Boxwood blight is caused by a fungus,
Calonectria pseudonaviculata,
that can infect
members of the boxwood family (Buxaceae)
including
Buxus
spp. and their cultivars, and
Pachysandra
and
Sarcococca
. The disease has devastated boxwood for many years in Europe and New
Zealand, but it is new to North America, including
Oregon. The fungus was found for the first time in late
2011 in three Oregon nurseries, and was detected in
a home garden in Coos Bay and in a few landscapes
in the Portland area for the first time in fall 2014.
Eradication efforts are ongoing.
The presence of boxwood blight in western Oregon
presents a new challenge to homeowners, commercial landscapers, professional gardeners, and
nursery owners alike, because boxwood is a popular
component of landscaping. We are unsure what the
ultimate impact of this disease will be for landscapes
in Oregon. If boxwood blight’s severity and destructive impact in Europe and New Zealand are any
indication, we can expect it to become a much bigger
problem in Oregon.
• Defoliation: leaves drop off, often soon after leaf
symptoms develop. This might start with one or
a few plants, but it can spread rapidly to others
(Figures 1 and 2, page 1; and Figure 3).
• Dark or light brown leaf spots, often in a circular or zonate pattern (Figure 4)
• Straw- to bronze-colored foliage (Figures 4
and 5)
• Dark brown or black lesions on stems, either
linear or diamond-shaped. These typically
progress from the bottom of the stem upward
(Figure 2)
• Downy white fungal growth MAY be seen on
the underside of leaves.
Defoliation is the most obvious symptom. Look
for fallen leaves (or defoliated areas) at the base of
the plant. Inspect the lower and interior canopy for
leaf spots and stem lesions by parting the canopy and
examining the interior leaves and stems. If the disease
has progressed, you may be able to see it on the outer
parts of the plant. Symptoms are best found in fall
through mid-spring; it is difficult to detect symptoms
in late spring through summer. The disease doesn’t
appear to infect the roots, only the aboveground portions of the plants.
The disease spreads over a long distance by people
moving infected nursery stock or infected plant
debris from one place to another. The boxwood blight fungus’ sticky spores are easily moved around by
animals, people (spores can stick to your clothes or
shoes), or splashing water. The fungus can also spread
from plant to plant or from yard to yard via contaminated pruning equipment and infected plant debris.
Western Oregon’s coastal climate is the perfect
environment for boxwood blight. Warm, wet conditions favor the spread of spores and infection of
healthy plants. Studies have shown that infection can
happen very quickly at temperatures between 64
°F
and 77°
F, but young leaves can be infected at temperatures at least as low as 54°F. Because it rains often
in the mild climate along the coast, there are many
opportunities for splashing water to spread infection.
In the Willamette Valley, plants can be infected during
new shoot growth in the spring and for a short time
in the autumn when the fall rains return. After infection, new leaf spots or stem lesions with a new crop of
spores can occur in as little as a week.
The fungus can survive on fallen leaves for up to
5 years and then produce its sticky spores when environmental conditions are favorable.
Because of its explosive and rapid life cycle, the
best practice for containing the disease is to remove
and destroy infected plants. Composting is not
recommended.
All infected plants and associated plant debris
(including all the leaves on the soil surface) must be
removed and destroyed. You can burn everything
on-site (but check local burn ordinances first!), or bag
everything and take it to the nearest waste disposal
site or landfill. Carefully bag and seal ALL the infected
leaf debris. Remember, moving infected plant material
is one of the main ways the disease is spread long-
distance! If you take the infected plant material to
a waste disposal site, DO NOT put it in the brush
pile! It MUST go into the “other waste” section—that
section is removed to a mined landfill at least weekly.
• Isolate new plants for at least a month to watch
for symptoms of the disease.
• Use drip irrigation, or water plants at the root
instead of wetting the foliage.
• Prune plants when conditions are dry.
• Sanitize pruning equipment between plants or
sections of plants and ESPECIALLY between
yards.
• Plant boxwood cultivars that are less susceptible
to boxwood blight. Less-susceptible cultivars
may harbor the fungus, but they develop
much less severe symptoms. (See Table 1 for
examples.)
• Consider planting other species that aren’t
infected by boxwood blight but could fulfill the
same function in the landscape. Some examples
are
Rhododendron migranthum ‘
Bloombux’,
Japanese holly (Ilex crenata), or Pieris japonica.
Boxwood blight isn’t the only disease that infects
boxwood. Boxwood plants are susceptible to a
number of other diseases, some of which can look like
boxwood blight. Volutella blight, Macrophoma leaf
spot, and boxwood decline can all be confused with
boxwood blight. Winter injury and sunscald can also
look like the disease. Some plants may have a combi
-
nation of ailments.
Contact your local OSU Extension Service office
if you are concerned that you have a boxwood blight
infection.
Pacific Northwest Plant Disease Management
Handbook.
2016. Oregon State University
Extension Service.
http://pnwhandbooks.org/
plantdisease
Douglas, S.M. 2011.
Boxwood blight—A new
disease for Connecticut and the U.S.
Connecticut
Agricultural Experiment Station Publication.
http://www.ct.gov/caes/lib/caes/documents/
publications/fact_sheets/plant_pathology_and_ecology/
boxwood_blight-_a_new_disease_for_connecticut_
and_the_u.s.__07-20-12_r.pdf
Ganci, M., D.M. Benson, and K. Ivors. 2012.
Susceptibility of Commercial Boxwood Varieties to
Cylindrocladium buxicola
. North Carolina State
University Extension.
https://plantpathology.ces.ncsu.
edu/wp-content/uploads/2013/05/final-2012-cult-susc-
summary.pdf?fwd=no<
