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Boxwood: Boxwood Disease

The telltale dark spots of boxwood blight; photo courtesy of Flickr cc/ Orgeon Department of Agriculture
The telltale dark spots of boxwood blight (Calonectria pseudonaviculata); photo courtesy of Flickr cc/ Orgeon Department of Agriculture

Plant discoloration and decline, in whole or in part, is frequently a problem of care and environment but also may be disease-rooted. Of current, intense concern, boxwood blight is an aggressive fungal disease caused by Calonectria pseudonaviculataIt affects all boxwood types as well as the related plants Sarcococca and Pachysandra and potentially others.

But boxwoods are susceptible to numerous diseases, some of which are common in the New York area. Most of these diseases, other than boxwood blight, affect plants that are weakened by environmental and cultural stressors. Caring for and maintaining your plants properly, as described on the first page of this guide, is the best defense against these diseases.

Keep in mind that a single plant may have multiple problems simultaneously, and display symptoms of diseases, insects and of poor care and conditions.

Boxwood Blight

Boxwood blight was first detected in the United States in 2011 and is gradually spreading across the country. Connecticut was among the states to first report a heavy incidence of the disease and our area continues to be strongly affected. It is now confirmed in 25 states and entire public gardens have been devastated.

Boxwood blight, an aggressive fungal disease caused by Calonectria pseudonaviculata, can be identified in your plants by the appearance of dark spots on the leaves in the spring or fall, following a period of rain. The pathogen is most active at around 75º F. and then becomes inactive at temperatures above 82º. The oily, leaf spots enlarge and merge, leaves turn brown or tan, followed by rapid leaf fall. Young branches may exhibit black lesions. The leaf spots and bark streaks differentiate this blight from other common boxwood diseases. Once blight is detected on your plant it cannot be cured, merely contained.

The same fungal pathogen may cause disease in other Buxaceae family plants, specifically in the genera Pachysandra and Sarcacocca. Recent trials by the American Boxwood Society have indicated that C. pseudonaviculata may be hosted by familiar garden plants as diverse as Alchemilla molis, Arctostaphylos uva-ursi, Brunnera macrophylla, Epimedium youngianum, Galium odoratum, Geranium sanguinium, Phlox subulata, Tiarella, cordifolia, Callirhoe involucrata, Ibiris sempervirens, Mazus reptans and Vinca minor.

Federal and State Agencies and horticultural research organizations have reacted with measures to intervene in the spread of this disease. A code of best practices has been developed and is in use amongst some growers; a strategy to avoid the widespread use of fungicides is being encouraged.

The spores of this disease are sticky and, while not as easily spread by splashing water as many fungal diseases,  adhere easily to garden tools, animals, clothing and garden debris. The most common sources of infection in a garden are through the purchase of an infected plant or the use of contaminated tools, though pets and wild animals can also spread it from plant to plant. It can also arrive on boxwood boughs in floral displays so avoid arrangements that incorporate boxwood branches.

What is the home gardener to do to protect their plant investment and contain the disease on their property? The American Boxwood Society recommends the following steps.

Buy carefully

Inspect plants before purchasing, always from a reputable seller. Ask the seller if fungicide was recently applied, as it can mask symptoms. Ask about the sourcing of the plants and if the grower is part of the Boxwood Blight Cleanliness Program. Make sure that you discuss the growing habit of available boxwood species with your nursery before making your selection as leaf size, branch structure, height and width can vary considerably. Choose a cultivar that is considered less disease prone. Quarantine your newly purchased boxwoods for a period of time; at NYBG we quarantine plants for an full growing season before installing them.

Keep your tools clean

Ethanol, bleach and Lysol™ are all effective in killing the fungal pathogen. Sanitize your tools and ask landscaping workers to use clean tools, free of soil and plant material from other properties, when working on boxwood in your yard. Boxwood pruning, thinning and shaping should be done at a time when conditions for transmission are at a low - a sunny and dry day when the leaves are not wet. More stringent controls (change of clothes, washing of vehicles) may be necessary in areas of known blight.

Remove affected plant material

Check plants periodically for signs of blight. If you are concerned by symptoms, confirm that your plant has boxwood blight through a lab test by your county's cooperative extension office. If the diagnosis is confirmed, you must remove affected plant materials. Cornell University Cooperative Extension recommends that you take a strong course of action and  remove all plant material as well as any fallen leaves and the top 1/2 inch of soil and replace with unrelated plant genera.  Merely removing branches of the affected plant is far less effective. Incinerate the diseased plant materials. Cover the soil with mulch after plant removal as a containment method for fungal spores remaining in the soil.

Fungicides should be used only as a last resort, according to directions and in concert with physical control measures, in areas of extensively damaged plants. This is a preventative, not a curative, measure to prevent spread of blight. Virginia Cooperative Extension Boxwood Blight Task Force, a leader in research and best practices, provides this list of fungicides for home grower's use. 

Volutella Blight

Volutella  is a stem blight of boxwood caused by the fungal pathogen Pseudonectria buxi. It is a very common and severe disease, particularly on the dense boxwood types common (American) boxwood and English boxwood. The disease most frequently affects already weakened plants in conditions of high humidity and poor air circulation.

This disease is identified by the growth of pink or rusty fuzz on the underside of the leaf, followed by the rapid discoloration of leaves, turning brown then tan. Dark, sunken lesions, and potentially cankers, form on stem material and may lead to die-back and leaf drop. More commonly, the plant simply remains discolored, with leaves staying on the plant. This differentiating factor can help separate volutella symptoms from those of boxwood blight.

The fuzzy pink to rust-colored spores, which appear and spread in wet weather, are the most identifiable characteristic. In contrast, spores for boxwood blight are white and crystalline in appearance. Both may be present on a plant simultaneously.

Avoid volutella blight by attending to the growing conditions, care and particularly air circulation of your boxwood plants and watering only at the roots. Control an established infection by cutting off plant material to below the infection site, during dry weather, and disposing away from the area.

Phytophthora Root Rot

This soil-borne disease is caused by Phytophthora water mold and is a problem for common (American) boxwoods, in particular. In the wet, cool weather of spring or autumn, Phytophthora attacks the plant's roots and progresses to causing lasting injury only when soil temperature rises above 75º. Roots become soft and brown; the trunk may show disintegration at the soil line. The fungal infection can cause a blockage of water and nutrients through the stem. The disease is usually detected only when leaves begin to turn pale, then darken or become straw-colored. The plant eventually dies.

Properly draining soil conditions are the best tool of prevention. If plants are affected, remove them and do not replant the area until drainage conditions are remedied.

Macrophoma Leaf Blight

Dothiorella candolei is a common but non-threatening boxwood fungal pathogen that causes macrophoma leaf blight. The fruiting bodies create numerous, tiny black dots on the leaf that are easily identified. The fungus particularly attacks plants already weakened by poor growing conditions and care, or other disease. It progresses from the interior of the plant to the outer, younger leaves, in cool, moist conditions with water as its primary means of spread from one leaf to another.

Overly dense growth, particularly of common (American) and English boxwood, make a plant more susceptible as does watering the plants from above. Cut away affected branches as soon as the black dots appear and improve air circulation to the plant. Look for other plant stressors that typically accompany this disease.