American Boxwood Blight or . . . ?
All 8 of the boxwood shrubs on the sides of our front steps looked fine until end of last summer or beginning of fall. Just one of the boxwood shrubs started turning yellow with a small branch at the bottom, which I cut out. Over the late fall and through until now more and more of that one shrub has turned yellow. All of the 7 other shrubs are healthy. Can you please tell me what this is and how to deal with it? Thank you in advance! John
Frederick County Maryland
There could be several factors contributing to this boxwood's decline. While it does not look symptomatic of boxwood blight, there are other "blight" diseases that commonly affect boxwoods in our area. This page is an overview of boxwood ailments including boxwood blight: https://extension.umd.edu/hgic/topics/boxwood-culture-and-diseases-including-boxwood-blight.
The most likely cause is from root dieback and/or infection from our overly-wet spring in 2019 as well as the weeks of drought and hot weather experienced later that summer. Root systems that suffered damage earlier were too weak to sustain plants through this swing and leaf and branch death ensued. Similarly, moist, cooler weather can delay the death of tissue that has already lost some of its water supply via stem or root damage, and when hotter and drier weather prevails, the plant can no longer maintain all of the above-ground growth. Even if this weather pattern didn't cause all of this damage, it set up stressful conditions for the plant that could have predisposed it to fungal attacks like those mentioned in the link. Wet conditions also promote the survival, spread, and infectious ability of fungal spores.
Voles chewing on plant stems at ground level (or roots below-ground) can also cause dramatic dieback such as this, because the chewing damages the tissue transporting water to the branches, which is just underneath the bark. You can check to see if any bark looks removed or if the plant pulls up easily when removed, although a loose rootball can also be explained by root death due to rot.
Our clay-based soils are easily compacted, and compacted soils tend to have poor drainage. It is hard to tell from the photo, but if this plant sits at the base of a slope (are those steps next to the plants?), then excess water draining into that spot would have contributed to the plant's stress. Are de-icing products used on the walk in winter? Sometimes excessive salt levels (not just sodium salts, and they can come from fertilizer too) can build up over time and cause root damage.
Depending on the determined cause, a replacement could be installed in that spot if drainage or soil quality issues are addressed, possibly through amendment with compost. Alternatively, an evergreen unrelated to boxwood could be used if the soil tends to stay moist or of disease problems reappear in the other boxwoods this year. The best approach is to keep the plants as healthy and stress-free as possible, as fungicides are not curative treatments and obvious dieback can occur well after the infection began.
In principle, relocating the plant may work, but in the photo it appears as though too much damage has already been done. Regrowth from what healthy tissue may remain is likely to be slow and impractical from an ornamental point of view, as the plant will look wimpy and misshapen for a long time. Plus, we do not want this ailing plant harboring pathogens that could easily spread to the others if they become susceptible.
If there is a downspout in the area that isn't shunted away from the beds, this additional water is also contributing to the challenges of the spot. Either way, you could try using a replacement plant that's more tolerant of wet soils. A small-statured shrub (evergreen or otherwise) or perennial may be a good choice. For evergreen shrubs, the newer varieties of dwarf Inkberry Holly (Ilex glabra 'Gem Box' and 'Strongbox') might be good options. They may grow a bit taller than the boxwoods eventually but are slow growers. There is a dwarf variety of Illicium (itself an uncommon shrub in our area, as we are on the northern edge of its cold hardiness range) named 'Swamp Hobbit' that would stay quite short (for a plant of this type) and tolerate the wetness if the site isn't in too much hot sun.
If you use a soil amendment, compost would be better suited than mulch unless it is the finest grade of pine bark mulch. Organic matter in compost helps to bind the fine particles of clay in such a way that makes it drain more effectively. Use amendments sparingly, however, as using two soil types next to each other can cause drainage problems of its own.