Symptoms on the cut branch are difficult to see clearly, in part because the dieback is more advanced, but you can use the information in our boxwood web pages (see link) to determine which issue(s) are more prevalent. Boxwood Blight tends to cause more widespread and drastic leaf loss than leaf browning (that would be more like Volutella Blight), which we do not see here. Environmental stress conditions tend to contribute to dieback symptoms or discoloration with boxwood shrubs, and also tends to be the precursor to infection. Volutella, for example, is often a secondary infection that arises when the shrubs were injured by winter weather, even if that injury took place the year prior. Boxwoods that are pruned regularly and maintained with an even, dense, manicured shape also are more vulnerable to infection and pest outbreaks since the dense outer layer of foliage impedes air circulation. When foliage doesn't get good airflow and remains damp for longer periods after rain, dew, or irrigation, those conditions are more conducive to spore infection.

We see what might be leaf blistering due to Boxwood Leafminer, a gnat whose larvae feed inside the foliage. This is a very common boxwood insect pest, and while certain insecticides may work to suppress them, they would require annual reapplication and may risk harming any pollinators that visit the plants. (Boxwood flowers may be small and not showy, but they do bloom and bees can visit them.)

Boxwood Blight would entail removing any infected plants. Volutella tends not to kill roots, so a drastic cut-back to leave branches just a few inches off the ground can allow them to regrow (gradually, since boxwoods have a slow rate of growth anyway), assuming the root system remains healthy. No fungicide can treat a disease that is already manifesting symptoms. If replacing the plants is your preference, we recommend either using different shrub species than boxwood, or selecting a boxwood variety that is a more modern introduction that is resistant to Boxwood Blight. (About a half-dozen such varieties are on the market these days. Several are also resistant to the leafminer, though no boxwood variety is immune to all potential problems.) Choose a variety that matures to the approximate mature size you want or can accommodate without relying on pruning. You can still prune to shape them into more even forms if desired, but instead of shearing, it's best to snip wayward growths individually with hand pruners, which won't cause the new growth to be too dense.

If you see symptoms upon closer inspect that you're unsure of as you compare them to the information pages above, feel free to send us additional close-up photos.

Miri

Hello Ken,

Yes, Volutella Blight and Boxwood Leafminer are so common that they are likely culprits, and they certainly can (and do) overlap on occasion.

Fungicides are preventative measures only; they cannot cure existing disease or reverse its damage. They also don't always work, since applications need to be made just before spores enter the leaf tissue but not so early that the fungicide wears off before infection. Rain events can weather-away applications over time, and depending on how long the window of opportunity is for the fungus in question (in what conditions it infects), multiple fungicide applications are often needed to cover that period of time. If an infection began but was still asymptomatic by the time a fungicide is applied, disease will still develop. Given those challenges, and the cost and hassle of fungicide use, it's generally not practical or recommended for use. (The cost of plant replacement could certainly be more worth the investment than a few seasons of rehabbing a damaged plant long after infection set it.) Fungicide residues can also harm other organisms, even though they target fungi (and in a few cases, also pathogenic bacteria).

Since Volutella is not as lethal to Boxwood as Boxwood Blight, you can try pruning in lieu of treatment. Plus, that will also physically remove the vast majority of juvenile leafminers that may be present as well. A cut-back can be done in spring, ideally closer to the time of the last frost date so any new growth that appears after the trim isn't zapped by frost, since it will be tender. Attached is an example of a "renewal"-pruned boxwood, though it's hard to judge the height it was cut down to. I saw this plant and would estimate it was a foot high in the photo (so, post-pruning). I don't know how much earlier the cuts were made compared to the new growth emerging, but would guess it was no more than one growing season prior. The response speed of new growth after pruning might vary from one cultivar to another, though boxwoods as a group are fairly slow.

Extension doesn't keep track of every pesticide brand available and we generally don't make brand recommendations, but as far as active ingredient goes, the primary options for Boxwood Leafminer include spinosad and (through a certified pesticide applicator for legal reasons) one of the neonicotinoid-class chemicals (imidacloprid and dinotefuran are common). Spinosad absorbs into the leaves the spray coats and may help to kill newly-hatched larvae. Leafminer adults are hard to kill since they fly about over the shrub's foliage and don't land often. (You're not going to want to spray the cloud of them.) The neonicotinoid insecticide group is soil-applied and absorbed into all plant parts, and poses a greater risk to pollinators and beneficial insects if they ingest it via nectar, pollen, or by other means. It could be applied after the shrubs flower to reduce that risk. In either case, larvae stop feeding in summer when the weather is hot (perhaps mid- to late June or so), so treatments will not be effective mid-season; they have to be used soon after egg-laying (when the adults are out, around late April or early May in average weather) or as the weather cools again towards autumn. By then, though, they've already done some damage, so it's better to catch them young.

We would still suggest trying the pruning option before resorting to insecticide. When abundant, leafminer damage can be an eyesore, but at least it doesn't seem to affect the plants too badly.

Empty areas may or may not regrow, because it depends on what killed those branches back and how far down the stem it progressed. (Since branch interiors on very dense plants naturally lose all of their leaves over time, it can be hard to tell what leafless growth on the plant's interior is dead versus just bare from lack of enough light.) As more light reaches the interior again after pruning, branches that still have living buds dormant underneath the bark may resume growth this coming season. With live, healthy wood, pruning triggers growth, so if stems are in good condition when pruned, they can be expected to begin growth the next growing season (mainly by the end of spring, when most of the plant's new growth is produced) and would only be delayed if there is still not enough light reaching those buds.

Fertilizer can support growth but won't necessarily trigger it by itself. Plus, ailing plants should not be fertilized; it can exacerbate plant stress (unused minerals that build-up in the soil can interfere with root function) and may actually worsen a pest or disease outbreak. If regrowth appears but seems weak and sparse for the first year, and if leafminers don't return in force, you can try a modest dose of a time-release fertilizer so nutrients are released slowly. Less is more in this case, especially since it's unlikely the soil itself is actually deficient in key nutrients. Follow package directions for the minimum amount to apply (if they give a range), and holly-tone is fine but need not be the only option. (Holly-tone is geared towards acid-loving shrubs, which boxwoods are not, but it does not markedly alter the soil's acidity.) You are correct that, of all the nutrients in fertilizer formulations, nitrogen is the one the plants will use most heavily, and the one that may leach out of the soil faster than the other major nutrients over time. Still, if you use an organic (biodegradable) mulch over the shrub root zones, that will also be providing a slow source of nutrients.

Miri

You're welcome.