I have been hearing about a new disease that affects boxwood, and it
concerns me because I have a lot of boxwood in my landscape. Can you
tell me more about it, and what I can do to protect my plants?
imagine you are referring to boxwood blight or box blight, which was
identified in the United States for the first time in 2011. This
disease was first discovered in the United Kingdom in the mid-1990s
and is now widespread in Europe. To date it has been found in
Connecticut, Maryland, Massachusetts, New York, North Carolina,
Ohio, Oregon, Pennsylvania, Rhode Island and three Canadian
caused by a fungus, Cylindrocladium pseudonaviculatum or
Cylindrocladium buxicola – both names refer to the same pathogen.
This fungus is extremely pathogenic and difficult to control;
avoiding infection is the best way to protect your plants. Other
members of the Buxaceae family include Japanese pachysandra
(Pachysandra terminalis), Allegheny pachysandra (Pachysandra
procumbens) and sweet box (Sarcococca spp.). Boxwood blight has been
identified in plantings of pachysandra, which complicates the
management of this disease when both hosts are present in a
landscape. Sweet box has been infected in the laboratory, but no
naturally occurring infections have been identified to date.
New & Old Boxwoods
Older, established plantings of boxwood are only at risk
if new boxwood plants have been installed since 2010. The spores of
this fungus are very sticky and are not easily spread by wind; long
distance transmission of boxwood blight is mainly due to the
unintentional movement of infected nursery stock and cuttings.
However, those sticky spores are easily moved short distances on
tools and equipment, hands and clothing, and by animals that have
been in contact with infected plants.
'Winter Gem' Boxwood
Boxwood Blight Symptoms
Symptoms of boxwood blight include light
to dark brown circular leaf spots that often have darker margins.
Those spots merge together and cause infected leaves to turn straw
brown and drop prematurely. The disease also causes dark brown to
black elongate or diamond-shaped stem lesions. Symptoms are
generally most severe low on the plant and where the plant is
shaded. Shade and warm (68-80 degrees), humid conditions favor
disease development, and defoliation can occur rapidly under those
conditions. Under high humidity, you may be able to observe fuzzy
white spores on the underside of infected leaves or on stems. Since
boxwood has other problems such as volutella blight or nematodes,
laboratory testing is necessary for a definitive diagnosis.
Preventing Box Blight
There are steps you can take to minimize
the chance of boxwood blight impacting your landscape. If you do
bring any new boxwood plants into your landscape, quarantine them
for at least a month (two is better) in an area well away from other
boxwoods. You may want to cover the area with black plastic and
place the pots on it; that will make cleaning up any fallen leaves
easier in case any of them are symptomatic. The fungus can remain
dormant in fallen leaves for years, and boxwood leaves do not break
Much of the research on boxwood blight
has been conducted by Kelly Ivors at North Carolina State University
and Sharon Douglas from the Connecticut Agricultural Experiment
Station in New Haven. North Carolina has done susceptibility trials
of commercially available boxwood varieties that rate boxwood from
most susceptible to least susceptible.
If you are adding
new boxwood to your landscape, it is wise to stick with those rated
as less susceptible or tolerant, such as:
Buxus ‘Green Gem’
‘Dee Runk’ or ‘Fastigiata’
Gem’ or ‘Golden Dream’
Buxus microphylla var. japonica
Unrelated Boxwood Problems
Remember that there are other common diseases and environmental
conditions that can cause dieback and defoliation of boxwood shrubs,
including poor drainage and winter injury. After the subzero, windy
weather we have experienced this winter,
winter injury on broadleaf
evergreens such as boxwood is likely to be quite common.
plant on a sandmound
Zucchini blossom end rot