Most of us enjoy evergreen shrubs like boxwoods because they hold steady throughout the year, providing a permanent framework to other plantings which often disappear in the winter. Plus, they take well to shearing. So why are your boxwood shrubs turning yellow? Good question.
Here are a number of possible causes:
1. Volutella fungus
2. Macrophoma leaf spot
4. Phytophthora root rot
5. Cylindrocladium buxicola
One that I want to point out, and give you some possible substitutes, is boxwood blight. This is not a fun problem to have. And, since it was first discovery in Great Britain in the mid 1990, is has also been found in Europe, New Zealand, Canada and as recently as 2011, in the U.S.
About Boxwood Blight
The genus Buxus is susceptible to boxwood blight which is caused by a fungus, Cylindrocladium buxicola. The fungus spreads by wind, water, birds and animals and can also spread by contaminated clothing and shoes. Unlike many fungal infections which spread when plant parts are injured and fungus then invades, the fungus doesn’t need a wound to infect the plant. It prefers high humidity, moderate temperatures, and poor air circulation. Like all diseases, this one also needs to have a susceptible host, the correct environment, and pathogen present. In the case of boxwoods, it has been found that the most susceptible to this blight are Buxus sempervirens (which includes American and English boxwoods: Buxus sempervirens and Buxus sempervirens ‘Suffruticosa’). Other species may be less susceptible, but the keyword is “less.” Withing the boxwood family, Buxaceae, plants such as sarcococca and pachysandra have shown susceptibility to the disease.
Here’s a good FAQ on boxwood blight from Cornell Cooperative Extension.
What to do if your boxwood has Boxwood blight?
One option is to try to save it. Here’s a PDF from Bartlett Tree Experts that has good information on plant health care recommendations for Boxwoods. See this about boxwood blight on Wikipedia . I think that a better option, is to replace your boxwood with a good substitute that is not susceptible to the blight and will thrive.
Considering that the blight thrives in humid conditions, opening up and thinning the boxwood to allow for more air to the its center is a good idea. This though is challenging when grown as a hedge designed to be full, closely planted and sheared regularly to be kept compact.
If you’re starting from scratch, or are dealing with yellowing, branches dying back and determine that you have blight, you may want to replace with other plants. Who wants to be spraying nasty fungicides and working that hard? No fun in that.
Here are a Few Alternatives
Ilex vomitoria ‘Nana’ – good dwarf form substitute for mild climates
Euonymus microphullus – susceptible to phytophthera
For a more complete list of boxwood substitutes from Cornell Cooperative Extension for American and English boxwood. The list is grouped in order of greatest similarity to boxwood form, texture, and function in the landscape.
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