Boxwood Blight Alert

Boxwood Blight Alert

  • By Melanie Feldman
  • /
  • November 2018 - Vol. 4 No. 11
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  • 0 Comments

Boxwood blight is a fungal disease caused by the pathogen Calonectria pseudonaviculata. It has become a serious issue in the Farmington/Ednam neighborhoods, in Lynchburg, and now in Charlottesville city (Rugby area). This disease causes rapid defoliation, decline, and death to American and English boxwoods.  This article is designed to give gardeners an overview of this disease and how to recognize it, as well as what steps to take to prevent it and treat it.

Prevention is the key since fungicides cannot eradicate the disease from infected plants.  Once boxwood blight has been introduced into a landscape, it is very difficult and costly to control with fungicides.  Currently, fungicide options for home gardeners are limited, although preventive fungicide treatments can protect uninfected plants.  Professional landscapers have other product options, though they can be expensive.  Scientists are working to develop effective control of boxwood blight for home growers with fewer fungicide applications. For a list of specific fungicides labeled for control of boxwood blight in the landscape for use by non-professional applicators, refer to Table 2 in VaCoopExt/Best Management Practices for Boxwood.

Boxwood blight is typically initially introduced into a new location when someone plants an infected boxwood or other susceptible plants (pachysandra and sweet box). There’s some evidence that boxwood blight was introduced in Virginia via an infected plant purchased from a national retail store.  Holiday greenery containing infected boxwood can also introduce the disease into a new location.  Sometimes the disease is spread accidentally by landscape equipment that carries the disease spores. If the disease has been identified in one’s neighborhood, then there is a heightened risk of local spread of the disease.

Seasons of extremely wet conditions, high humidity, and warmer temperature ranges — which we’ve had a lot of recently — are ideal for fungal spore generation. The spores are sticky and must attach to an object for transport. This could be anything from grass cuttings, leaf litter, debris of infected plants, clothing, and any gardening or lawn maintenance equipment. Birds, insects, animals, wind, rain, and runoff of irrigation water are also capable of moving the spores between properties. The spores can hide and can remain viable for several years even after leaves have fallen from the plant. Therefore, the soil under an infected plant can serve as inoculum for new infections in the following seasons.

Defoliation
Photo: A. Bordas

 

Boxwood blight has two very distinct symptoms — sudden, severe leaf drop and black streaking on stems.  If you see both of these symptoms, you are very likely seeing boxwood blight.  You may also see brown spots on the foliage.  Virginia Tech’s Plant Disease Clinic is recommending that we gardeners do the following in our yards:

1. Inspect any boxwoods and related plants in the Buxaceae family (Sarcococca spp. and Pachysandra spp.)

a. Look for discolored and unhealthy areas with sudden defoliation

Close-up of leaf spots; note light center and dark border.
Photo: A. Bordas

 

 

 

 

 

 

 

 

b. Examine stems for black streaking 

Steaking on stems.
Photo: M.A. Hansen

 

2.  If  you suspect you may have found boxwood blight,

a.  cut samples of infected stems and double bag them in Ziploc bags, and then spray the outside of the bag with Lysol or a similar product (to kill any hitchhiking spores).

b.  bring  1 foot of diseased and healthy plant along with 2 cups of soil that includes feeder roots to the local Virginia Cooperative Extension Office.  

c.  Contact the local Extension Office if you suspect boxwood blight.  We need to help the Virginia Cooperative Extension keep track of this disease and its spread!

If the Plant Disease Clinic confirms your worst fears, the following actions are recommended by Virginia Tech AFTER A CONFIRMED DIAGNOSIS:

1. Remove diseased boxwood and leaf litter promptly and very carefully. Remove leaf litter from soil surface by vacuuming, raking/sweeping. If leaf debris has been incorporated into the soil, removing soil to a depth of 8″ to 12″ may help eliminate inoculum of the pathogen. Diseased boxwood, leaf debris and soil should be double-bagged and removed to the landfill OR buried 2′ deep in soil AWAY from boxwood plantings. Do not compost boxwood debris or plant material. (Note that if English and American boxwood are nearby, they are very susceptible to the disease.)  If you decide to hire someone to remove diseased boxwood, be sure they are knowledgeable about boxwood blight and how to prevent its spread.  Ask questions!

2. Because the fungal spores can stick to tools, equipment and just about anything, sanitize all tools, equipment, tarps, shoes, gloves, etc., used in removing infected plants.  Solutions of bleach or Lysol are the recommended sanitizers for home gardeners.  You’ll find precise directions on mixing and applying these santizing solutions at Va.Coop.Ext/Best Management Practices for Boxwood Blight in the Virginia Home Landscape.  This sanitizing step is essential to prevent spread of fungal inoculum to healthy boxwood in your yard and your neighborhood.

3. Promptly begin a preventative fungicide spray program on any remaining susceptible boxwood in the landscape to prevent further disease outbreaks. (Note that if healthy-appearing plants are located near affected plants, they may already be infected.) Repeated fungicide applications (7 to 14-day intervals, according to product label) to susceptible boxwoods in the vicinity that are not yet showing symptoms will be necessary to protect them from infection by this pathogen. Plants already infected will not benefit from fungicide treatment. Products containing the active ingredient, chlorothalonil, and labeled for use on landscape ornamentals, have been shown effective as preventative applications on boxwood. Professional landscapers have additional active ingredient options. Refer to the fungicide information on the Virginia Boxwood Blight Task Force website for more fungicide information. Va.Tech.edu/boxwood-blight/fungicides-table.pdf.  Monitor other boxwoods in your yard for development of symptoms.  Some gardeners may choose to avoid the preventive fungicide spray program and simply remove any other susceptible boxwoods in their yards.  See the discussion of options at VaCoopExt/Best Management Practices for Boxwood Blight.

4. Be aware that pets, children, and other animals can also potentially move the sticky spores of this fungus to new locations.

5. Purchase boxwood only from reputable sources (e.g. nurseries that participate in the Boxwood Blight Cleanliness Program). Carefully monitor any new boxwood plants that are introduced into the landscape prior to planting. If plants have any suspect symptoms (circular leaf spots, black streaking on stems, leaf loss), do not plant them.

6. Other plants in the boxwood plant family, Buxaceae, including Pachysandra spp. and Sarcococca spp., are also susceptible to the disease and should be removed if boxwood blight has been diagnosed on nearby boxwood plants. Plants outside of the Buxaceae family can safely be planted into areas where boxwood blight was diagnosed because these plants are not susceptible to the disease. If you decide to replace infected boxwood plants with boxwood, you’ll want to plant only varieties which are resistant to boxwood blight (though that’s no guarantee).  Consult this helpful list that rates boxwood varieties according to their resistance — all the way from “highly susceptible” to “most resistant” at VaCoopExt/Susceptibility of 23 Commercial Boxwood Cultivars to Boxwood Blight.

7. Holiday greenery that contains boxwood could also be a potential source of spores of the boxwood blight pathogen. Double-bag and discard holiday greenery after the holidays. Do not compost boxwood wreaths or other greenery.

Prevention is the Key

The Virginia Cooperative Extension makes the following recommendations to gardeners wishing to avoid a boxwood blight infestation:

  •  If you purchase new boxwoods, be sure they come from a grower that adheres to the Boxwood Blight Cleanliness Program (see details in the box at the end of this article).
  •  Minimize leaf wetness and promote good air- circulation in boxwood plantings to minimize disease pressure. Examples include:
    – Choose cultivars that have a more open-growth habit (e.g. Buxus microphylla cultivars as  opposed to B. sempervirens ‘Suffruticosa’).
    – Avoid overhead irrigation.
    -Ensure good air circulation in plantings by providing adequate spacing between plants. In general, growers may want to avoid close spacing of boxwood and, therefore, hedges.
  • Mulch boxwood plantings to reduce the spread of boxwood blight inoculum to foliage by splashing water.
  • Avoid working in boxwood plantings when the foliage is wet and fungal inoculum is more likely to be spread.
  • Practice good sanitation practices to avoid moving infested soil or plant material to landscape locations where boxwood are located.
    – Sanitize pruning tools and other tools/equipment/ clothing/tarps between boxwood plantings and also between other members of the Buxaceae family.
    – Bag and dispose of all boxwood debris (including holiday greenery) in the landfill or bury 2’ deep in soil away from boxwood plantings.
    – Be aware that allowing boxwood tippers onto your property to collect greenery may increase the risk of introduction of boxwood blight if the tippers visit multiple boxwood plantings and do not follow good sanitation practices.
    – If you hire landscape professionals to spray or otherwise maintain landscape boxwood, discuss your concern about boxwood blight with them to learn about management practices they may have in place to avoid movement of boxwood blight from one client’s landscape to another. Then you can decide if their approach is acceptable to you.

 

 BOXWOOD BLIGHT CLEANLINESS PROGRAM,

Va.Dept.Agriculture&Consumer Services/Boxwood Blight

One of the ways that consumers can protect against introducing boxwood blight onto their home landscapes is by dealing with retail or production nurseries that have met the compliance requirements of the Boxwood Blight Cleanliness Program. This is a voluntary program geared for production nurseries. If a nursery grows its own stock, or part of its own stock, it could voluntarily sign on to the Compliance Agreement for Production Nurseries and adhere to the very stringent requirements for production cleanliness outlined in the agreement (to view the requirements, go to Compliance Agreement through the highlighted link above). A retail nursery that does not grow its own plant stock should require that the production nurseries that supply its plant stock are in compliance with the Boxwood Blight Cleanliness Program.

Consumers should consult the list of production nurseries that are in compliance. If you deal with a retail nursery, you should ask who supplies their plants in the Buxaceae family, and then check to make sure that this production nursery is on the list of Virginia Nurseries Participating in the Boxwood Blight Cleanliness Program. You’ll find the list at Va.Dept.Agriculture&Consumer Services/Participating Virginia Growers.pdf.  Remember, commonly-purchased Buxaceae family members include pachysandra (Pachysandra terminalis) and sweet box (Sarcococca), in addition to all the many boxwood varieties.


Sources:

“Update on Boxwood Blight in Virginia,” VaTech/ag-pest-advisory/update-on-boxwood-blight-in-virginia/ (9/8/2016)

“Best Management Practices for Boxwood Blight in the Virginia Home Landscape: Version 2, September 2016,”pubs.ext.vt.edu/PPWS-29-pdf

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