It is rare that a single disease affects multiple plants that are not related to each other. In the case of diseases that do have a very wide host plant range (such as certain fungal wilt diseases and root rots from fungus-like pathogens), it's usually a case of chronic or acute stress or injury predisposing the plant to infection by opportunistic pathogens that would otherwise coexist in the environment without being a direct threat to the plant. Sources of stress often revolve around root moisture, whether from drought or excessive wetness (whether from over-watering or excessive rain in areas with poor drainage and/or soil compaction).

Trees sometimes take a while (weeks, months, or even years) to manifest obvious symptoms of decline, which makes tracing-back the issue to the original cause(s) with certainty hard to do. Dieback usually cannot be reversed, though how quickly it progresses depends greatly on the agent(s) responsible and how vigorous any still-healthy parts of the tree may be. With maples, one common issue they are prone to developing, at least if they are planted too deeply or over-mulched, is girdling roots. Over time, unaddressed girdling roots can "strangle" the trunk and kill parts of the canopy. Different maple species also have different tolerances, though they do share some vulnerabilities (for instance, they tend to not fare well in soils high in salts, such as from over-fertilization or heavy exposure to ice-melting products). Red Maple, for instance, handles temporary root zone flooding quite well, while Sugar Maple does not. Red Maple is also more heat-tolerant overall, while Sugar Maple prefers cooler climates and nears the southern end of its native range in coastal and piedmont Maryland. We can't tell from the photo which species this is.

Dying and dead trees can still be quite valuable to wildlife, but in cases where branch breakage and falling wood can endanger people or property, it's safer to have them removed. (If you wanted, you could keep the downed wood if you have the space, though, for a brush pile, wood chip mulch, and other uses.)

The shrubs pictured do not look like boxwood; instead, they look like Japanese Hollies (botanical name Ilex crenata), another commonly-grown evergreen. (This species of holly has spineless leaves similar in size and shape to boxwood. In comparison, though, they tend to be glossier, have very subtle scalloped/toothed leaf edges, and are arranged alternately on the stem. Boxwood leaves are arranged in opposite pairs on the stem, the leaves are usually more matte, and their edges are entirely smooth.) Boxwood and holly don't share the same pests and diseases save for broad-impact pathogens like root rot.

In our experience, a common cause of Japanese Holly dieback is Thielaviopsis, a type of fungal root rot (which isn't quite the same as the fungus-like "water molds" typically behind root rot conditions). Information about the pathogen and conditions that predispose plants to infection is in the linked page. There is no practical fungicide treatment for suppressing the fungus, and no cure for existing disease. Dead branches would need to be pruned out as they appear. Planting sites near a home foundation and/or concrete sidewalk might have soil that is too alkaline (higher pH) for the preferences of hollies, which need acidic soil (lower pH) to thrive. The overall yellowed appearance of its live foliage suggest that the soil is outside of the ideal pH range, and/or that they are being kept too wet or getting too dry, which affects the ability of the roots to absorb nutrients. If you would like to check the pH of the area where the hollies are planted, the most accurate method is to have a laboratory soil test taken; we can help to interpret the results if desired.

Soil moisture extremes (too wet, too dry) can also lead to branch dieback. Are the hollies being monitored for watering needs in this ongoing drought? While Baltimore County isn't as extremely dry as other parts of the state, planting locations near surfaces that reflect heat (walls, pavement) and where plants have limited root space can experience more pronounced affects from even moderately dry stretches of weather.

Although it's far from extensive, we noticed a bit of English Ivy beneath the hollies. If you are able, we recommend its removal, since it is an invasive species and can spread rampantly. The fallen maple leaf if that photo, which we presume is from the nearby tree in decline, looks to be either a Sugar Maple or Norway Maple, since the two look nearly identical. (This guide can help separate the two using various traits.) Norway Maple is invasive and, if a tree needs replacing, should not be planted, though it used to be sold more widely at nurseries a couple decades ago.

Miri