Ali, the white coating on the stem and patches on the lower leaves **look consistent with powdery mildew**. It can affect dahlia stems as well as foliage, but it seldom kills the plant. There is still plenty of useful foliage here, so I wouldn’t cut the whole plant down now. ([dahlia.org](https://www.dahlia.org/docsinfo/articles/fungus-control-for-dahlias-and-the-garden/?utm_source=openai)) **I’d take these steps:** 1. **Remove only the worst-affected leaves**, particularly those already yellowing or dying, and collect fallen leaves from the pot. Don’t strip off every leaf with a light dusting. Removing infected material helps reduce further infection. ([rhs.org.uk](https://www.rhs.org.uk/disease/powdery-mildews)) 2. **Keep the compost evenly moist, not waterlogged.** Check beneath the surface before watering; if it is drying out, water thoroughly and let excess drain away. Dahlias are more susceptible to mildew when they suffer dry spells at the roots. I can’t judge the compost’s moisture from the photograph. ([rhs.org.uk](https://www.rhs.org.uk/plants/dahlia/growing-guide?utm_source=openai)) 3. **Give the pot more air and sunshine.** It appears close to a fence: if practical, move it out a little and leave space around the foliage. Still, humid air encourages mildew. Avoid extra high-nitrogen feed, which encourages susceptible soft growth. ([rhs.org.uk](https://www.rhs.org.uk/disease/powdery-mildews)) **For the coated main stem:** I would leave it in place if it is firm and supporting healthy growth rather than sacrifice the remaining canopy. My first approach here would be the measures above, not spraying; the RHS recommends non-chemical management and advises against homemade treatments. ([rhs.org.uk](https://www.rhs.org.uk/disease/powdery-mildews)) As it is late September, keep the remaining green growth going until it naturally dies back or frost blackens it. Then remove the affected top growth and fallen leaves, and protect the tubers from frost for winter. **Mildew alone isn’t a reason to discard the dahlia.** ([rhs.org.uk](https://www.rhs.org.uk/plants/dahlia/growing-guide?utm_source=openai))