Cimicifuga leaf edge browning
Although, there will be a few problems with leaves browning due to different reasons, I notice your plant is a bit isolated in the area. I would expand the area a couple of feet or more around the plant to make a more agreeable woodland environment, and amend the soil with a good amount (3") of compost (you can buy in bags) The soil around it may be taking moisture or containing it around the plant. Check the soil with your trowel and feel what's going on. Moisture is good, but of course well-drained soil is also needed. Plants will react to the weather changes, dormancy will set in slowly as well shown by browning leaves. More below:
Cimicifuga prefers humusy, moisture-retentive, mulched soils in a site that is protected from strong, drying winds (compost will help a great deal). Although generally problem-free, there are a few problems that could be causing your leaf blotches. Dark spots on the leaves of your bugbanes could be caused by leaf spot diseases, anthracnose fungus, summer scorch (dead and browning leaf margins) or tarnished plant bug sucking damage (brown sting marks). These potential causes all have different visual signs that can be used to help narrow down the root cause of your problem. By looking at the location, size, extent and color of each of these leaf problems, you should be able to rule out some of them. Give this plant time to acclimate and establish itself, once you give it what it needs to flourish. Hope this is helpful.
Possible leaf blotch causes:
Leaf spot diseases: medium-sized, dark brown or black rounded areas of dying leaf tissue, often between the leaf veins and frequently along leaf margin. Not symmetrical. The University of Maine has some online images of the Ascochyta sp. leaf spot fungus on bugbanes. Other references to Ascochyta actaeae leaf spot fungus on Cimicifuga (in PA and NY) can be found on page 201 in the Diseases and Pests of Ornamental Plants by Pascal P. Pirone (1978 John Wiley & Sons Publisher) and on page 618 in Westcott’s Plant Disease Handbook by Cynthia Westcott and Ralph Kenneth Horst (2001 Springer Publisher). Bruce Watt of the University of Maine has put a photograph of Ascochyta fungi on a black cohosh leaf online for easy comparison.
Anthracnose fungus disease: large, irregular brown or black patches of dying leaf tissue.
Summer scorch: brown, drying, dying leaf tissue progressing from the leaf margins inward, somewhat symmetrical. A physiological/cultural problem.
Tarnished plant bug: small brown sting marks, deformed stems, wilting new growth.
In diagnosing your bugbane leaf blotch problem, one of the key clues is the fact that your bugbane is having the same problem as others growing wild in your region. This would suggest that it is an environmental problem instead of a fungal or insect attack. It is unlikely that a fungus or insect attack would be so prevalent that it would cover such a broad area. More likely, it is an environmental situation since the weather does manifest itself widely and would affect numerous plants. To help protect your bugbanes from future drought causing leaf scorch, amend your soil with humusy, organic, moisture-retentative soils and mulch the plants well. Shredded leaves or leaf mold would be ideal as a nutritious mulch to conserve soil moisture. Also take a look at the amount of direct sunlight your plants are receiving. Bugbanes prefer part shade to full shade.
The Missouri Botanical Garden, on their plant finder website, say that black cohosh (Actaea racemosa (syn. Cimicifuga racemosa)) leaf margins may brown up (scorch) and growth may slow down if soils are not kept consistently moist. They also say that it is easily grown in average, medium moisture soils in part shade to full shade. It prefers humusy, organically rich, moisture-retentive soils. The foliage tends to scorch and otherwise depreciate if soils are allowed to dry out. It is best sited in locations sheltered from strong winds. This is a slow-to-establish plant.
Additional cultural information for black cohosh can be found on the Cornell University Growing Guide website.