Landscape & Garden Editor Kevin Johnson Warns of a New Disease Threatening One of Our Most Beloved Shrubs
The Boxwood (Buxus) is considered our oldest garden ornamental. Introduced in the US in the 1600’s, Buxus is extremely hardy, resilient and trouble-free. These attributes earned the English Boxwood top honors, which is why you’ll find this evergreen rubbing elbows with royalty in select palace gardens around England. Boxwoods typically do well in drought situations and require little fertilizer. They have many uses from hedges to individual specimens, they’re also grown in containers, used in topiary and in bonsai gardens. And now one of our favorite plants is in trouble.
Boxwood Blight, caused by the fungus, Calonectria pseudonaviculatum (syn. Cylindrocladium pseudonaviculatum and Cylindrocladium buxicola) has been confirmed in multiple commercial and residential landscapes. Often the source of the introductions is unknown. In some cases new boxwood plants already are diseased, in others cases, the disease may be spread from pruning or other garden maintenance operations.
Boxwood blight was first identified in the USA in the fall of 2011 in NC and CT. Since then, it has been identified within nurseries and/or landscapes in numerous states (AL, CT, DE, FL, GA, KS, KY, MA, MD, MO, NC, NJ, NY, OH, OR, PA, RI, TN, VA, WV) and several Canadian provinces. The disease is spreading to new states each year (FL and WV were added to the list in 2015). Spread is most likely through the movement of infected plants. The disease has been known for over decade in the UK and Europe.
Once a plant becomes infected with this pathogen, the disease can be very damaging. The American Boxwood and Dwarf English Boxwood are very susceptible to the disease. Causing rapid defoliation of the leaves, the disease leaves the plant severally damaged or dead. There are no fungicides that can be applied to help this problem once the plant has become infected. Preventative fungicides may work but this approach is not always practical. Boxwood Blight begins initially with circular tan leaf spots with brown or purple borders. Blackening of the stems is also common. Humid, warmer weather promotes the spread of this disease. This pathogen requires 24-48 hours of leaf wetness to infect the plant. As the disease progresses, the leaves will turn tan and fall off the plant. This disease in readily transferred in many ways because the spores of the pathogen are very sticky and can stick to worker’s tools, clothing, or even animal fur (cats, dogs, rabbits, etc.) that may move through the garden. Once introduced, the disease can be devastating to boxwood in landscapes and nurseries. Here are some tips if you’re concerned about your Boxwoods.
- Proper identification is important. If you suspect Boxwood Blight, take a cutting to your county extension agent. They can send a sample to U.G.A. for testing.
- Be sure not to introduce this disease through new nursery stock. Be sure the plants that you’re purchasing are disease free.
- Don’t allow landscape maintenance companies to prune your plants with contaminated tools.
- Avoid pruning plants when they are wet.
- Implement a preventative fungicide spray program.
- If discovered, remove infected plants and leaf litter from your property. It is recommend that you also remove the soil that the plant was growing in.
Fungicide efficacy trials have shown that those containing chlorothalonil (Daconil, Spectro, Concert II) and fludioxonil (Medallion, Palladium) provide the best control. Fungicides containing azoxystrobin (Heritage), pyraclostrobin (Pageant), trifloxystrobin (Compass), and thiophanate methyl (Cleary 3336) provide fair to good preventative control. Spraying after the disease is present will not control it. ACLM
Kevin Johnson is the owner of Green Leaf Lawn and Ornamental, LLC, based in Blue Ridge. For more information about the devastating hemlock woolly adelgid and treatment options, Kevin can be reached toll free at 866.883.2420 or by email at firstname.lastname@example.org. Or check out his web site at www.wetreatlawns.com or visit www.hemlocks.org for more info.