LOCATED IN NEW ENGLAND

I planted dinnerplate dahlia tubers back in the late Spring. They have taken off since the first little leaves popped through. Thrilled to see, as gardening was a new hobby I chose to focus on during my time off from work.

Now about 2.5' tall, leaves are huge, strong and green. Base & stems are super thick and sturdy. I will note these have no buds/blooms...yet.

I've spent a lot of time tending to these new plants, which have been thriving so far this Summer... until the last week or so. I will note, it has been extremely warm and humid. I am now seeing a few issues:

1. I am noticing some more mature leaves around the middle of the plant that are starting to brown and blacken.
2. Smaller leaves towards the base of the plant are yellowing and wilting off before they get to even a fraction of the size that the other leaves are.

I have googled these issues and have seen things from over-watering to certain fungal issues. Any thoughts? And more importantly, am I right in assuming these plants are still able to be saved?? I have attached photos to this post. I have invested a lot in these plants and am so excited to achieve my first blooms. Any and all tips, tricks, diagnostics and advice is welcomed!

Thanks!

Sarah