We're not certain what is causing the yellowing and poor growth, but suspect environmental conditions over a disease issue. Few horticultural references make any mention of serious infections of Calycanthus, but the bacterial crown gall and powdery mildew occasionally listed do not appear to be present in the plants pictured, since their symptoms manifest differently.

You mention that supplemental watering is provided when needed. Are the plants being monitored for watering needs by feeling the soil about six inches deep, next to the root zone? Or are watering needs being determined another way? This might be a situation of accidental over- or under-watering. (Last year, for example, most of Maryland experienced drought conditions for much of the growing season.) Our watering recommendations for trees and shrubs is to feel the soil at that six-inch depth, more or less, and water when the soil becomes somewhat dry to the touch at that depth; if damp when checked, watering is probably not needed.

Unfortunately, the only way to verify root loss from overwatering is to dig up a plant and inspect the roots, which isn't necessary just yet unless you wanted to replace one of the shrubs since a stressed plant might not recover well from the added stress of being dug and replanted. If decline continues and you do dig one up, look for roots that are mushy/soft and which pull apart fairly easily. The outer sheath will detach with little effort and leave the stringy inner core of the root behind. Roots dead from drought or other causes will instead snap more cleanly in half, and will take more effort to break.

Were the root balls well-loosened, if they were tangled or matted, when removed from the pot for planting? If not, that can slow establishment and make accurate monitoring for watering more difficult.

For now, just monitor the plants for different or worsening symptoms, and feel free to share more photos if that happens. Make sure any mulch is not burying the crown of the plant (where stems emerge from the roots) so it gets good air circulation and so the plant isn't installed too deeply. (The point where roots branch off of the main stems should sit just at the soil surface, like the root flare on a tree.)

While yellowing leaves sometimes indicate a nitrogen deficiency, there is little reason to suspect one unless there were lots of undecomposed wood (mulch, sawdust, tree stump grindings, etc.) in the immediate area, since microbes decaying that wood use-up some of the surrounding soil's nitrogen. Those impacts, though, tend to be highly localized, and the violets growing near the Calycanthus appear unfazed. Only a laboratory soil test would reveal any other nutrient deficiencies (not that they are suspect here), but testing does not measure nitrogen since its levels fluctuate so regularly in the soil. While testing can also reveal the acidity level (pH), since this can play a role in nutrient availability to roots, Calycanthus is so adaptable with regard to pH tolerances that it's unlikely the soil here is that far outside of its acceptable range (around 5.0 to 8.0, according to Cornell).

Miri