Boxwood Blight


As noted in last week' s BYGL (2012-01), boxwood blight was discovered in Ohio, making it the 10th state to find this disease and the invasive fungal pathogen Cylindrocladium pseudonaviculatum.  Here is some additional information about this disease.

All boxwood species appear to be susceptible, though new studies will provide information on relative susceptibility of different boxwood species and cultivars within these species.  As far as we know, this fungus does not have wide host range outside the genus Buxus, although recent inoculation studies indicate that Pachysandra and Sarcococca, other genera in the boxwood family (Buxaceae) can be infected by Cylindrocladium pseudonaviculatum.   

Symptoms of boxwood blight include:  brown spots or lesions on leaves, sometimes with dark borders; spots eventually growing together, often with a zonate or target-like appearance; areas of straw-colored foliage and overall stem dieback and blighting of shoots; significant leaf drop when the disease is severe; dark-brown to black angular spots on stems; and stem dieback if disease is severe.

Signs of the fungal pathogen and the disease cycle include "Sporodochia "with masses of spores may become present on the undersides of leaves and stem cankers with these spore masses having a whitish, downy appearance.  The microscopic spores are easily dispersed by wind and splashing water, causing new infections especially when boxwood plants are growing closely together.  The pathogen thrives at temperatures from the mid-60s to mid-70s and with high humidity.  The fungus survives during and between seasons in cankered areas on the stems and in leaves and leaf debris as fungal strands (mycelia) and as resting bodies known as microsclerotia which allow the fungus to survive long periods in the soil.  

As noted last week, diagnosis can be tricky.  If you suspect that you are seeing boxwood blight, Nancy Taylor with the CWEPPDC urges the samples be submitted for diagnosis.  Do not rely on field diagnosis.

Diagnosis of other boxwood problems include:

VOLUTELLA BLIGHT, in which you will see salmon-colored spore bodies on undersides of leaves.  Though you will also see leaf discoloration and lesions, this disease, caused by the fungus Volutella buxi is not as severe as boxwood blight.  SUNSCALD and WINTER BURN on boxwood are also common.  Symptoms include blanching of leaves due to water loss in the winter and spring.  Soil-borne diseases are also a factor from PHYTOPHTHORA ROOT and CROWN ROT disease to NEMATODE problems resulting in long-term decline and stunting which require soil assays to determine if microscopic plant parasitic nematode populations have reached damaging levels.  Again careful monitoring and sending samples to the CWEPPDC is essential.

Management options for boxwood blight by growers include:

1.  Sanitation by removing infected plants and by cleaning up fallen and decomposing leaves.

2.  Buying from reputable sources that practice good plant health management.

3.  Growers should isolate and monitor new boxwood purchases for several months.

4.  Avoid overhead irrigation to the extent possible.

5.  Use of fungicides.  Check labels for legal use of fungicides containing, e.g. mancozeb, chlorothalonil, azoxystrobin, and fludioxonil products and rotate to limit fungicide resistance issues.  The dilemma is that these products are not curative and must be applied before symptoms occur and of course we never see infections occurring prior to these symptoms and thus preventive schedules must be developed.

For more detailed information, check out: "Boxwood Blight – A New Disease for Connecticut and the United States" by Dr. Sharon Douglas of the Connecticut Agricultural Experiment Station.  This is easy to find if you Google-it, but as always be careful with your googling practices.  For example, if you use Google Images for "boxwood blight", you will get a potpourri of images, including boxwood blight of interest here due to Cylindrocadium pseudonaviculatum, but also images of everything from Volutella blight of boxwood, a different disease (see above) to Volutella blight of pachydsandra, a totally different disease on a different host caused by a different fungus.  Go figure.