What is boxwood blight

Everything You Need to Know About Boxwood Blight

Boxwood is, quite possibly, the most versatile and perfect plant that you can add to your landscape.  Many varieties are compact by nature but with the ability to take large amounts of pruning, can be kept at any size necessary.  Boxwood can be either formal or kept natural.  It is the one plant that you can easily integrate into any style of landscape, regardless of form.  But what happens when the much-loved boxwood appears to be under attack?  Here’s what you need to know about Boxwood Blight.

 

What is Boxwood Blight

While Boxwood Leafminer has been and on-going boxwood issue, a new problem has been found to do equal or more damage.  Boxwood Blight.

Boxwood Blight is a fungal disease (Calonectria pseudonaviculata) that affects all top growth of the plants that it has infected.  While some boxwood appears to be more susceptible than others, all species can be affected by this pathogen as well as Pachysandra (Japanese Spurge) and Sarcococca (Sweetbox).

History of Boxwood Blight

The origin of Calonectria pseudonaviculata is unknown but was first reported in the United Kingdom in the early 1990s.  By 1998, the disease had spread through Europe and to New Zealand.  Boxwood Blight was first introduced to two nurseries in the United States in 2011.  Although uncertain of how it came to be in the United States, it has since spread to over 20 states as well as 3 Canadian provinces.

Boxwood blight in the United States
Image Courtesy Margery Daughtrey, Cornell University

Symptoms of Boxwood Blight

Boxwood Blight infects leaves and branches but not the root system of boxwood.  Although this may seem beneficial, the continual defoliation and continuation of the pathogen can cause younger or smaller plants to die.  Mature boxwood will often be left unsightly and vulnerable to additional diseases.

Infected plants first show dark brown/black leaf spots.  This damage is often confused with other diseases.  The difference between Boxwood Blight and other fungal threats is the narrow black streaks that develop on the green stems.  Clusters of white spores visible to the naked eye will appear during humid conditions.

This disease causes severe defoliation and eventual dieback.  Defoliation will begin on the lower branches and work its way up the plant.

What dies boxwood blight look like
Image Courtesy Margery Daughtrey, Cornell University

The Spread of Boxwood Blight

This disease thrives in 70-degree temperatures with high humidity.  It can easily be transferred from one boxwood to the next by rain or splashing water.

Although the pathogen will go dormant during cold temperatures or during the winter, the disease will still persist in the plant, including in the leaf debris.  Numerous states have found Boxwood Blight to be carried into the state via boxwood wreaths sold during the holidays.

This disease can also be transferred long distances from the movement of infected plants, soil, and equipment used for pruning.

What are the symptoms of boxwood blight
Image Courtesy Margery Daughtrey, Cornell University

Management of Boxwood Blight

There are no magical treatments for Boxwood Blight.  Fungicides are used as a means to prevent and control the disease but not to cure it.  Here are some other forms of Boxwood Blight management:

  • Space plants to add better air movement.
  • Use drip irrigation instead of overhead irrigation.
  • Do not shear or prune the boxwood while wet.
  • Collect and remove ALL debris after pruning, do not compost.

This disease can persist in the soil for 5 years or more.  If you’ve removed your boxwood due to Boxwood Blight, re-plant with alternative shrubs that are not affected by this disease.

Many growers from states that are affected are currently required to follow management practices and have proven to be free of Boxwood Blight before shipping.  They then ship their boxwood to companies like ours along with a certificate indicating they are in compliance with the Boxwood Blight Cleanliness Program.

Can you treat boxwood blight
Image Courtesy Margery Daughtrey, Cornell University

What’s Next for Boxwood

Boxwood Blight isn’t going to go away, but growers and plant breeders are working on creating varieties that show better immunity to this disease.

A new breed of boxwood, presented by Saunders Brothers, is being previewed at trade shows January 2019.  NewGen™ Boxwood will be available for a limited market in late 2019 with full availability expected in spring of 2020.

Can you treat boxwood blight
NewGen™ Boxwood Image Courtesy Saunders Brothers

Saunders Brothers say that the “NewGen™ Boxwood is to be the standard bearer of a distinctively better family of boxwood.  These plants have proven through years of testing and trialing to have better tolerance of Boxwood Blight, better resistance to Boxwood Leafminer, and overall WOW factor in the landscape”.

While we have not seen complete decimation of boxwood populations in the United States, Boxwood Blight is a problem that still needs to be monitored and controlled at all cost.  If you suspect your boxwood may be infected with this disease, contact the local Cooperative Extension Office in your area.