It’s not every day you get to discuss your problems with an international expert. But Lynn Batdorf is the real deal. Batdorf is the world’s top resource on everything boxwood, including all of the diseases and pests that affect this diverse species. Recently he spoke to me about how to deal with the latest threat to our gardens, the dreaded boxwood blight.
WHERE DID BLIGHT COME FROM, ANYWAY?
Given its rapid spread, you could be forgiven for assuming that boxwood blight is a recent arrival. But in fact, the fungus was first reported in the United Kingdom in the mid 1990s. Following its discovery, the disease quickly spread across Europe and New Zealand before landing in North America in 2011. Since then, it has been decimating landscapes across the world at alarming rates.
Boxwood blight was first discovered in the United Kingdom in the mid 1990s.
SIGNS AND SYMPTOMS OF BLIGHT
Boxwood blight is caused by the pathogen Calonectria pseudonaviculata, which induces leaf spots and stem cankers, followed by defoliation and eventual death of the shrub. The fungus usually starts on the bottom branches and moves up the plant via splashing water.
The initial leaf spots are dark with a black ring. Batdorf refers to these as ‘frogs’ eyes.’
Boxwood blight leaf spots
Soon after the spots appear, they expand to cover the entire leaf and the blighted leaves begin dropping from the plant. The disease next progresses to the stems where it forms narrow black streaks called cankers. As the cankers enlarge, they girdle the stems and the twig dies.
Photo: Landis Lacey & Kelly Ivors, NCSU Dept. of Plant Pathology
Blight infects all the above-ground parts of the plant and will cause defoliation of the entire shrub in less than 10 days. As Batdorf puts it, the plant simply dies from exhaustion. Indeed, each time the boxwood tries to recover and produce a new leaf, blight spores hop back up on it. Eventually the shrub succumbs, deprived of its source of life-giving energy.
Row of infected boxwood in Connecticut/Photo: Sharon Douglas, Ph.D., CAES
Although it can at first resemble other boxwood diseases such as Volutella and Macrophoma, the main thing that sets blight apart are its spores. Said to live up to 5 years in the soil, they are nearly impossible to eradicate. As a result, once boxwood blight has appeared on a plant, it will most likely keep reappearing.
To make matters worse, during growing season transmission of the disease is usually splash-related. This means it generally occurs through irrigation or rainfall, both of which easily spread blight to nearby shrubs.
Irrigation helps spread the disease.
Even worse, boxwood blight can overwinter on infected plants or hide in leaf litter. It is also transmitted by tools, lawn mowers, leaf blowers, vehicles that contain infected leaves and pets. And host plants include all members of the boxwood family such as pachysandra and sarcococca. Recently, these two popular ground covers have also shown signs of being infected.
A new host, pachysandra can also spread blight/Photo: CAES
STRATEGIES FOR COPING WITH BLIGHT
So what’s a gardener to do? Up until now, there have been few options. More recently, instituting a preventative fungicide spray program has shown some promise in protecting non-infected plants. However, since the spores live for so long in the soil, this involves expensive, bi-weekly spraying during the growing season for at least five years.
For those who aren’t comfortable with or able to invest in such measures, Batdorf (who is not a fan of spraying) offers some suggestions.
First, only purchase from reputable nurseries. And don’t buy English box. It is the most susceptible of all.
English box is the most susceptible of all varieties.
Second, don’t shear plants when they’re wet. Boxwood blight thrives in humidity and moisture.
Avoid pruning in wet conditions.
Third, collect and remove all leaf debris. It may very well be harboring spores. And don’t compost dead leaves in the vicinity of boxwood plants.
Don’t compost near boxwood plants.
If you do diagnose blight on your boxwood, remove and destroy the plants. Then dig out the infected soil and replace with clean soil. Replant with a different species. Unfortunately, other boxwood, even so-called resistant varieties, are no longer candidates for that space.
OTHER NON-HERBICIDAL STRATEGIES
Whether or not you’ve decided to use fungicides, here are Batdorf’s three main recommendations for helping to prevent boxwood blight on healthy shrubs. (The key thing to remember is that blight loves to spread by splash from soil to plant.)
Remember, boxwood blight spreads primarily by splash.
1. APPLY 1″ OF PINE BARK MULCH
Apply 1″ of mulch (not 2″ or 3″, which can hold too much moisture) around the plant’s drip line to prevent splash. Batdorf recommends pine bark over hardwood for its better aeration. Hardwood can suffocate boxwood’s delicate surface roots.
2. LIMB UP THE BRANCHES
Limbing up the branches by 6 to 8 inches will help prevent splash up from the soil.
3. PRACTICE SANITATION
Regularly vacuum up the area under your boxwood to remove all dead leaf debris, which is a major cause of infestation.
Clean up all dead boxwood debris.
BEST DISEASE-RESISTANT BOXWOOD CULTIVARS
Finally, although there are no known resistant boxwood, some have been found to be more tolerant than others. One of the best ways to limit your exposure is to plant less susceptible varieties. Below are some recommendations.
DO NOT PLANT. At the top of the list are ENGLISH BOX, Buxus sempervirens Suffruticosa and Buxus sinica var. insularis Justin Brouwers. Other highly susceptible cultivars include Buxus sempervirens Marginata and Buxus sempervirens Elegantissima.
Buxus sempervirens Elegantissima
BETTER choices are Buxus microphylla Jim Stauffer, Buxus X Green Mound and Green Mountain.
BETTER STILL are Buxus microphylla japonica Winter Gem, Buxus sempervirens Dee Runk, Buxus hybrid Green Gem and Buxus microphylla John Baldwin.
BEST OF ALL and recommended for all new plantings are Buxus sinica var. insularis Nana and Buxus microphylla japonica Green Beauty.
For more information on Lynn Batdorf, the U.S. Arboretum’s National Boxwood Collection (which he curated for 36 years) and his many books, articles and lectures, click here for his website. He also provides on-site consultations.