The leaf spot disease symptoms on the Turtlehead can be quite common on a variety of perennials and shrubs, and might be Cercospora or a similar fungus. Although they can be an eyesore, they rarely causer serious problems for the plant they infect. While use of a fungicide as a preventative, used starting earlier in spring, might suppress infection, there are several reasons we generally don't recommend their use:

• they need yearly applications, often several times per year to maintain protection of healthy leaves
• they can fail to work if applied too late (like after infection took place but symptoms weren't yet present) or if a spray schedule is interrupted by rain (prolonged wetness of leaf surfaces give most disease spores an easier opportunity to infect leaves)
• their use might pose a risk to pollinators or other organisms, since their impacts aren't always limited to just fungi and a few bacterial pathogens
• they can be a costly treatment option for infections that are more cosmetically damaging than serious, especially considering that plant replacement (if a disease became too extensive or damaging) would be much cheaper

With the rapid increase in the use of a variety of native plant species in home gardens in recent years, we are still catching up with learning about the typical pathogens that can affect them. Cercospora and similar fungi are quite common, though, and the good news is that recurring infection actually tends to demonstrate a plant's tolerance for the disease, since if it were a more serious infection, the affected plant likely would have died out long before now.

Watering drought-stressed roots can help plants tolerate other stressors, and while native plants (when grown in conditions compatible with their tolerances) can handle moderate drought well enough if they're not a moisture-loving species (which Turtlehead often is), not being watered might worsen any other condition they are currently facing, or may lower plant vigor overall when it comes to regrowing the following year. If you are unable to water due to their location or limits on water availability, that's understandable, but if possible, see if they can be irrigated periodically during this prolonged drought, especially since much of 2023's growing season also experienced notable drought conditions. In general, Black-eyed Susan is more drought-tolerant than Turtlehead. The organic matter you've added over the years with layers of mulch that have self-composted and mixed-into the topsoil by the actions of soil life (worms, insects, etc.) will help any applied water be absorbed and then retained.

While Black-eyed Susan can contract its own leaf spot infections (bacterial angular leaf spot and fungal Septoria are two of the most common), the stem dieback is unrelated, and the foliage on the plants in your photo looks great. The scattered stem wilting and browning might be due to stem-boring insects using it as a host plant, a different fungal infection near the soil line, or just physical damage from wildlife. We can't tell from the pictured symptoms what the cause was, but at least the recourse is the same: pruning it off.

Miri

You're welcome.