Disease name: Boxwood blight
Pathogen name: Calonectria pseudonaviculata (fungus).
Hosts: The primary host for boxwood blight is boxwood (Buxus species), but it can also attack Pachysandra and Sarcococca (sweet box).
Boxwood blight is caused by a non-native fungal organism.
The disease was first reported in the United States in 2011. It is now found in most states in the eastern half of the U.S., as well as California, Oregon, and Washington state.
Boxwood blight can quickly kill entire plants, and is considered a serious disease in both production nurseries and landscapes.
Symptoms
Any or all of the aboveground parts of the host plant can show symptoms of boxwood blight.
Leaf symptoms often show up first. Dark leaf spots will form and may merge together. These spots may be mistaken for the spots caused by the boxwood leafminer. If leafminer is present, holding the leaf up to a strong light will reveal the insect inside.
Another leaf symptom of boxwood blight can occur if weather conditions are humid. In this situation, white fungal spores may develop on the lower side of the leaves.
Leaf symptoms of boxwood blight move quickly, starting on the lower part of the plant and moving upward. Infected leaves will turn brown and begin to drop off. Severe defoliation may take only a few weeks.
Stems of plants infected by boxwood blight will develop elongated black cankers. These distinctive cankers can help distinguish boxwood blight from other boxwood diseases. During humid weather, white, fuzzy masses of spores may be seen developing from the cankers.
Infected boxwoods will die. Larger shrubs may take longer to die, but will decline to the point where they no longer have any ornamental value as landscape plants.
Disease Cycle
Spores of boxwood blight will overwinter on infected plant parts, on fallen leaves, and in the soil.
The spores are often dispersed by rain or irrigation water hitting them and splashing them to other plants.
Infection can occur during warm, wet weather in spring and fall.
The disease can also be spread by transport of infected plants from nurseries, and on contaminated tools and clothing.
As symptoms develop on the leaves and stems, new spores are produced. These can overwinter and provide a source of infection in the next growing season.
Spores can persist in the soil for five years.
Management
Cultural Management
Inspection and sanitation are key measures in managing this disease. Inspecting new boxwoods at the time of purchase can help prevent introduction of the disease into the landscape. Quarantining new plants for a month will also help with this. Remove and destroy infected plants. Also remove all fallen leaves from the infected plants. Do not compost them. After handling infected plants, sanitize clothing, shoes, and tools.
Resistant Cultivars
Boxwood species and cultivars differ in their susceptibility to boxwood blight. See this link for specifics.
Planting species other than boxwood is a useful technique as well. The Morton Arboretum website provides help on selecting trees and shrubs.
Chemical Management
Fungicides are available for management of boxwood blight, but they are preventative rather than curative.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. Use pesticides safely and wisely; read and follow label directions. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
For more information, contact The Morton Arboretum Plant Clinic at 630-719-2424 or plantclinic@mortonarb.org.