In the Garden: Bad News for Boxwood

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2046

by Charles Kidder

If Virginia were a country, boxwood might be the Official National Plant. True, dogwood would be a strong contender, especially if the choice were restricted to native plants. Notwithstanding, few plants conjure up Ol’ Virginny more than this familiar evergreen shrub that was actually brought here from Europe and Asia.

Boxwood does have some unique qualities that recommend it. Its dark green leaves have a matte finish that gives them a less plastic look than other broadleaf evergreens. Dwarf varieties stay neat and compact without frequent shearing. And they can withstand more shade than most people realize. Many might find it hard to believe that boxwood can have its troubles, pointing to majestic old specimens that appear to have been planted by Thomas Jefferson. As always, the rule “right plant, right place” applies. Some boxwood varieties are tougher than others; for example, the so-called American Boxwood (Buxus sempervirens) is less prone to some diseases than its dwarf cultivar, English Boxwood (B. sempervirens ‘Suffriticosa’). And any boxwood demands good drainage, prefers afternoon shade and wants to be sheltered from drying winds. So a boxwood growing in those conditions has at least a fighting chance. But in the spirit of full disclosure, they are indeed beset by a host of problems, including a new one that has only recently come onto the scene in the United States. So, let’s take an unflinching look at boxwood and its issues. Just promise not to shoot the messenger.

The average home gardener will have difficulty in making an exact diagnosis of what may be attacking his boxwood. It’s best to take some plant material, ideally both leaves as well as stems, to either the offices of Virginia Cooperative Extension or to a nursery. A major note of caution: particularly when taking diseased plant parts to a nursery, enclose them in a sealed plastic bag. They don’t want you spreading your problem to their plants! Here is a brief rundown of boxwood pests and diseases and how to spot them.

Insects and Other Critters

Boxwood Leaf Miner is the larval stage of a tiny fly. They burrow within the plant’s leaves, creating irregularly-shaped yellow blotches and/or blister-like structures. A heavy infestation can cause defoliation and death.

Boxwood Mites are tiny critters that feed on both leaf surfaces. This results in yellow stippling, or a silvery, dingy appearance if their numbers are high. Natural predators can often control them, so insecticides should be avoided.

Boxwood Psyllids are 1/8-inch long insects that resemble tiny cicadas. Both adults and larvae feed on boxwood, leading to a cupping of the leaves. Plants generally outgrow the injury.

Nematodes are tiny worms that attack the roots of boxwood, as well as many other plants; fungi then enter the damaged roots. This will lead to leaf bronzing, stunting, and a general decline of plant health. Predatory nematodes and bacteria can attack the parasitic species and limit damage. American boxwood are fairly resistant. Organic matter in the soil will encourage beneficial organisms.

Diseases

Volutella Blight is a fungus that may be identified by salmon-colored fruiting bodies on leaf undersides. Damaged leaves will turn straw-tan; affected stems should be pruned and destroyed, i.e. burned or bagged and put in the garbage. Also, remove any dead leaves that are on the ground or lodged in the branches.

Macrophoma is another fungus, diagnosed by black fruiting bodies on leaf undersides, which ultimately turns them a straw color. It is actually considered a weak pathogen and generally attacks dead leaves.

Phytophthora Root Rot is yet another fungus; it causes poor growth and off-colored foliage.

Boxwood Decline is a catch-all term for the declining health of older boxwoods. It may manifest through stunting, wilting, chlorosis (or yellowing) of leaves, and bronzing of foliage. It may be caused by a combination of all these pests and diseases and aggravated by drought.

Boxwood Blight is a new fungus that first appeared in the U.S in 2011, although it has been in the UK since the 1990s. It can be identified by black fruiting bodies on leaf undersides. Leaves will turn straw-colored and drop off the plant. The root system is not damaged, and the plant can recover; however, fallen leaves can lead to a re-infection. Fungal spores can survive on dead leaves for up to five years, making eradication difficult unless all infected leaves are removed.

Box Blight is most likely to spread in warm, humid conditions, although sustained temperatures above 91F will kill it. Ideal conditions for its growth—warmth, moisture, and crowding—are more likely to occur in production nurseries than in the home landscape. Still, it could definitely decimate boxwood in Virginia gardens.

Growers and retailers are taking steps to control boxwood blight. Saunders Brothers, a Nelson County wholesaler of boxwood as well as other plants, is now being inspected monthly by the Virginia Department of Agriculture and Consumer Services. A Phytosanitary Certificate, posted on their website, certifies that their boxwood show no signs of the blight.

I talked with Heidi Crockett of Ivy Nurseries to see what they were doing about this problem. She said they closely monitor the health of their stock, and if any infected plants are found they are isolated and treated. She emphasized that proper care of boxwood by gardeners is important to minimize boxwood blight, as well as other diseases. When watering your plants during a dry spell, make sure that the root system and not the foliage receives the supplemental water. So, think “hose,” not sprinklers and pop-up irrigation systems.

If you want more information, as well as some excellent, albeit disturbing, pictures of boxwood pests and diseases, Google “boxwood diseases,” and it will take you to the websites of Clemson, Virginia Tech and the Connecticut Agricultural Experiment Station.

And good luck with those boxwoods.