Boxwoods are one of the most iconic and ubiquitous garden design choices for shrubs. They are almost like a lawn in that they give the eye a rest from the activity of the rest of the garden, while providing year-round structure and interest when plants are out of bloom. They can be clipped into fanciful shapes or simply into crisp outlines around a little wildness within their confines.
So how did such an essential, charming plant end up with so many possible diseases? Boxwood blight is a fungal disease (Cylindrocladium pseudonaviculata) that features brown leaf spots with dark edges, black cankers on stems and severe dieback and defoliation. Fungal diseases such as Phytopthoras can also cause root rot, resulting in yellowing of leaves as entire parts of the plant die. Macrophoma leaf spot caused by a parasitic fungus (Macrophoma candollei) causes reddish brown spots. Volutella stem canker, caused by the fungus Pseudonectria buxi, can take out entire parts of the plant. There is seemingly no end to the fungal pathogens that can cause problems for boxwoods. Nematodes can cause problems as the microscopic worms eat through the roots. Boxwood even has its own spider mite, Eurytetranychus buxii. Now, if all the other diseases weren’t enough, there are boxwood leafminers (Monarthropalpus flavus).
Are your boxwoods looking unhappy and not so healthy? Do you see yellow to brown spots on the leaves? Is the canopy sparse? Is there premature leaf drop? It’s possible you have leafminers. These unfortunate pests do a lot of damage and since boxwood grows slowly, removing infected plants and putting in new ones makes for a rather dowdy hedge; some big and some small is not a good look for a hedge you’d like to be all one size and uniformly trimmed.
The culprit is a tiny orange fly that looks a bit like a mosquito. They insert their eggs under the leaves of this larval food, which hatch into larvae or maggots and cause the leaf to blister as they tunnel through, munching as they go. After all this eating they’re exhausted and ready for the next stage so they pupate and hang from the leaves to await transformation into adulthood. The newly hatched females emerge from the leaves leaving a hole behind when they exit. The adults look like a swarm of orange gnats when they fly and then they mate. The next generation of eggs is safely ensconced in the leaf tissue and, their biological destiny fulfilled, the adults die. The larval stage will overwinter and become active in the spring.
The blisters mess with the vascular tissue and cause leaves to drop as the flow of nutrients is disrupted. Damaged shrubs grow slowly, have sparse canopies, tip dieback, and drop leaves prematurely. These pests rarely kill the plant but can make it pretty unattractive.
What to do? Select less susceptible cultivars such as Suffruticosa, Pendula, Argenteo-variegata, Handworthiensis, Pyramidalis, or Varder Valley. Encourage green lacewings and spiders and other natural controls, which improve the overall health of your garden. Healthy plants will tolerate insect damage better. Try to reduce the population and hard prune before the adults hatch out. Clean up debris under plants in fall and winter to get rid of the pupae still in the leaves. Some pesticides such as avermectin and imidacloprid have shown promise, but they’re not environmentally friendly and could be responsible for contributing to honeybee decline. Research is ongoing, so currently UCCE recommends using the hard pruning method and designing your new boxwood garden installations with pest-resistant varieties as listed above as opposed to using pesticides.
With so many threats out there, if you have healthy boxwoods consider yourself lucky or diligent and enjoy this delightful and important garden staple.