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by Caroline Hooks

Boxwood leaves can drop for a number of reasons. This may be caused by boxwood blight, which is a fungal disease that was first discovered in 2011. Once boxwood blight is present in the landscape, it is very difficult to control. Hosts consist of plants in the Buxaceae family, including all boxwoods, Japanese spurge, Allegheny spurge, and sweetbox. Partially resistant cultivars of boxwood can harbor the boxwood blight pathogen as well, yet show no symptoms. Mead Tree & Turf Care’s plant health care program scouts every client’s landscape for boxwood blight. It’s trifecta of symptoms are unique from all other boxwood diseases:

A dark leafspot, sometimes with lines in it
Dark canker lesions on green, one year old stems
Defoliation, which most often starts at the bottom and works its way upward

               

The disease is most commonly introduced by infected plant material and/or contaminated tools, equipment or other items. It is very difficult to control because the survival structures produced inside the leaves remain viable for 5-6 years! These leaves can be easily transported and blown by wind and leaf blowers. Spores are produced in warm, humid conditions, and are not well adapted to long-distance spread by air currents. However, the microscopic spores can adhere to all surfaces (for example clothing, equipment, insects, animals), and can be splash-dispersed by rain or watering. When taking samples, potential spores can be killed by sanitizing tools, shoes, with 70% alcohol and washing clothing.

There are steps you can take to control boxwood blight.

Do not use overhead sprinkler irrigation
Promote good air circulation by pruning to a more open-growth habit
Choose cultivars that have a more open growth habit (B. microphylla vs B. sempervirens ‘Suffruticosa’
Purchase from nurseries in a Boxwood Blight Cleanliness Program
Prior to purchase, carefully inspect plants for symptoms
Do not introduce new boxwoods to landscapes with historic boxwood plantings

To make a positive identification of the disease, Mead Tree & Turf Care sends samples to University of Maryland’s plant diagnostic lab. The lab cultures the sample to verify if the pathogen spores are present. Once diagnosed, diseased boxwood and leaf litter should be promptly removed. It is recommended to “remove leaf litter from soil surface by vacuuming, raking, or sweeping. If leaf debris has been incorporated into the soil, removing soil to a depth of 8” to 12” may help eliminate fungal inoculum of the pathogen. Diseased boxwood, leaf debris, and soil should be bagged and removed to the landfill OR buried 2’ deep in soil away from boxwood plantings. Do not compost.”*

Because the spores can stick to spray guns, tools, equipment, and clothing, these items should be sanitized after removing plants to prevent spread of fungal inoculum to healthy boxwood. Mead Tree & Turf Care takes all recommended precautions when addressing boxwood blight on a client’s property. Removing diseased boxwoods and leaf debris will not eradicate the pathogen from the location, because of its long-lived survival spores. An effective preventative fungicide spray program can protect plants that have not yet been infected, and requires repeated applications (at 7-to 14-day intervals, depending on product label and environmental conditions) throughout the growing season. Fungicides are useless if sanitation is not practiced and cannot eradicate the disease from infected plants. If your boxwoods show any of the signs of boxwood blight, call Mead Tree & Turf Care to evaluate your plants and have a culture taken if necessary. We can then advise you on options for intervention and treatment.

Resources:
https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/PPWS/PPWS-29/PPWS-29-pdf.pdf
https://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/ipmnet/Boxwood%20Blight-UnivOfMaryland.pdf

 

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