Sick boxwoods, possibly spreading

Question: Hi, we have two very sick boxwoods, and possible spread to a third. We have a number of others and are trying to protect the rest, so are wondering what’s causing it and how to stop it spreading. 

The two sick plants are on the north side of our house, in a fairly well drained and mainly shaded spot (see first picture; the third plant with possible spread is in the background). We only noticed they were sick 2 or 3 weeks ago, and it was already quite far along, more or less the same as in the photo. The third bush started showing discoloration in the last week or so (see 2nd photo). It gets a bit more sun but is also at the outlet for our downspout so a bit worse drainage. It’s separated from the first 2 plants by about 3 feet. Third photo shows a close up of the first plants. Thanks I’m advance for your help.

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied June 26, 2023, 12:48 PM EDT
response: The color change on the foliage indicates substantial stress, but from what primary cause is hard to determine. Have you experienced vole issues in the yard in recent years? Sometimes drastic branch dieback like this is caused by their gnawing on the lower branch bark, either over winter or any other time of year. We think there might be some holes and a surface tunnel/"runway" in the soil around the base of the shrub in the first photo, but it's hard to tell for certain. Is there mulch underneath the shrub(s)? We don't see any, but if there is around its base, make sure it's not touching the stems themselves so it doesn't hamper air circulation or provide a sheltered spot for voles to feel safe from predators.  Several commonplace ailments can affect boxwoods regardless of age, so root rot from saturated soil (perhaps for the plant you mentioned was closer to a downspout) is possible, along with Volutella infection or stress related to some other environmental condition. You can explore the linked page to see if any of those scenarios seems to match what you're seeing. We do see some old mite damage, but it's not serious and not responsible for the branch discoloration symptoms or dieback. We do not see indications of Boxwood Blight, so this is not a suspect.  Given how much of the plants are affected, you would need to either consider replacing them or severely cutting them back to force regrowth. (How successful the latter option is will depend on how healthy the roots are.) If you opt to replace the plants and prefer to keep growing boxwood (as opposed to installing a different species of shrub), look for a cultivar with noted resistance to Boxwood Blight and Boxwood Leafminer. There are a handful now that are available. Even if neither of those particular ailments is an issue in the yard now, they are widespread in our region and will pose a threat to most cultivars grown, so it's practical to select the most resilient cultivars available.  Miri

responder: The Question Asker Replied June 27, 2023, 12:15 PM EDT
response: Dear Miri:  Thanks for the response. I don't think we've had any voles, and I didn't see anything looking under the first plants (holes, tunnels, bite marks on the bark). THose two are actually the only ones that don't have mulch--for the others we have noticed that something is digging up the mulch a bit, but I think it might be birds looking for food.  Good to know that it's not boxwood blight. From the info you shared it seems most likely it's root rot or Volutella. Is either of those more likely to only affect some branches on the plant? The discoloration isn't uniform, and seems spread a bit randomly throughout the branches.  Thanks,Bill

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied June 28, 2023, 2:50 PM EDT
response: Hi Bill,  Either condition can cause patchy dieback at first, but we find Volutella to be one of the most common diseases that boxwoods develop. Ironically, despite causing significant dieback once established, it usually arises as a secondary problem in that other stress factors or injuries (comparatively minor stem damage from winter weather, for instance) predispose the shrub to Volutella infection, potentially as long as a year or two prior to symptom appearance.  Given that stress is a predisposing factor and that Volutella development is sometimes gradual, an affected plant might be able to recover once all symptomatic growth is removed (though we don't know how often this happens or how long sufficient regrowth to fill back in again will take). However, with root rot, usually decline progresses and can't be remedied. If you do wind-up removing any of the boxwoods entirely, you can examine the roots to see if root rot is suspect. Dying roots will be mushy, not firm, and the outer sheath (the mushy part) will easily detach from the tougher, stringy inner core of the root. Roots dead from other causes take a harder tug to snap and tend to just break in half all the way through rather than sloughing-off a layer like that. Root rot fungi tend to be opportunistic as well, infecting roots already stressed by soil that stays too wet or which is too compacted since that deprives roots of sufficient oxygen for healthy growth.  Miri