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Into the Garden: Thinking inside the boxwood?

CHARLESTON, W.Va. -- Traveling through Virginia recently, I noticed many historic, stately homes with immaculate boxwood hedges. While this sort of landscaping doesn't match the gardening style at my house, I love the look. I've got boxwoods in my garden beds, but they are left unclipped and natural.

While boxwoods are not native to the United States, they are widely cultivated as ornamentals in two major production areas: the Pacific Coast from Washington to north-central California, and in the East from Maryland to South Carolina and west to Tennessee. The three main species grown as ornamentals in the United States are Buxus sempervirens (common or American boxwood), Buxus microphylla and Buxus sinica var. insularis.

A friend said she's had trouble with her boxwoods. After some research, I found there are several major diseases that affect American and English boxwoods.

Mary Ann Hansen, extension plant pathologist at Virginia Tech, said the most common and most important diseases observed in boxwoods are root diseases that cause a gradual and irreversible decline of the plant. A few minor stem and foliar diseases occasionally affect boxwoods. Hansen explains the major diseases of boxwood below.

Phytophthora root rot

Both English (Buxus sempervirens cv. 'Suffruticosa') and American boxwood (B. sempervirens cv. 'Arborescens') are susceptible to this disease, which is caused by the fungus Phytophthora parasitica. The disease has also been observed in littleleaf boxwood (Buxus microphylla) in Virginia. Aboveground symptoms include poor growth and off-color foliage. Leaves are at first light green and may turn yellow or bronze. Leaves turn upward and the edges curl.

Leaf symptoms may appear on just a few branches or on the entire plant, depending on the extent of infection of the roots. Usually, the bark at the base of the infected plant dies and can be easily separated from the wood. By the time foliar symptoms are observed, roots are few in number and many are brown in color. The lack of functioning roots precedes the yellowing and death of the top of the plant.

Plants growing in soils that have become waterlogged following over watering or heavy rains in summer are predisposed to infection by Phytophthora parasitica. Abundant moisture allows spores of the fungus to move in the soil, infecting new roots on the same or adjacent plants. New plantings should always be made with healthy-appearing plants in well-drained soil. Avoid planting a susceptible plant in infested soil unless drainage can be improved before planting. Planting on raised beds may help improve drainage around plants.

English boxwood decline

The disease called English boxwood decline can best be described as a slow but progressive decline occurring commonly in large plants 20 years or more in age. Decline symptoms resemble those of root rot caused by Phytophthora parasitica. However, Phytophthora root rot is primarily a problem in wet soils, whereas English boxwood decline often follows drought stress.

A complex of fungi has been associated with English boxwood decline, but the fungus Paecilomyces buxi is believed to be the primary pathogen. Plant parasitic nematodes have also been recovered from the roots of dying plants, but not consistently enough to explain the disease.

Every effort should be made to maintain plants in a high state of vigor. Because drought stress is thought to be one of the main factors that predisposes plants to disease, it is especially important to water plants deeply and regularly during drought. Do not replant infested areas to English boxwood. Both American boxwood and several cultivars of Buxus microphylla, tested under field conditions, have been observed to be resistant to decline in field tests and can be planted in areas where English boxwood decline has been diagnosed. No fungicides have been found to be effective in controlling this disease.

Root-feeding nematodes

Damage to roots of both American and English boxwoods can occur from the feeding of several types of plant parasitic nematodes. The most common nematodes that feed on boxwood roots in Virginia are ring, lesion and spiral nematodes. These nematodes obtain nutrients by inserting their syringe-like mouthparts into root cells and removing the contents.

When populations of any of these nematodes are high in soil, their feeding can cause severe damage to roots and may also predispose roots to infection by fungi. Symptoms on roots include stunting and browning. Aboveground symptoms resemble those caused by root rot fungi: The plant undergoes a gradual decline characterized by yellowing and bronzing of the leaves and dieback of large sections of the plant.

According to the University of Massachusetts-Amherst Center for Agriculture, a boxwood blight (Cylindrocladium pseudonaviculatum) was found in Massachusetts and is found mostly in English boxwood. Other extension services, in Connecticut and in other states, are also reporting on the blight.

At Purdue University, scientists explained that this blight was first observed in the United Kingdom in the mid-1990s, moved to New Zealand in 1998, and is endemic to Central America, the Caribbean and Mexico. The blight recently was found in the northwest region of North Carolina. The worry is that asymptomatic plants will be shipped to other uninfected regions, spreading the blight.

The best management strategy at this point, before more is known about the pathogen, is to not introduce any boxwoods from unverified sources, either into the nursery or landscape.

Identifying another mystery

Jim in Braxton County has a thought about the last mystery plant.

"Just a guess, but this may be a [crop failure] of a kind of celosia. Had one a few years back. Sometimes the seed company will go ahead and use them to help fill the seed pack."

Thanks, Jim!

Reach Sara Busse at sara.busse@wvgazette.com or 304-343-3909.


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