Diseased Boxwoods: Prevention and Management Strategies

Diseased boxwoods can be a real challenge for gardeners and landscapers, but there are steps you can take to prevent and manage the problem.

The most common causes of diseased boxwoods are overwatering, poor air circulation, and compacted soil, all of which can be addressed through proper care and maintenance.

Regular pruning can help prevent the spread of disease by removing infected leaves and branches, and it's also a good opportunity to inspect the plant for signs of disease.

By taking a proactive approach to caring for your boxwoods, you can reduce the risk of disease and keep your plants healthy and thriving.

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Boxwood disease, also known as box blight, is a serious fungal disease caused by the fungus Calonectria pseudonaviculata. This disease results in defoliation and decline of susceptible boxwood.

Boxwood blight can affect any type of boxwood, including European or common boxwood, Korean littleleaf boxwood, and Japanese littleleaf boxwood.

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The disease was first identified in Virginia in 2011 and has since been found in other parts of the United States, including Wisconsin, where it was first detected in 2018.

There are two types of boxwood blight: Volutella blight and Calonectria blight. Volutella blight causes light green-yellow leaves to turn bronze and yellow-tan, while Calonectria blight causes dark leaf spots that form brown blotches.

To identify which type of blight you're dealing with, send a sample of the plant to the WVU Plant Diagnostic Clinic for diagnosis.

Here are some key signs of boxwood blight:

Prune out infected branches and destroy them by burning or hauling them away in a trash bag. Sanitize pruning tools between each cut to prevent the spread of the disease.

Boxwood blights are caused by two fungal pathogens: Volutella buxi and Calonectria pseudonaviculata.

The fungus can survive on infected plants or fallen leaves on the ground from one year to another, known as overwintering. This means that the disease can persist in the environment even after the initial infection has been removed.

Both fungal pathogens can be introduced into an area on nursery plants that are infected, but not showing symptoms. Holiday wreaths containing boxwood sprigs have also been documented as a source of the boxwood blight fungus.

The fungus can be spread through splashing water, wind, or contaminated gardening tools, such as pruners, shovels, and gloves. This makes it essential to sanitize pruning tools between each cut and remove all infected fallen leaves and litter.

Environmental stress, such as drought, excessive water, or winter injury, can make boxwood plants vulnerable to infections. This means that boxwoods may be more susceptible to disease during periods of stress or extreme weather conditions.

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Boxwood blight originates from a specific fungus called Calonectria pseudonaviculata.

This fungus thrives in humid, warm conditions and can be introduced into an area on nursery plants that are infected, but not showing symptoms.

Holiday wreaths containing boxwood sprigs have also been a documented source of the fungus.

The fungus can survive and produce spores in dead boxwood leaves and branches for several years.

The survival and spread of the boxwood blight pathogen is a complex process that can be influenced by various factors. Both fungal pathogens, Volutella buxi and Calonectria pseudonaviculata, can survive on infected plants or fallen leaves on the ground from one year to another, known as overwintering.

Environmental stress, such as drought, excessive water, or winter injury, can make boxwood plants vulnerable to infections. This stress can weaken the plant's defenses and make it more susceptible to the pathogen.

The spores produced on infected leaves and stems during the growing season can be splash-dispersed through irrigation, rainfall, wind, or pruning tools. This can spread the disease within a plant or to nearby boxwood shrubs.

Recent outbreaks of Volutella blight in West Virginia and neighboring states may have a connection with the cold stress from the arctic blast that occurred in December 2022. This highlights the importance of considering environmental factors when assessing the risk of boxwood blight.

The pathogen can also be introduced to new areas through the nursery trade, where infected plants may be unknowingly purchased and planted. Homeowners and landscapers should be cautious when selecting new plants to ensure they are not infected with the pathogen.

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Boxwood blight can affect plants beyond just boxwood. Pachysandra terminalis, also known as Japanese spurge, can also get the disease, showing symptoms like brown leaf spots.

Other plants in the Buxaceae family, like Sarcococca species, are susceptible to boxwood blight as well. Researchers are still learning about this disease, but it's likely limited to members of this family.

American, English, and Korean boxwoods are all susceptible to blight, but disease severity varies among cultivars. Some boxwood cultivars, like Green Velvet and Green Mound, have some resistance against Volutella blight.

However, other cultivars, like Suffruiticosa and Arborescens, are highly susceptible to Calonectria blight. It's essential to identify the disease accurately, as chemical control methods differ between Volutella and Calonectria blight.

If you suspect boxwood blight, it's crucial to send samples of the suspected plant to the WVU Plant Diagnostic Clinic for diagnosis.

To diagnose diseased boxwoods, it's essential to identify the type of blight they're suffering from. This can be Volutella blight or Calonectria blight, two fungal diseases that can be fatal if left untreated.

The first step is to look for distinctive symptoms on the leaves and stems of the boxwood. For Volutella blight, you'll notice orange or salmon-colored fungal fruiting structures on the stems or underside of leaves. These can be observed by placing suspected leaves in a zip-close plastic bag with a moist paper towel for three to four days.

For Calonectria blight, the undersides of infected leaves will show white sporulation. This can be a key indicator of the disease.

If you're still unsure about the diagnosis, consider sending a sample of the plant to the WVU Plant Diagnostic Clinic for expert analysis. This will help determine the type of blight and provide guidance on the best course of action.

Prune out infected branches and destroy them to prevent the disease from spreading. Sanitize pruning tools between each cut to prevent the spread of infection.

Fungicides cannot eradicate the disease from infected plants, so it's essential to implement a preventative fungicide management program to control boxwood blight. This involves repeated applications of protectant fungicides at 7- to 14-day intervals throughout the growing season.

To control boxwood blight, you'll need to thoroughly cover the boxwood foliage with fungicide, which can be challenging. However, it's crucial for protection from the disease.

Prune out infected branches and destroy them by burning or hauling away in a trash bag. Prune dense shrubs to improve air movement, sunlight penetration, and keep canopy humidity low. Sanitize pruning tools by dipping in 10% bleach or 70% ethyl alcohol between each cut.

Here are some sanitizers you can use for equipment/tools:

Fungicide management is a crucial aspect of controlling boxwood blight in the home landscape. Fungicides cannot eradicate the disease from infected plants, so it's essential to use them preventatively.

To implement a preventative fungicide management program, you'll need to use protectant fungicides, which must be applied at 7- to 14-day intervals throughout the growing season. This means you'll need to spray your boxwood plants regularly to ensure thorough coverage of the foliage.

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The key to effective fungicide management is to start early, before the disease takes hold. Research studies have shown that fungicide applications begun after the disease is already present do not provide acceptable disease control for susceptible boxwood. So, don't wait until you see symptoms – start spraying preventatively.

Thorough fungicide coverage of boxwood foliage is difficult, yet necessary for protection from the disease. To achieve this, you may need to use multiple applications and take care to ensure the fungicide reaches all areas of the plant.

If you're using a fungicide, make sure to follow the label directions and precautions. Also, be aware that warm temperatures with leaf wetness result in high boxwood blight disease pressure, so a preventative fungicide spray should be in place post-season as well.

Here's a summary of the key points to keep in mind:

Doing nothing when it comes to management and control is not a recommended approach, as it can lead to unforeseen consequences.

Read and follow label directions for all products, as commercial products are named in this publication for informational purposes only.

The authors of this publication do not endorse these products and do not intend discrimination against other products that also may be suitable.

Label directions are crucial to ensuring safe and effective use of products, so it's essential to take the time to read and understand them.

Note that the authors' intention is not to promote or demote specific products, but rather to provide information in a neutral and informative manner.

To minimize the chance of boxwood blight, purchase boxwood from nursery producers in the Boxwood Blight Cleanliness Program. This program ensures that the boxwood you buy is less likely to be infected with the disease.

Mulch the soil surface under boxwood to a depth of approximately 2 inches to prevent splash dispersal of boxwood blight spores. This simple step can make a big difference in keeping your boxwood healthy.

To avoid introducing boxwood blight to your landscape, be aware of how it spreads. The disease is most likely to be introduced through accidental introduction of infected plant material, contaminated tools, equipment, wildlife, or domestic animals, and wind-blown infected leaves.

Here are some sanitizers for equipment and tools that you can use to prevent the spread of boxwood blight:

To minimize the chance of boxwood blight, purchase boxwood from nursery producers in the Boxwood Blight Cleanliness Program. This program ensures that the boxwood has been grown in a way that minimizes the risk of disease.

Choose boxwood blight-resistant cultivars, such as those with glossy green, evergreen foliage and growth habits. These cultivars have varying levels of resistance to the disease and can thrive even if infected.

Minimize leaf wetness and promote good air circulation in boxwood plantings. Avoid overhead irrigation and ensure good air circulation by providing adequate spacing between plants. Close spacing of boxwood and hedges can increase disease pressure.

Mulch the soil surface under boxwood to a depth of approximately 2 inches. This can effectively prevent splash dispersal of boxwood blight spores to lower leaves.

Avoid working in boxwood plantings when the foliage is wet, as this can spread fungal inoculum. Practice good sanitation practices, such as sanitizing pruning tools and equipment, and disposing of boxwood debris in the landfill or burying it 2 feet deep in soil away from boxwood plantings.

Here are some sanitizers for equipment and tools, as recommended by the Virginia Cooperative Extension:

Follow the label directions and precautions on all labeled products, and ensure surfaces are free of soil and other organic matter for sanitizers to be effective.

Accurate records are crucial for tracking boxwood cultivar names and locations on your property. Keep a log of where and when you purchased or obtained plants.

Keep records of any diseased boxwood that are removed, noting their location. This will help you identify potential problem areas.

Keep a record of fungicide applications, including product information and the date of application. This will help you track the effectiveness of your treatments.

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Removing diseased boxwood is crucial to prevent the spread of boxwood blight.

First, identify the type of boxwood you have. If it's English boxwood, it's best to remove it promptly, as it's extremely susceptible to the disease.

For American boxwood, pruning and removing diseased branches is an alternative approach, but it may not be as effective in the long run.

Sanitize all tools and equipment used after removing plants to prevent the spread of fungal spores.

Mulch the soil surface under existing boxwood to a depth of approximately 2 inches to prevent splash dispersal of boxwood blight spores.

If leaf debris has been incorporated into the soil, removing soil to a depth of 8 to 12 inches may help eliminate the pathogen, but this is often impractical.

Apply preventative fungicides to the infected and non-infected boxwood in the vicinity whenever weather conditions are favorable for disease development.

Monitor other boxwood and members of the Buxaceae family in the landscape for development of boxwood blight symptoms.

Here are the steps to follow for remediation:

Boxwood blight is a fungal disease that can be fatal if not treated early. It's caused by two fungal pathogens: Volutella buxi and Calonectria pseudonaviculata.

If you suspect boxwood blight, send samples of the plant to the WVU Plant Diagnostic Clinic for diagnosis and help determining your next steps. This will ensure accurate identification of the disease, which is crucial for effective management.

Volutella blight turns boxwood leaves a light green-yellow color, then bronze and finally yellow-tan. It can also infect stems, resulting in loose or peeling, yellowish bark.

Calonectria blight, on the other hand, causes dark leaf spots that form brown blotches. Infected leaves drop off quickly, giving the boxwood plant a barren appearance.

To manage boxwood blight, prune out infected branches and destroy them by burning or hauling them away in a trash bag. Sanitize pruning tools between each cut to prevent the spread of the disease.

Chemical control options are available, but they will depend on which disease your boxwoods have, which is why it's critical to identify the type of blight accurately.

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