Green Thoughts

Box Blight: A New Disease of Boxwood

Posted by Kent Pierce on Fri, Mar 02, 2012 @ 01:44 PM


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Photo Provided by Conneticut Agricultural Experiment Station


In the fall of 2011 a significant new disease of boxwood, Cylindrocladium pseudonaviculatum, was found in Connecticut and New York, as well as Maryland, North Carolina, Oregon, Rhode Island, and Virginia. The common name for this disease is Box Blight.  It was found in the United Kingdom in the 1990s and in New Zealand in 2002, but hasn’t been seen on the North American continent until the last several months.  It’s not known how the disease was introduced to North America.

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Photo Provided by Conneticut Agricultural Experiment Station

This is a serious disease.  It affects all boxwoods tested so far.  It spreads long distances by introduction of infected plants from wholesale nurseries to retail nurseries to individual properties.  Infected boxwoods can be asymptomatic at time of sale. Once an infected plant is introduced into a landscape spores can move to healthy plants in a variety of ways.  The spores are sticky so they can be moved on pruning tools, birds, mammals like deer, clothing, etc..  Microsclerotia form on fallen leaves and infect healthy plants. Spores can be carried by wind or wind-driven rain over short distances.

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Photo Provided by Conneticut Agricultural Experiment Station

Box Blight has a very rapid disease cycle that can be completed in one week.  Infections can spread very quickly under optimal conditions with temperatures ranging from 64-77 degrees F, especially if humidity is high. The full temperature range is 41-86 degrees F so this is a disease that can spread under humid conditions at almost any time of year.


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Photos Provided by Conneticut Agricultural Experiment Station

Symptoms include light or dark brown spots on leaves that coalesce, often in a concentric pattern.  After infection leaves drop and bare twigs often show black cankers.  Symptoms can be similar to two other fungal blights of boxwood, Volutella blight and Macrophoma blight.  To be sure which of these is fungi are present it is necessary to submit samples to a plant diagnostic clinic.  Healthy plants can be infected and killed in a matter of days.


What to do?  For the moment, don’t introduce or transplant any new boxwoods to your property.  Follow proper sanitation measures by destroying infected plants, cleaning pruning tools, and raking up and destroying any leaves fallen from infected plants.  Infected plant parts should not be composted or allowed to come in contact with any healthy plants.  There are a few chemical fungicides that have proven effective in protecting established plants.  As yet there are no known organic control measures.  

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Photo Provided by Conneticut Agricultural Experiment Station


With this new disease boxwoods have moved to the top of our list of plants we recommend people avoid planting.  Japanese Holly, Ilex crenata, has a very similar look and very few insect and disease problems.  It’s a reasonable substitute although is doesn’t have exactly the same character and can’t be pruned in the same way, at least not with the same look.  


For specific advice on how to protect your boxwoods in areas we service, please call us.


All Photos provided by S.M. Douglas, CAES

Tags: Boxwood blight, Connecticut Agricultural Experiment Station, microsclerotia

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