The Latest on Boxwood Blight
Blight found on a boxwood in Forsyth County NC.
Boxwood Blight is a fast-moving, destructive, and incurable fungal disease. Boxwood Blight reached the United States in 2011, initially affecting North Carolina and Connecticut. It can now be found in at least twenty states and several Canadian provinces. Boxwood Blight already has a strong foothold in Winston-Salem, and is beginning to be seen in the Greensboro area. It is devastating to all boxwood types (English, American, Korean/Japanese), including varieties and cultivars initially thought to be resistant. New Garden Landscaping & Nursery feels that Boxwood Blight could have a major impact on landscapes containing boxwood in the near future. Here’s what you need to know about the disease.
Disease Spread
Boxwood Blight spreads very easily via the sticky fungal spores. These spores can cling to and be spread by people, tools, clothing, and wild (deer, birds, rabbits etc.) and domestic (dogs, cats, horses etc.) animals. It can also be spread by water droplets or flooding. Often it is introduced from boxwood being added to a landscape, where rain-splashed droplets move it from infected additions to the original plants. Due to their stickiness, spores are not thought to be spread by wind.
Infected plants become symptomatic in a matter of weeks. The disease causes defoliation of affected stems, and most importantly, if an area/landscape becomes infected, boxwood can never be planted there again as it will inevitably succumb to the disease. It also can become established with minimal symptoms on boxwood relatives Pachysandra and sweetbox (Sarcococca). This can create a disease reservoir on a property that does not have boxwood on it that can affect neighboring properties.
Boxwood Blight can infect pachysandra (left) and sweet box or sarcococca (right)
Leaf lesions caused by Boxwood Blight
Boxwood Blight Symptoms
Initial symptoms of boxwood blight include circular, tan leaf spots with a dark purple or brown border. The leaf spots may or may not have yellow to reddish halos surrounding the spot. Black stem lesions or blackening of the stems is often seen. Infected leaves become tan and drop from the stems in sections, with defoliation happening rapidly. The speed of defoliation is one characteristic that indicates Boxwood Blight rather than another boxwood disease. One reason that it has spread so quickly is that some boxwood, particularly varieties that until recently were thought to be “resistant”, show only the stem lesions for a period, which remain hidden within the shrub’s leaf canopy. Without close inspection, these infected carrier plants may be planted and spread the disease.
Black stem lesions and defoliation caused by Boxwood Blight.
Shady, warm and humid conditions seem to favor development of Boxwood Blight, but it has been seen in almost every combination of conditions. It may slow during periods of hot, dry weather or when fungicides are applied, but it is not stopped. Proper hand pruning (by opening the leaf canopy to allow improved air movement and better light penetration into the interior of the shrub) may improve resistance and slow spread of the Blight in two ways:
Better light and air movement makes conditions somewhat less favorable for Boxwood Blight to take hold by reducing humidity at leaf level.
This type of hand pruning (rather than shearing) improves the overall health of the shrub, increasing disease resistance. If shearing is unavoidable for reasons of plant quantity or formal design, we recommend additional hand pruning at least once a year to maintain plant health.
New Garden’s Response Plan
There is no cure; control measures are limited, expensive and must be continued indefinitely. Once infected, boxwood will never recover and will eventually need to be removed. For this reason, New Garden Landscaping & Nursery is implementing a Boxwood Blight response plan:
We will no longer use boxwood in our landscape designs. If a customer is set on using boxwood despite being informed of the risks, we will plant them, but will not warranty them, nor will we be responsible for loss of any boxwood already existing on the property.
New Garden Gazebo will no longer carry English boxwood as it is hugely susceptible to Boxwood Blight. American and Korean boxwood will be offered, sourced only from a nursery that has an extensive disease control plan in effect.
Boxwood of any type sold at New Garden Gazebo will not be covered under our Hardy Plant Warranty. Customers choosing to plant boxwood do so at their own risk.
Barring a breakthrough in disease eradication options, we will cease selling boxwood altogether by 2022.
New Garden Gazebo will offer and educate customers about boxwood alternatives.
Our account managers are undergoing training to identify Boxwood Blight and maintenance crews are instructed to bring any suspicious boxwood to their manager’s attention.
We will continue to educate clients about Boxwood Blight, and offer proper pruning of boxwood to minimize disease spread.
Through Nature’s Select we will facilitate fungicide treatments to those who wish to implement a preventive fungicide program for their boxwoods. We cannot ensure the long-term effectiveness of preventive spray programs, particularly under heavy disease pressure.
Protective Measures
If you have boxwood in your landscape, you can take the following measures to keep your boxwood as healthy as possible, hopefully slowing development of the disease.
Have your boxwood properly pruned to increase airflow and light; avoid shearing.
Mulch regularly with organic material. Mulching keeps soil moisture even for shallow-rooted boxwood, and increases organic content of soil as it breaks down. Additionally, mulch may reduce spore-laden water splashing onto leaves during rain.
Test pH and apply a calcium product to reach a pH of 6.5 to 7.2. It may take several applications to reach ideal pH. We recommend Solu-Cal or Bonide Turf Turbo, please follow the label directions for application rate and frequency.
Test pH every year until the target range is achieved, then every other year to keep it in range, applying calcium as needed.
Boxwood are relatively heavy feeders, so fertilize with a non-acidifying slow release organic fertilizer around Thanksgiving. Boxwood roots are most active at this time and will make the best use of the available nutrients.
If means allow, consider a professionally-applied preventive fungicide program to prolong the health of your boxwoods.
Remove any boxwood with confirmed Boxwood Blight immediately. Place affected boxwood in trash bags before moving them through/out of the landscape to minimize spore dispersal. Dispose of in the trash-do not compost plants with Boxwood Blight.
Do not replant with boxwood if Boxwood Blight is present. Boxwood alternatives can be found here.