This does appear to be a type of crown rot, though the causal agent can't be determined by symptoms alone, though for remediation options, it won't matter much. Mycelium is just the general term for fungal body (as opposed to a mushroom, the "fruiting" body of a fungus), but it's not visible here, unlike with some fungi. Sclerotia are also just fungal structures that help a fungus to survive adverse conditions, like drying out or winter dormancy.

Many different genera of fungi (and a few bacteria) can cause root, crown, and stem rot. None can be treated or cured once they manifest, and affected plant parts will need to be removed and disposed of (don't compost). Maintaining good air circulation by spacing-out crowded plantings can help to reduce the risk of future infections, though admittedly the dense nature of a groundcover (to suppress weeds and erosion) is the reason they are grown, so that prevention method is hard to accommodate in those situations.

Ajuga can be invasive (plants have been documented growing in natural areas in most Maryland counties), so if you are open to it, we encourage you to replant with a different species of groundcover (or a mix of species). (What appears to be Vinca minor next to those Ajuga is invasive as well.) While any lush or dense planting runs a greater-than-average risk of future instances of crown rot or Southern Blight (a high summer disease; not the issue here), at least using a mix of species helps the planting to be more resilient overall, since some species might be more resistant to infection or other sources of environmental stress.

Miri

Thank you so much, Miri! Is Mazus reptans invasive? That’s my first thought for replacing the ajuga and vinca minor. Maybe sweet woodruff? No point in replacing an invasive with an invasive. There seems to be so much conflicting information on the internet about invasive status. I’ve seen natives listed as invasive and that doesn’t make sense. I’ve also seen people claim that any non-native plant is an invasive species. Do you have a recommendation if those two options are kosher? Maybe a thyme? It’s under a sugar maple and it gets very dry there. I had Eurybia divaricate under there, but they were very thirsty and aren’t thriving. Deer is also an issue (of course).

 

Mazus reptans is not listed as invasive in Maryland (or in the Mid-Atlantic Invaders Tool database), though in garden settings, it is very aggressive. Sweet Woodruff is vulnerable to dieback from similar blights (including Southern Blight in summer), and is similarly non-native (if that matters).

Not just any non-native plant is invasive; there is a distinction in how aggressively plants spread and displace others in natural habitats to give them an "invasive" designation. We don't know what reference would classify any native plants as also invasive, though some native species can be aggressive spreaders.

Thyme needs full sun to thrive and is not very hardy in our heavy soils that can compact easily; they need excellent drainage. The groundcovers web page we linked to earlier provides plant lists for various conditions, including dry shade. White Wood Aster should do okay in those conditions, though as you noted, sometimes deer or rabbits eat them, and they do need more regular moisture during establishment (the first year or two after planting). Sugar Maple is hard to grow anything under due to its shade and root competition, and the younger a plant you can start with (like plugs), the more easily they should be able to establish, though any new plant will need careful attention to watering (even if a drought-resistant species), especially given the competition with the tree.

Miri

These are excellent resources! Thank you so much!!

 

You're welcome!