While the foliage damage resembles a leaf infection, we think the issue instead lies with the crown (the base of the plant, where growth emerges from the roots). It's hard to see clearly in the photo, so we're not certain, but the plant might have contracted Southern Blight, a fairly common fungus that can kill above-ground growth. If you would like to share a close-up photo of the crown, we can try to confirm the diagnosis. We'd be looking for the characteristic "mustard-seed"-like spore capsules along with white mycelium (fungal filaments) growing over the stem bases, as pictured on the linked page.

If Southern Blight is responsible, all you can do is to remove the plant, scrape away some of the surface soil where it was growing, and replant. Even though the fungus doesn't tend to kill roots, we can't be sure the plant will regrow after infected top growth was removed, and leaving the soil at its base may risk a rebound infection if the spores remain. Southern Blight can thrive in hot temperatures and humid weather, which is unfortunately the trend for Mid-Atlantic summers. You didn't necessarily do anything wrong if it occurs, since it's a widespread fungus that takes advantage of the weather and conditions of plant crowding, poor air circulation, and/or too much mulch laid close to the crown, and not all plants in such conditions succumb to this blight.

If not Southern Blight, then we're not certain which infection this is, as more information about Turtlehead vulnerabilities to pests and diseases needs to be studied, with the increase in the planting of native species in recent years. Our reference books thus far do not list any serious issues specific to the plant, but we have encountered some suspected leaf spot diseases before, though haven't identified them to genus/species. (Usually, leaf spot infections are an eyesore but not a threat to long-term plant health. There are many generalist leaf-infecting fungi and bacteria, and Turtlehead can contract Alternaria Leaf Spot and likely several others. The symptoms pictured don't appear to align with Alternaria.)

Since fungicides are not warranted in this situation, all that could be done for now would be to remove and dispose of some of the more heavily-symptomatic leaves, and keep monitoring the plant for watering needs to keep the roots healthy. (Fungicides can't cure existing disease, only protect healthy growth; they don't always work and the cost of the multiple treatments usually needed might outweigh the cost of a new plant; and their use might risk harm to pollinators or other beneficial insects and other organisms. Since Turtlehead tends to be grown for wildlife benefit, we would not recommend using a fungicide on the plant.)

Miri

Hello Pat,

Thank you for the additional photo. Unfortunately, as you might suspect, this looks like the classic signs of Southern Blight fungus activity. Since the plant is expected to decline in that case, as there is no cure for Southern Blight, it would be best to remove it before it spreads to nearby plant crowns.

If you wanted to try experimenting with rooting cuttings of the upper stem tips, assuming any look asymptomatic (not that the pathogen is present there, just that the loss of access to enough moisture and nutrients from the dying tissues below might mean it's too weak to root), you could try to salvage the plant that way, and discard the rest. We're not sure how easily Turtlehead would take to rooting this time of year (or how easily it roots in general, as some perennials are harder to propagate than others), but it might be worth a try if it's not a hassle to watch the cuttings closely for watering needs (keep them in shade) while they root. Using rooting hormone powder or gel would probably help, not only to speed-up rooting itself, but because some brands include a fungicide ingredient to reduce the risk of stem rot in the cut end.

Miri

You're welcome!