More sinister complications can arise in your dahlias which can sometimes kill the plant permanently.
These may include:
Stem rot – Stem rot occurs when dahlias are growing in heavy, poorly drained, wet soil. Look for a white ring in the soil around the stem. The rot will creep in and kill the stem and advance down into the soil to kill the tubers.
Mosaic virus – Mosaic virus dwarfs plants and distorts leaves. You must destroy the plant, as there is no cure.
Botrytis – Botrytis blight is almost as bad and causes flowers/buds to rot and covers the plant with powdery gray mold. Remove any affected part and destroy it.
Aster yellows – Leafhoppers, which are frequently found on plants, are the vector for Aster yellows, a disease where leaf-like tissue forms where you should be getting flower buds. Plants are sadly a loss.
Viral issues – Problems with dahlias also include verticillium wilt and necrotic spot virus. Infected soil causes the former and leaves become black, brown or greenish-brown. Necrotic spot virus is caused by the feeding of thrips.
Powdery mildew – This fungal disease is easy to spot by a mealy, powdery growth that appears on leaves, usually late in the growing season. Although powdery mildew is rarely fatal, it can definitely affect the appearance of the plant
Wilt – Fusarium wilt and verticillium wilt are fungal diseases that cause wilted, yellowing leaves before the plant turns dark brown or black and eventually dies. Verticillium often appears when weather turns warm after a cool period, while fusarium is most severe when the soil is warm. Never plant new dahlias in affected soil