.
3. Do not set the new plant any deeper than the soil level in the container or the soil line in the nursery. Firm the soil beneath the soil ball so that the plant will not settle in the bed.
4. In areas where root-rot susceptible plants have died, replant with species that are not susceptible to the disease.
5. Resistance. Within cultivars of various types of plants, some are highly susceptible (very likely to be killed by the fungus) and others are resistant. Juniper cultivars Andorra, Bar Harbor, Parsoni, Sargents', Shore and J. procumbens Nana are known to be highly susceptible to root rot. These cultivars should be avoided in wet areas or where root rot has been a problem in the past.
6. Fungicide Drenches. The spread of Phytophthora into or among plants also can be reduced through the use of fungicides. Mefenoxam (Subdue Maxx) should be used at the rate of 1-2 oz per 100 gallons of water to treat 400-800 square feet or 3 cc per 10 gallons of water to treat 40-80 square feet of bed. Two to three applications should be made during the growing season. Fosetyl-Al (Aliette 80% WP) may also be used at the rate of 1-2 pounds per 1000 square feet of bed as a drench using 0.5-1.5 pints of the diluted mixture per square foot, or 2.5-5.0 pounds per 100 gallons as a foliar spray at monthly intervals, or as needed.

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:

1. Avoid poorly drained areas.
2. Plant in raised beds except in deep sandy soils.
3. Use highly resistant or tolerant cultivars.
4. Use chemical control as a last resort in the landscape.

Nematode Diseases

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.

Nematode Control in the Landscape

Presently there are no effective chemicals registered for control of nematodes on existing landscape plants. Thus control of nematodes must be achieved by careful planning before planting. Where possible, it is best to avoid using highly susceptible plants on sites where damaging nematodes are known to occur. It is also very important to purchase nematode-free plants from a reputable nursery.

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

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.

Other Problems

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.

In addition 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.

Table 1. Response of selected woody ornamentals to nematodes.

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

Other Links

Plant Disease Information Notes Home Page
North Carolina Insect Notes
North Carolina Agricultural Chemicals Manual
CALS Home Page
NCCES Home Page
NCARS Home Page

For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service

Recommendations 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

Reformatted Feb. 2001 by Plant Disease and Insect Clinic