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Boxwood Blight - Researchers Looking Outside of the Box for Solutions
By Dr. Jill Calabro, Ph.D., AmericanHort


Boxwood blight causes circular black lesions on leaves, ultimately defoliating the plants. Mulch applied to the surface of the soil should theoretically reduce disease development via rain splash, a primary cause of the blight's spread. Researchers at Virginia Tech are investigating the potential mitigation of boxwood blight using various mulches.


The large, tacky spores of this fungus would not disperse far without human intervention. The spores cling to and contaminate pruning tools, clothing, equipment and anything that might have had contact with infected plants.

Boxwood blight (BB) was first identified in the U.S. in the fall of 2011 and has since been detected in at least 22 states across the U.S., in both nursery and landscape settings. Two closely related fungi, Calonectria pseudonaviculata and C. henricotiae, cause boxwood blight on three plants: boxwood (Buxus), Pachysandra, and sweet box (Sarcococca).

The Horticultural Research Institute, a leading force in the BB fight, continues to monitor, support, and communicate research activities to the industry.

Sanitation and Disinfection
Soil in nursery beds and benches can harbor BB microsclerotia, overwintering structures in dead leaves that will be viable for years. Several commercially available sanitizers were evaluated for their efficacy against microsclerotia. Ethanol was very effective in Dr. Nina Shishkoff's trials and completely killed conidia in leaf debris in studies conducted by Norm Dart. Studies have shown bleach to be effective against BB spores as well.

Mulching can either help or hinder development of a plant disease, depending on the system. When it comes to BB, mulch theoretically should reduce disease development by reducing pathogen dispersal via rain splash (a primary cause of BB spread). Studies are being conducted in operational nurseries and residential landscapes to best gauge impact on disease development.

Few data exist on what fungicides best control BB. Dr. Jim LaMondia is comparing commercially available products and has identified some that are effective, including pyraclostrobin and propiconazole. Both products have demonstrated good preventive control. Propiconazole, in particular, shows promise for early curative control. In fact, most triazole fungicides (also known as DMI's) control BB preventively, as do preventive sprays of chlorothalonil.



Symptomatic plants, as evidenced by this infected boxwood dying from the bottom up, and associated leaf debris should be isolated and removed immediately. The fungus can remain in soil for over five years.

Biological Control
Dr. JoAnne Crouch and Dr. Chuan Hong are screening large numbers of microorganisms in the hopes of finding candidates for biological control of BB. Endophytes, microorganisms that live in or on a plant without causing disease, have been identified associated with boxwood leaves and roots.

Heat Therapy
Dr. Marc Cubeta's lab found that after exposure to hot water at 47.5 degrees C (117.5 degrees F), conidia are either killed or impaired in their ability to cause infection. Preliminary results suggest that certain boxwood varieties are still able to root after exposure to these temperatures. Studies are ongoing to determine whether the pathogen can be eliminated from infected cuttings in this manner.

A breeding program to identify disease resistant varieties and exploration into fungicide sensitivity are two other key components of this collaboration. More information can be found at

This article had to be edited to fit the allotted space. It can be read in its entirety at Outside_The_Boxwood.aspx.

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October 13, 2019, 6:42 pm PDT

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