University of Maryland Extension

Boxwood Culture and Diseases - Including Boxwood Blight

Author: David L. Clement, University of Maryland Extension Specialist Plant Pathology
Article originally appeared in the Spring 2013 issue of the HGIC eNewsletter

        boxwood foundation hedge
Back to common problems - shrubs

Boxwoods have always been a popular shrub in Maryland and recently they have become more widely planted because of their deer resistance. However, boxwoods are often overly pruned into tight shapes that limit air circulation within the canopy and favor foliar diseases.

Boxwoods grow well in full sun to partial shade. Sites exposed to full winter sun can cause winter burn on boxwoodfoliage to “burn” and turn orange. Boxwoods planted with a south or southwest exposure suffer winter burn more than plants with an east or north exposure due to increased sun exposure. As with all evergreens normal leaf drop does occur, but leaves normally remain functional for 3 years. Boxwoods are broad leafed evergreen shrubs that are typically used as foundation plantings and backdrops for planting beds, topiaries, and formal gardens.

Boxwoods require adequate drainage, ample amounts of organic matter and grow best within a soil pH range of 6.5 to 7.2. Excessive yellowing of older inner foliage or premature leaf drop may indicate a lack of nutrients. Do not cultivate deeply near the boxwoods or shallow roots will be damaged.

Boxwoods require only light applications of mulch. Do not apply more than one inch of mulch over the root zone and keep it clear of the main stem. Excessive mulch may encourage vole (meadow mice) activity and production of adventitious roots in the mulch layer which are very prone to desiccation (drying) damage.

The most appropriate pruning method for all boxwoods is thinning. Thinning allows the center of the plant to receive adequate sunlight and air circulation. Properly pruned boxwoods will have leaves along the entire branch length. Dense foliage encourages fungal diseases such as macrophoma leaf spot and volutella canker. Shearing stresses plants and should only be used in boxwood topiaries. The best time to thin boxwoods is December through February.

Cultural Problems

Winter Injury

snow on boxwood

Winter injury may be confused with early stages of the fungal diseases phytophthora root rot or volutella blight. Leaves turn from bronze to reddish brown as a result of exposure to cold, dry winter winds. Tissue death is caused by the removal of water in the leaves faster than the plant can replace it through root uptake from frozen water in the soil. Bark splitting can be caused by a rapid temperature drop caused by a mid-winter thaw. Dead twigs and branches in the spring may be the result of ice and snow damage from the winter. 

Winter damage can be reduced by locating plants in partially shaded areas protected from winter winds. Physical barriers, placed about 18 inches from the plants on the windward side, de from materials such as burlap or plastic, can also lessen winter wind damage by reducing wind velocity. Maintain adequate soil moisture in the fall to prevent winter desiccation. To avoid damage from falling snow and ice do not plant boxwoods under roof eaves. For established boxwoods, tie a string or twine at the base of the plant and spiral the twine up and down the plant to hold it together and gently brush snow off plants as soon as possible. This will help prevent damage from falling ice and snow. Inspect plants for winter damage in the spring and prune out affected areas.

Boxwood Diseases

Root Rots
Phytophthora spp. and Ganoderma lucidum

The fungi, Phytophthora spp. and Ganoderma lucidum, can cause root rots in many boxwood cultivars. Symptoms include poor growth, loss of healthy foliage color (leaves eventually turn light yellow), upward turning and inward rolling of leaf margins, dark brown discolored wood at the base of the stem for 2 or 3 inches above the soil line, and loosening and separation of the dead lower bark. As a result of the fungal infection, the root system is reduced and dark brown.

Although there are no chemical cures for these diseases they can be prevented by proper planting. Avoid planting boxwoods in poorly drained compacted soils or in low areas where water collects. Also avoid placing boxwoods near downspouts. Construction of raised beds or grade changes may be needed to ensure proper drainage. Root diseases on older established plants can result from changes in water drainage patterns.

Volutella Stem Blight or Canker Pseudonectria rouselliana

Both American and English boxwood are highly susceptible to this disease caused by the volutella stem canker on boxwoodfungus, Pseudonectria rouselliana. The imperfect stage is Volutella buxi . Before new growth appears in the spring, leaves on the tips of infected branches turn red, then bronze and finally yellow. Infected branches die back. Examination of affected branches reveals loose bark and girdling at varying distances from the tips and discoloration of the wood. In moist weather the fungus produces salmon pink fruiting bodies on leaves and stems.

In home landscapes proper pruning and thinning is the most effective way to manage this disease. In contrast, shearing boxwood foliage often leads to disease problems because of dense branching and reduced air circulation within the foliage canopy. Thin boxwood foliage so that when brushed by hand the branches flex and move and light can penetrate into the center of the plant. Diseased branches should be pruned out when the foliage is dry. Old fallen leaves and diseased leaves that have accumulated in the crotches of branches in the interior of the plant should be shaken out and removed.

straw colored leaves

Straw colored leaves

Macrophoma Leaf Spot Macrophoma candollei

black fungal spores

Most boxwoods are susceptible to infection by the weakly parasitic fungus, Macrophoma  candollei. The most obvious symptoms are the many tiny black raised fruiting bodies found on dying or dead straw-colored leaves (see photo above). Again in home landscapes proper pruning and thinning is the most effective way to manage this disease.

Boxwood Blight Calonectria pseudonaviculata, syn. (Cylindrocladium psedonaviculatum)

A new disease called Box Blight or Boxwood Blight has been causing defoliation of boxwoods throughout Europe since the late 1990’s. In October 2011, the disease was found in North Carolina and Connecticut in both nursery, and landscape plantings. The disease was also found in a Virginia nursery. Since this first US report the disease has been identified in a number of northeastern states and also in Oregon, and British Columbia. The first Maryland case of Boxwood Blight was confirmed on plants from a landscaper’s nursery in, 2011, and last year in 2012, in a landscape.

The disease is caused by a fungus called Calonectria pseudonaviculata. The first symptoms begin as leaf spots followed by rapid browning and leaf drop starting on the lower branches and moving upward in the canopy. The key symptoms that differentiate Boxwood Blight from other boxwood diseases, such as Volutella Blight and Macrophoma Leaf Spot, are numerous narrow black cankers (black streaks) that develop on the green stems. The pathogen does not attack the roots, so larger plants may produce new leaves during the growing season, but may lose ornamental value as defoliation becomes severe.

Repeated defoliation and dieback from stem cankers has killed small rooted cuttings in nursery propagation. The causal fungus can remain alive in fallen leaves which can then serve as the source of infection for subsequent years. The spores of the fungus can be splash dispersed through irrigation or rainfall resulting in spread of the disease within a plant or to nearby boxwoods. The primary method of long distance disease spread is most likely shipping of infected plants, use of contaminated tools and transport vehicles that contain fallen infected leaves.

Recent research on the susceptibility of various boxwood cultivars has shown promising results for resistance in some cultivars. Within the last year, Japanese spurge, Pachysandra terminalis , and Allegheny spurge, Pachysandra procumbens, have also been shown to be susceptible to boxwood blight in the landscape. Sweet box, Sarcococca, another ornamental that is in the boxwood family (Buxaceae) has also been infected under experimental conditions. Research is under way to test the effectiveness of fungicides for management of Boxwood Blight.

Boxwood blight photo gallery:

stem cankers caused by boxwood blight infected stems infected boxwood hedge
Boxwood blight stem cankers
Close-up of infected stem
Photo: Landis Lacey & Kelly Ivors NCSU Dept. of Plant Pathology
Row of infected boxwood
Photo: Sharon Douglas Ph.D.,CT Ag Experiment Station

 Suspected Boxwood Blight cases should be reported to University of Maryland Extension.

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